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Scientific literature for mineral soil nitrogen content (ammonium and nitrate) measurements usually differs regarding the chosen soil sample conservation method (fresh, frozen or air-dry) before soil extraction and analysis. In addition, most of the commonly used methodologies focus on the definition of the analysis processes, regardless of the previous sample conservation methodology. With the aim to fill this gap, we performed an analysis of the conservation method effect on the nitrate, and ammonium content in frozen or air-dried samples, comparing the results with direct fresh extraction. Moreover, the effects of additional soil parameters, such as soil texture, organic matter content or mineral nitrogen content range were also studied. The results showed that both, frozen and air drying soil samples, were capable of reliably preserving the N content of soil samples in most cases. However, some ammonium losses may occur in frozen samples when a high N content (>30 mg kg−1) is present. It was also observed that air-dried soil samples can reduce the soil nitrate and increase ammonium content in samples with a high N content. It was also observed that significant amounts of organic matter in soil can alter the mineral N measured depending on the conservation method chosen. On the other hand, the soil texture presented small effects on the mineral N measurements. In any case, a broader range of soils conditions (including i.e. pH or different natural organic matter content) should be further tested to confirm our findings.

Urbanisation induces nitrogen (N) enrichment in tropical soils. This enhances soil acidification, leading in turn to a decrease in effective cation exchange capacity (CECe) and base saturation (BSe). Here, we investigated if and how these effects of urbanisation-induced N inputs affect the soils of tropical urban agroecosystems. Our aim was to quantify the impact of urbanisation intensity (UI) on total N (TN) contents and stocks, and to detect possible adverse N effects on soil pH, CECe, and exchangeable bases in rainfed urban arable soils of Kumasi, a tropical city in Ghana, West Africa. We analysed 618 topsoil samples (0–10 cm) from arable fields within the urban perimeter, categorised into four classes forming a gradient from low to high UI. For baseline comparison, 9 samples each from both rural arable fields and an urban forest were included. Urban arable soils generally had higher TN levels than their rural counterparts. Among the urban arable soils however, TN levels decreased with increasing UI, possibly as crop production and related N export through harvest intensified. Conversely, increasing UI correlated with increased levels of exchangeable Ca2+, K+, and Na+. No adverse N effect on soil pH, CECe, and BSe was detected. This could be explained with the strong TN-SOC relationship (R2 ≥ 0.9), and an increasingly strong relationship between exchangeable bases and TN and SOC (R2 = 0.5–0.8) with increasing UI. These relationships suggested a major common source of TN, SOC, and exchangeable bases. As the soils are neither chemically fertilised nor limed, these findings supported earlier studies that this source is provided by dumped biodegradable household waste, charred matter, ash, and construction materials. We concluded that soil N enrichment through these materials in Kumasi is accompanied by abundant bases in the same materials, which overcompensate any theoretic acidification. Our findings imply that not only can the fertility of tropical urban arable soils in cities like Kumasi be maintained, but also that the crops and ground water may not be at risk from the mobilisation of trace elements and heavy metals due to the hindered soil acidification.

Agroforestry systems are often promoted as solutions to address land degradation and climate change. However, agroforestry is an umbrella term for a large variety of systems and it is not clear how their degree of complexity influences their provision of soil-based ecosystem services, such as soil organic carbon (SOC) storage and nutrient cycling. Furthermore, a knowledge gap remains whether agroforestry systems perform equally well on all soil types. The objectives of this study were 1) to assess the links between agroforestry complexity, nutrient cycling and SOC fractions, and 2) to assess how soil texture influences these relationships in Brazilian agroforestry systems. We sampled 59 agroforestry plots across 30 sites in Sao Paulo state, Brazil, and 8 monocrop sites (6 pastures and 2 crop monocultures). The 38 sites represented a soil textural gradient, ranging from very sandy to very clayey (clay content range 25 – 620 g kg−1). An Agroforestry Complexity Index (ACI) was defined based on tree species richness, stem density and pruning management. Nutrient (N, P, K, Ca, Mg) and C contents were determined in litter and soil (0–30 cm depth) samples, and mineral-associated organic C (MAOC) and particulate organic C (POC) in soil samples were assessed as well. ACI was positively associated with C, N, P, Ca and Mg stocks in litter, and these litter nutrients were in turn positively associated with the corresponding soil nutrient stocks. Associations between soil nutrients and MAOC were stronger on sandy soils than on clayey soils, particularly for P, Ca and Cation Exchange Capacity (CEC). For POC, robust relationships with nutrients were only found on sandy soils. Structural Equation Models indicated causal relationships between agroforestry complexity, P and Ca cycling, and MAOC and POC stocks in topsoils. Our results indicate that nutrients effectively cycle from in situ mulch into plant-available soil pools and highlight the synergies between nutrient cycling and stable C stocks that can be achieved in complex agroforestry systems. These synergies seem to be particularly strong on sandy soils (

• Constant and variable manure rates impact GHG emissions within commercial silage barley fields. • Landscape position and its catchment area size impacted N 2 O, CH 4 , and CO 2 emissions. • Phosphorus-limited crop growth increased yield- and value-scaled N 2 O, CH 4 , and CO 2 emissions. • Solid cattle manure did not influence ecosystem respiration or the CO 2 -equivalent emissions. • Variable rate manure application reduced N 2 O emissions and increased CH 4 sink strength. Solid cattle manure amendments provide a low-cost alternative nutrient source to inorganic fertilizers, while providing a carbon input to the soil. The augmented soil organic carbon levels, however, may be largely offset by manure-related greenhouse gas (GHG) emissions. Soil nitrous oxide (N 2 O), methane (CH 4), and carbon dioxide (CO 2) emissions were measured at the landscape-scale in a Canadian prairie agricultural field supporting silage barley (Hordeum vulgare L.) production. Manure was applied to meet barley P requirements, while total N rate was supplemented using anhydrous ammonia. A non-manured control (NMC) also was included, to calculate N 2 O emission factors. The NMC zone consisted of an annual application of anhydrous ammonia at 80 kg N ha−1. In addition to solid cattle manure at a constant (CRM; 45 Mg ha−1) or variable (VRM; 0–72 Mg ha−1) rate, the manured treatment zones also received 80 kg N ha−1 of anhydrous ammonia. The VRM treatment included set-backs from the watershed basin centers in ephemeral wetlands that did not receive solid cattle manure. Gas samples were collected using chamber-based methodology, with chambers installed at 130 locations across six watershed basins (n = 2 per zone) during 2019–2021. Cumulative N 2 O emissions were 76 % (CRM) and 62 % (VRM) higher following manure addition. The normalized N 2 O emissions for CRM were 24 % greater than VRM and NMC, with CRM having 31 % larger manure-induced N 2 O emissions than VRM. Though all soils were net CH 4 sinks, manure application reduced CH 4 consumption by 33 % (CRM) and 25 % (VRM) compared with the NMC. Manure addition did not impact cumulative CO 2 emissions. Although VRM application mitigated manure-related GHG emissions, enhanced GHG intensity following manure addition highlights the importance of ensuring balanced soil fertility, to support optimal crop growth and maximize yield-scaled GHG performance metrics in manured landscapes. [ABSTRACT FROM AUTHOR]

Conventional soil chemical analyses are time-consuming and laboratory-based. Spectroscopic techniques have been introduced to make soil measurements more rapid and cost-effective. While portable X-ray fluorescence spectrometry (pXRF) has been increasingly adopted for advanced soil analysis in temperate regions, comparatively few applications have been developed for tropical soils. Using PXRF, soil total elemental composition can be obtained in seconds, with properties inferred from the collected data. In the present study, pXRF was used to screen samples (n = 252) from tropical Cerrado agricultural areas under different land uses in Brazil. The objective was to determine the total elemental composition of soil samples and predict soil fertility properties using simple or multiple linear regression with pXRF data as a proxy. Air-dried and sieved soil samples (<2 mm) under laboratory conditions were screened using a pXRF, then compared to soil chemical characterization by conventional methods. pXRF data properly corroborated the known soil mineralogy and geochemical background of Cerrado soils. Based exclusively on pXRF data, clay content was successfully predicted. Also, soil organic matter and plant-available forms of Ca, Mn and Cu were reasonably predicted from pXRF results. • Cerrado agricultural soils chemically characterized by proximal sensor. • Clay content can be accurately predicted by pXRF. • pXRF data elucidate soil mineralogy and geochemistry of Cerrado soils. • Several plant-available elements were successfully predicted by pXRF. • 252 highly weathered Cerrado soils of Brazil were evaluated. [ABSTRACT FROM AUTHOR]

Biochar is used as a soil amendment to improve soil quality and fertility. The performance of biochar changes as it ages in fields. Artificial biochar aging methods (i.e. chemical oxidation) are considered proxies for natural biochar aging and help to evaluate the long-term effects of biochar amendment in soil. However, few studies have focused on how close artificial biochar aging methods are to biochar aging in fields. A meta-analysis of 42-studies was conducted to quantitatively compare the effects of soil incubation (natural field biochar aging), chemical oxidation (chemical biochar aging) and freeze-thaw cycling (freeze-thaw biochar aging) on biochar properties, including surface and bulk element compositions (C, H, O, N), specific surface area, cation exchange capacity and pH. The results showed that the artificial biochar aging methods cannot yet simulate biochar aging in the soil. In comparison with natural field aging, chemical aging tended to have a higher degree of oxidation at the surface of the aged biochar and posed the problems of oxidation inside the biochar and the import of exogenous elements from oxidants to the biochar. Freeze-thaw aging changed only the porous structure of the biochar with no significant alterations to element compositions, in contrast with natural aging. • Effects of artificial and natural biochar aging methods on biochar properties were studied using a meta-analysis. • Chemical oxidation aging had a higher degree of oxidation at the surface of the aged biochar. • Chemical oxidation aging posed the problems of oxidized biochar interior. • Chemical oxidation aging imported exogenous elements from oxidants to biochar. • Freeze-thaw cycling aging changed only biochar porous structure. [ABSTRACT FROM AUTHOR]

Classification of fields into management zones based on variability of soil fertility parameters is under use in precision agriculture. The study was conducted in west Wollega zone of Ethiopia covering nearly 40 km2 of agricultural land, with the aim to explain variability of soils in the field, classify soils into mapping units and produce a map of these soils at a scale of 1:10000 using geostatistics. In this paper, soil mapping units (SMUs) were interchangeably used with management zones. Ten SMUs were identified in the study site. The SMU mean pH value varied between 5.3 in the SMU6 and 6.4 in the SMU7. Variation in soil texture, pH, exchangeable acidity, exchangeable potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), cation exchange capacity (CEC), organic carbon (OC), total nitrogen (TN), and plant available phosphorus (AvP), were observed within and among SMUs. Variability of soil parameters ranged from 1.06% (Na) in the SMU5 to 172.94% (AvP) in the SMU8. The OC and AvP content of all SMUs were found in the very low and low critical ranges. Intensive grid sampling had helped to produce digital soil maps and exposed soil heterogeneity across the landscape. We suggest that classification of fields into management zones was a remedy to low crop production occurred from uniform applications of nutrients and chemical amendments to significantly varying soils. • Digital soil maps aid variable rate technology and site-specific management soils. • Geostastical models are used to show soil-environment interactions. • Acid soils have a wide range of nutrient deficiencies and imbalances. [ABSTRACT FROM AUTHOR]

The influence of Trichoderma fungi on growth, yield and nutrient status of food crops has been widely studied. However, a lack of knowledge still exists on the effect of agricultural soils with different geological nature, land use and management on Trichoderma -plant interaction. This study aims to evaluate the influence of three soil types, with different physicochemical properties and under fertilization by NPK-based nutrient solutions, on the efficacy of two T. harzianum (T22 and M10) strains on the growth, nutrient uptake and glucosinolate concentration in Brassica rapa subsp. sylvestris cv. esculenta. Overall data showed that the biofertilization by Trichoderma strains is very effective to enhance the fertility and productivity of the nutrient-poor soils even in the absence of fertilization by NPK-nutrient solutions, as well as to stimulate the production of glucosinolates by foliar biomass of B. rapa , although this effect was strongly dependent on the soil type. Unlabelled Image • Trichoderma -plant interaction was influenced by soil type and extent of fertilization. • Trichoderma was more effective to enhance the productivity of nutrient-poor soils. • Trichoderma ameliorated biomass production, K and glucosinolate concentrations. • Highest plant-growth stimulatory effect induced by Trichoderma in unfertilized plants • Glucosinolate concentration in leaves raised by Trichoderma , lowered by fertilization [ABSTRACT FROM AUTHOR]

Soil organic matter has direct relationship with soil fertility and quality. However, its estimation in laboratory generates chemical residues which can contaminate the environment, and more ecological methods to determine the soil organic matter present high costs to the laboratories. This study aimed to evaluate the accuracy of artificial neural networks (ANNs) in estimating soil organic matter content from soil chemical attributes and to indicate whether network complexity affects estimation accuracy. A database was created containing 8556 samples, and 75% of the data were used for calibration and 25% for validation of the models. The variables used were: pH, potassium, phosphorus, calcium, magnesium and potential acidity. However, potassium and phosphorus were removed from the input variables. The ANNs were from the Multilayer Perceptron class, with two hidden layers, each of which had number of neurons ranging from 4 to 20. The 15 ANNs with lowest root mean square error (RMSE) were randomly presented by the program Statistica7®, and 6 of them were chosen for accuracy assessment. The fits were tested by analysis of variance (F test) and accuracy was assessed based on the coefficient of determination (R2), RMSE, mean error (ME), index of agreement (d) and confidence coefficient (c). The ANNs showed high accuracy to estimate soil organic matter in the phases of both calibration (R2 = 0.92; RMSE = 1.82 g kg−1) and validation (R2 = 0.76; RMSE = 1.98 g kg−1). Less complex networks can be trained and show the same accuracy as more complex networks. • Soil organic matter can be estimated with high accuracy based soil chemical analysis. • The soil pH, Ca, Mg and potential acidity are the best for estimating SOM. • Artificial neural networks (ANN) presents high accuracy in the estimation of SOM. • Less complex ANNs can be trained and show the same accuracy as more complex ANNs. [ABSTRACT FROM AUTHOR]

While laboratory methods of elemental analysis of soil nutrients are used frequently to support soil studies, the implementation of more portable and cost-efficient methods lingers behind. The portable (handheld) X-ray fluorescence spectrometer (XRF) is one such tool enabling onsite elemental analysis in a straightforward manner. However, in soil studies the use of XRF often remains cumbersome, following the poor performance of the method for low-Z elemental analysis and the complex nature of the soil matrix, introducing background noise. Here, we therefore evaluate how the potential use of a portable XRF for predicting potassium (K), phosphorus (P), magnesium (Mg) and calcium (Ca) can be improved through the analysis of XRF spectral data with the Random Forest (RF) machine learning method. A total of 105 soil samples from a wide range of soils collected from 10 different countries (D.R. Congo, Belgium, Ivory Coast, Italy, The Netherlands, Saudi-Arabia, South Africa, Spain, Switzerland, and Zimbabwe) were scanned using an Oxford XMET8000 XRF spectrometer (Oxford Instruments, UK). Spectral data of the calibration set (n = 74) were pulled in one matrix alongside measured elemental concentrations and subjected to RF analysis. Resulting models were validated using an independent validation set (n = 31). The best RF prediction result was obtained for K followed successively by Ca, Mg and P with coefficient of determination (R 2) values of 0.83, 0.76, 0.69, and 0.47, and root mean squared error of prediction (RMSEP) of 2283.8, 6818.7, 1511.8, and 538.08 mg kg−1, respectively. The RF modelling procedure provided improved prediction performance compared to the calibration models provided by the manufacturer (R 2 = 0.65, 0.75, 0.65, and 0.22, for K, Ca, Mg and P, respectively). Our results suggest that portable XRF instruments coupled with spectral data analysis by RF allows for rapid and low-cost analysis of soil K and Ca with remarkable accuracy. Still, the lower measurement accuracy for P and Mg suggests further work is needed to test whether the prediction can be improved by better calibration models, and how such approaches can help overcoming instrumental limitations. • The potential use of XRF for predicting soil elements (K, Ca, Mg, and P) was investigated. • Samples from a wide range of soils collected from 10 different countries were used. • Random forest (RF) coupled with XRF spectra were used for developing the calibration models. • Results suggest that RF-XRF is a promising tool for rapid analysis of soil K and Ca. [ABSTRACT FROM AUTHOR]

The straw residue amendment to soil is documented as an effective measure for integrated fertility management, and residue's decomposition dynamics are closely related to the physical, chemical and biological properties of soil. This paper aimed to investigate the effects of soil fertility on residue-derived carbon (C) decomposition, turnover and retention, and to identify the linkages between these processes and soil aggregation as well as microbial community. Arable soils following 27-year applications of compost and inorganic NPK along with a control (no fertilizer) were collected to divide into the corresponding high, medium and low fertility levels, and an in situ incubation was set up with these soils amended with and without 13C-labeled maize residues. During the whole incubation, the residue decomposition was significantly affected by the fertility level, and the retentions of residue-C in soils were ranked as high fertility soil > medium fertility soil > low fertility soil. Adding straw residue and improving the fertility level were favorable for the formation of water-stable macroaggregates. Following the addition of 13C-labeled residues, although the residue-C was preferentially incorporated into the large macroaggregate (>2000 μm) and then translocated to smaller aggregates, the large macroaggregate made the greatest contribution to the residue-C retention. The distinct microbial community dynamics determined their residue-derived 13C incorporations into microbial biomass. The increasing 13C incorporations into gram-positive (G+) bacteria, fungi and anaerobes and decreasing 13C incorporations into gram-negative (G−) bacteria, actinobacteria and aerobes corresponded to improving fertility levels. Consequently, soil aggregation and microbial community composition collectively explained 92% of the variation in residue-C retention, and their interaction exerted the largest effect. It can be concluded that residue return could effectively enhance soil fertility, which in turn contributes to residue-C retention by associating with macroaggregation and microbial community in a sandy loam soil. • Straw residue decomposition is significantly influenced by soil fertility level. • Residue-C is preferentially and predominantly incorporated into macroaggregates. • The amount of residue-derived PLFAs is primarily determined by soil microbial biomass. • Soil macroaggregation and microbial community collectively affect residue-C retention. [ABSTRACT FROM AUTHOR]

An adequate supply of micronutrients is essential for balanced plant growth. In Africa, micronutrient constraints are known to restrict crop productivity and impair human welfare and health. We analyzed a total of 816 topsoil and 796 subsoil samples from Sub-Saharan Africa, to provide baseline information on their readily available Al, B, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, V, and Zn concentrations. This work builds on previous global micronutrient surveys. The studied soils were low in hot water-extractable B, acid ammonium acetate-EDTA-extractable micronutrients, and potentially harmful metals. In comparison with tentative critical level values, B, Cu, Zn, Mo, and Fe in a considerable number of samples seemed to be at a low level for crop production. Apart from Al, Mo, V, Pb, and Cr, readily available element concentrations were higher in topsoil than subsoil. Concentrations of harmful metals (Cd, Ni, Pb) and of Mn and Mo were enriched in cultivated soils in comparison with semi-natural sites. Among the 11 Reference Soil Groups represented in the study, Arenosols were discernible as poor in Cr, Cu, and Fe. Other significant differences in microelement concentrations between soil groups were found only for Ni and V. • We present novel information on microelement status in Sub-Saharan African soils. • The soils were overall poor in easily available B, Cu, Zn, Mo and Fe. • Concentrations of most microelements were higher in topsoil than subsoil. • Levels of harmful metals were higher at cultivated than semi-natural sites. [ABSTRACT FROM AUTHOR]

Abstract The agronomic management of vineyards, aimed at the maximization of grape yields, is intensive and includes repeated tillage, crop residues removal, strict weed and pest control treatments. In viticulture, healthy soils and a suitable climate are the key factors that influence the quality of the produced wine. Even under intensive agronomic practices, typical of conventional viticulture, the choice of appropriate soil management strategies can preserve soil quality in the long term. The use of biochar as soil amendment has been suggested as a potential practice that allows restoration of the functionality of degraded soils in terms of water retention, chemical and biological fertility. In this paper we report the results of a long field trial on biochar amendment of a vineyard, assessing the impact on soil functionality, VOC emission and fertility. Our results show that biochar effects on soil functions and fertility are maintained in the long term (seven years) after a one-time application. Highlights • Biochar application is an effective practice for restoration of the functionality of degraded soils. • Changes in soil fertility after a single application of biochar persist after seven years • Biochar incorporation into soil caused substantial changes in the metabolic pathways of soil microbial activity. • Biochar is an environmentally-friendly solution for soil management in vineyards. [ABSTRACT FROM AUTHOR]

Abstract Soil fertility is vital for agricultural productivity, yet poor soils and erosion remain a management challenge in many parts of sub-Saharan Africa. One challenge is that soil scientists and farmers often evaluate soil fertility using different knowledge systems and the implications have not been clearly reconciled within the literature. In particular, whether farmers are observing similar aspects of structure and function as classified in soil science. If so, what can we learn about how soil fertility is evaluated and communicated in terms of developing a hybrid approach that improves communication of ideas between different stakeholders. This paper addresses this challenge by examining the similarities and differences between farmers' qualitative evaluation and soil science quantitative analysis for soil fertility classification, and how location of soils influence farmers' evaluation of soil fertility. Empirical fieldwork was carried out in two villages in Kitui County, Kenya with 60 farmers using semi-structured interviews and focus group discussion. Based on farmer perception, 116 soil samples of the best and worst soil fertility taken and analysed for physiochemical factors. Farmers had a consistent classification system and primarily relied on texture and colour as indicators for good soil fertility and texture alone for poor soils. Soils with fine texture under the local semi-arid climate were associated with higher pH, TOC and WHC and fertile black and red soils were associated with pH, TOC, WHC and AP based on differences in bed rock. Poor soil fertility was associated with sandy soils and soils with no colour in their local name. Spatial location is an important consideration in farmers' evaluations, reflecting awareness of local diversity in soil and historical social or environmental factors. Local historical narratives reveal the importance in changes to humus, consistent with technical knowledge about the role of soil organic matter for soil fertility. The paper provides a better understanding of farmers' soil classification, evaluation processes and perspectives that help to inform scientists working with alternative frameworks for assessment and, in doing so, supports the development of local tailor-made soil assessment systems. Highlights • Key local soil indicators are texture and colour which consistent with soil analysis results. • Location of soils influences the difference of fertility and fertility evaluation by farmers. • The classification of colour is different between Kitui farmers and scientific standard. • Qualitative farmer and quantitative soil science indicators could be used by extension officers. [ABSTRACT FROM AUTHOR]

Abstract The current study seeks to assess sustainability of agricultural land use by identifying the effect of land use change on soil quality using cross-sectional data collected through a household survey among 525 farm households in densely populated areas of Kenya. Soil samples were collected, analyzed and compared across three land use types: undisturbed, semi-disturbed and cultivated. To achieve these objectives, descriptive, Nutrient Index approach and Classification and Regression Tree (CART) analysis methods were used. Results indicate that within a period of five decades, agricultural land use has led to a decline in Total Organic Carbon (−72%), Magnesium (−65%) and Boron (−61%), Iron (−22%) and Total Nitrogen (−15%). The drivers of deterioration identified were cutting across inherent properties such as soil chemical (pH), physical (soil mapping unit) and biological (organic carbon) attributes, farmer practices (agricultural commercialization) and exogenous factors (population density and Agro-ecological zones). The study concludes that indeed conversion of land from natural vegetation is associated with deterioration in soil quality and therefore policy needs to create incentives for the build-up of soil organic matter, replenishment of soil macro and micro nutrients. Blending of commercial fertilizers with targeted micro-nutrients, maintenance of soil conservation techniques and long term fallowing are encouraged. Highlights • Agricultural land use has led to decline in OC, Mg, B and total N. • Deterioration driven by inherent soil properties, farmer practices and exogenous factor • Policy action: increasing soil organic matter and replenish macro and micro nutrients • Mandatory blending of commercial fertilizers with micro-nutrients desirable [ABSTRACT FROM AUTHOR]

Abstract Nitrogen deposition is notable in China, even in the high altitude region of Qinghai-Tibet Plateau (QTP). To identify the effects of nitrogen deposition on soil nutrient and ecological stoichiometry of the QTP's alpine grasslands, we treated three major types of grasslands (alpine meadow, alpine steppe, and cultivated grassland) with 6 different nitrogen addition levels: 0, 8, 24, 40, 56, 72 kg N ha−1 yr−1 through the years of 2015 to 2016. We sampled the soils at two depths, 0–10 and 10–20 cm in the peak-growing season (July) of each year. Results showed that: (i) soil total carbon and phosphorus are relatively stable under nitrogen deposition. (ii) Soil total nitrogen increases under low nitrogen addition rates while decreases under higher rates, and soil available nitrogen increases under nitrogen deposition in surface soil while decreasing in deeper soil. (iii) C:N, C:P, N:P had no significant difference between different nitrogen addition levels while there were significant differences between the two years under nitrogen addition. We speculate that nitrogen addition could alter soil nutrient conditions but there were certain limit of soil nutrient under N deposition. Highlights • Soil total C and P are stable under N deposition. • Total N increase under low N addition rate while decrease under higher rate. • Available N increase under N deposition in surface soil while decrease in deep soil • Stoichiometry had no significant difference between N addition levels. [ABSTRACT FROM AUTHOR]

Abstract The soil organic matter (SOM) content is strongly related to soil fertility and greenhouse gas emissions. Knowledge of the soil SOM content is therefore necessary for efficient and sustainable management practices. In this study, we compare the performance of five machine learning techniques for the prediction of SOM contents using remote sensing, proximal soil sensors and topographic data as environmental predictor. The methods used were kriging, Cubist, Random Forest and regression-kriging (Cubist- kriging and Random Forest kriging). 285 soil samples were collected from a 10 ha field in Denmark. 75% of the data were used for training and 25% for validation. NDVI (Normalized Difference Vegetation Index), RVI (Ratio Vegetation Index), DVI (Difference Vegetation Index) and SAVI (Soil-Adjusted Vegetation Index) were derived from Sentinel 2 images with 10 m resolution as auxiliary variables. Topographic variables were derived from a digital elevation model (DEM) with 1.6 m resolution. The performance of the methods was compared based on the coefficient of determination (R2) and root mean square error (RMSE). The results showed that kriging achieved the best performance, followed by regression-kriging. The models using only Cubist and Random forest had the poorest performance. The results, therefore, demonstrate that kriging can predict SOM contents without the need of auxiliary variables for fields with high sampling densities. Highlights • Kriging, Cubist, Random Forest and regression-kriging were used to predict soil OC. • 19 covariates were used in regression-kriging to predict soil OC. • Kriging with no covariates showed better OC prediction compared to other methods. [ABSTRACT FROM AUTHOR]

Abstract Nitrogen (N) fertilization and straw incorporation strongly influence nitrous oxide (N 2 O) emissions from agricultural fields. An in-situ micro-plot experiment on intensively farmed winter wheat (Triticum aestivum L.) was conducted to investigate the source and rate of N 2 O emissions from soils following labeled 15N fertilization with and without straw incorporation. Four treatments, i.e., no N fertilizer and no straw incorporation (N0S0), straw incorporation only (N0S1), N fertilizer only (N1S0), and N fertilization plus straw incorporation (N1S1), were established in the experiment. The N 2 O emissions mainly occurred after N fertilization and lasted for approximately 1–2 weeks, accounting for 60%–67% of the wheat seasonal N 2 O emissions. Within the 6 days after basal fertilization and 2–4 days after top-dressing, most of the N 2 O fluxes (>50%) were derived from fertilizer. Thereafter, soil-derived N 2 O dominated the total N 2 O emissions and within 10–20 days after N fertilization, fertilizer-derived N 2 O became negligible. Fertilizer N and soil N both accounted for 40%–60% of the seasonal N 2 O emissions, which may be explained by the high soil N stock due to long-term high N fertilization in the region. This implies the similar roles of soil N pool and fertilizer N in N 2 O generation under intensively farmed soils. The N fertilization had a significant priming effect on the turnover of soil N, which contributed 21.02%–50.47% of the total N 2 O emissions. During the basal fertilization/first irrigation event, straw incorporation significantly (P < 0.05) stimulated CO 2 fluxes both in N-fertilized and non-N-fertilized plots; however, after the top-dressing/second irrigation event, the significant increase of CO 2 fluxes induced by straw incorporation was only observed in the N-fertilized treatment. Straw incorporation interacted with N fertilization, and tended to enhance N 2 O emissions in the basal fertilization and lower N 2 O emissions in the top-dressing period. In N-fertilized plots, the seasonal N 2 O emissions from straw-incorporated and straw-removed treatments were similar, indicating that straw incorporation enhanced the N supply without increasing the N 2 O emissions. Our study highlights that there are significant benefits of straw incorporation to soil fertility improvement; however, the long-term impacts of straw incorporation on greenhouse gas emissions should be further examined. Graphical abstract Unlabelled Image Highlights • Direct fertilizer-derived N 2 O emissions occurred within 2 weeks. • Soil N and fertilizer N similarly contributed to seasonal N 2 O emissions. • N fertilization had a significant priming effect on soil N turnover. • Straw incorporation enhanced N input without increasing seasonal N 2 O emissions. • Crop straw incorporation interacts with N fertilizer on N 2 O emissions. [ABSTRACT FROM AUTHOR]

Abstract In the present study, the long-term effects of crop rotation and fertiliser (manure, harvest residues and mineral fertilisers) use on the presence of phosphorus and its fractions at different soil depths was monitored and analysed in relation to the long-term use of particular maize growing systems. The 270 analysed soil samples were obtained as a part of a long-term two-factorial trial with four replications initiated in 1965. The plan of divided plots (split-plot design with randomised variants) was adopted and the following factors were analysed: (1) Fertilising systems: single-crop system (control, NPK, NPK + maize remains, NPK + manure) and two-crop rotation – maize / barley (NPK + manure) and (2) Soil layers at different depths: 0–20 cm, 20–40 cm, and 40–60 cm. The highest value of readily available phosphorus was found in all studied soil layers in the two-crop rotation treatment with organic and mineral fertilisers. On the other hand, the treatments involving manure application resulted in the highest content of total and organic phosphorus, whereby a 27–70% increase was observed, depending of the variant. The greatest presence of Ca-P fraction was obtained in treatments based on organic matter use, ranging from 364.4 to 482.8 mg kg−1. Nonetheless, in the treatments involving application of organic matter, fraction related to this group had a significantly lower contribution to the total inorganic phosphorus, ranging from 56% to 86%, depending on the variant. Correlations obtained via principal component analysis (PCA) showed clear demarcation among treatments and control samples based on the use of manure and mineral fertilisers. Highlights • Changes in soils properties are studied after 40 years of agricultural practice. • Clear variation in basic soil properties between studied treatments • Sequential analysis of P fractions showed dominance of phosphates bound to calcium. • PCA analysis indicates obvious demarcation among fertilising treatments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Seven key soil health indicators (SHIs) were identified from a 168 SHI dataset. • Identified key SHIs were sensitive to long-term crop rotation and fertilization. • Despite some differences, the key SHIs were consistent with other publications. • A site-specific soil health score was significantly correlated to crop yields. Long-term agricultural management practices affect soil health, but identifying measurable soil properties that reflect soil health (soil health indicators – SHIs) is a challenge. Five long-term (>40 years) crop rotations with varying cropping frequency, crop diversity, fertility and lime treatments were sampled as part of the Soil Heath Institute's North American Project to Evaluate Soil Health Measurements (NAPESHM) at the University of Albert Breton Plots research site in 2019, and additional samples were collected in 2020 for additional indicator measurements. Many soil health frameworks such as the Cornell Assessment of Soil Health (CASH) generate soil health scores based on key SHIs identified from a large, multivariate database using principal component analysis (PCA). For our dataset, we adopted the PCA-based approach with the following questions in mind: 1) Were the key SHIs that explain a large portion of the total dataset variance also the key SHIs that were most sensitive to the rotation, fertilizer and lime management? 2) Were the key SHIs identified with PCA associated with soil health, soil fertility or inherent soil characteristics? We identified seven key SHIs with PCA that explained 86.8% of the database variance. Results of a permutational MANOVA suggested that crop rotation and fertilizer management significantly influenced the total variance of the identified key SHIs. Four of the seven identified key SHIs primarily reflected soil health and three SHIs primarily reflected soil fertility and/or inherent soil properties but were also positively associated with soil health at this site. Overall, the PCA-based approach used to develop the site-specific soil health score (SSpeSHS) proved to be a helpful screening tool for the identification of key SHIs that are sensitive to soil-health-promoting management practices from a large dataset. [ABSTRACT FROM AUTHOR]

• A method for estimating SOM content based on spectral indices constructed by the improved Hapke model was proposed. • The SOM-Hapke model was constructed, which is an optical radiative transmission models that can be used to invert SOM content. • The optimal forms and sensitive bands of the three-dimensional spectral indices were preferred. • The preferred TBIs exhibit good inversion accuracy for both field hyper-spectral data and field broad-band multispectral data. • In terms of both accuracy and stability, TBI (r) constructed based on the improved Hapke model outperforms TBI (R) constructed based on spectral reflectance. Soil organic matter (SOM) content is an important indicator to measure the degradation degree and fertility of soil. However, most current SOM prediction methods are based on statistical learning theory, overlooking the transmission process and physical mechanism of reflectance spectra, and lacking the physical basis of soil remote sensing. In this study, a method for estimating SOM content based on spectral indices constructed by the improved Hapke model was proposed, which started from the radiative transfer process of soil reflectance spectra and used the converted reflectance r and single scattering albedo ω as means to construct spectral indices. The prediction accuracy of these spectral indices with sensitive bands selected from laboratory-measured data (Data 1) was validated using field high-spectral data (Data 2), and the potential application in remote sensing of spectral indices was validated using multispectral data (Data 3). As expected, these spectral indices exhibit good prediction accuracy for both field hyper-spectral data (TBI 37 : R2 P is 73.88; RPD is 2.02) and field multispectral data (TBI 17 : R2 P , is 67.19; RPD is 1.78). The comparative results indicate that, in terms of both accuracy and stability, spectral indices constructed by the improved Hapke model outperform those based on spectral reflectance. This study reduces the complexity of model calibration effectively, and the constructed spectral indices have clear physical meaning and good potential for fast and high accuracy prediction of SOM content at large scales. [ABSTRACT FROM AUTHOR]

[Display omitted] • Converting primary forest to agricultural production decreased fungal, bacterial, and total microbial residue C. • Soil fungal abundance increased microbial residue C accumulation. • Soil bacterial abundance declined fungal and total microbial residue C accumulation. • Soil fertility improved fungal, bacterial, and total microbial residue C accumulation. Soil microbes mediate soil organic carbon (SOC) storage by affecting microbial residue carbon (C) in terrestrial ecosystems, a main source of SOC. However, how microbial residue C accumulation changes during the conversion of subtropical primary forest to plantations and the role of soil microbial community structure in regulating these changes remain unclear. Here, effects of conversion from forest on microbial residue C accumulation in three plantations, including tea (Camellia sinensis var. assamica), walnut (Juglans regia), and macadamia (Macadamia integrifolia). We assessed amino sugars content and the relative importance of soil microbial community structure, soil fertility, and aboveground biomass in four forest types in southwest Yunnan province, China. Our results show that total microbial residue C accounted for 27.8 % of SOC on average across all forest types, and fungal residue C contributed more to SOC than bacterial residue C across different forest types. Conversion from primary forest decreased fungal, bacterial, and total microbial residue C content in all plantation types except tea. Microbial residue C contents was significantly lower in the macadamia plantation than in the primary forest and the tea plantation. Fungal and total microbial residue C content increased with increasing soil microbial community composition, soil fungal abundance, and soil fertility. Soil fertility regulated fungal, bacterial, and total microbial residue C accumulation, which was major determinant of microbial residue C via its influence on soil fungal and bacterial abundance. Overall, we found that soil fungal abundance contributed more to microbial residue C accumulation than change in soil microbial community composition. We also found that soil bacterial abundance was negatively correlated with fungal and microbial residue C, while soil fungal abundance was positively related to microbial residue C accumulation. Ascomycota was most important phylum regulating microbial residue C. These findings suggest that soil fungal and bacterial abundance and soil fertility are the principal regulators of microbial residue C accumulation during the conversion from primary forest to different plantations. [ABSTRACT FROM AUTHOR]

• N fertilizer significantly promoted SIC dissolution in the 0–80 cm soil profile. • DMPP had no significant effect on SOC and δ13C SOC. • DMPP application slowed SIC dissolution in the 0–40 cm soil layer. • DMPP alleviated PIC dissolution and increased PIC stock. In carbonate soils, soil inorganic carbon (SIC) dissolves and releases CO 2 due to exogenous application of nitrogen fertilizer, which makes the soil an important carbon source. Application of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is likely to be an effective measure to alleviate SIC dissolution. A 5-year field experiment was conducted to observe SIC dynamics under three treatments: No N fertilizer (CK), N fertilizer (NF), and N fertilizer combined with DMPP (N + NI). Soil samples were collected from 0 to 20, 20–40, 40–60, 60–80, and 80–100 cm in depth, and soil organic carbon (SOC), SIC, total carbon (TC), total nitrogen (TN), and carbon isotope composition (δ13C) of the soil were analyzed. The proportion of pedogenic carbonate (PIC) in SIC was estimated using an isotopic mass balance equation. Results showed that: (1) compared with the application of N only, DMPP had no significant effect on SOC content and δ13C SOC in the 0–100 cm soil profile; (2) the application of N fertilizer significantly reduced the SIC content by 10.0 g·kg−1 (58.3 %) in the 0–80 cm soil profile. In the 0–20, 20–40, and 60–80 cm soil layers, the SIC content was reduced by 2.9, 1.9, and 3.8 g·kg−1, respectively. However, the SIC content significantly increased by 3.2 g·kg−1 (58.9 %) in the 80–100 cm soil layer. DMPP application significantly increased the SIC content in the 0–20 and 20–40 cm soil layers by 1.3 g·kg−1 (50.9 %) and 1.2 g·kg−1 (48.2 %), respectively; (3) DMPP combined with N significantly alleviated PIC dissolution in the 0–40 cm soil layers and increased PIC stock in 0–100 cm soil by 6.2 Mg·ha−1 (64.6 %). Applying DMPP to farmland soil containing carbonate significantly alleviated the dissolution of N fertilizer on PIC, which may decrease CO 2 emissions in the field. [ABSTRACT FROM AUTHOR]

Abstract Soil base saturation percentage (BSP) plays an important role in the assessment of soil taxonomic classification and soil fertility. Conventionally, soil BSP measurement methods are fraught with many drawbacks such as being laborious, time consumptive, destructive to the samples, and can lead to the underestimation of true cation exchange capacity (CEC). Recently, proximal sensors such as portable X-ray fluorescence (PXRF) spectrometry have proven to be effective for rapid physicochemical analysis of soils. In this study, we proposed and examined a PXRF-based method to predict BSP using 300 soil samples from the active agricultural lands in six states across the USA; Colorado, California, Minnesota, Nebraska, Oklahoma, and Texas. An Olympus Vanta series PXRF analyzer was employed to measure Mg, Ca, and K for BSP prediction. Results were validated using four different multivariate models [generalized additive model (GAM), multiple linear regression (MLR), random forest (RF), regression tree (RT)] via R 3.5.1. Predictive model performance was assessed via root mean squared error (RMSE), coefficient of determination (R2), residual prediction deviation (RPD), the ratio of performance to interquartile (RPIQ) range, and bias. While predicting BSP from PXRF quantified elements, models exhibited R2, RMSE (%), and RPDs as follows: GAM = 0.58, 9.0, 1.6; MLR = 0.45, 10.4, 1.4; RF = 0.62, 8.7, 1.6; RT = 0.68, 7.9, 1.8, respectively. Soil cation exchange capacity was also predicted using a similar approach, with similar and moderate predictive performance; GAM produced R2, RMSE (cmol c kg−1), and RPD of 0.69, 5.6, 1.8, respectively, relative to laboratory data. This study showed that the PXRF elements can be used to predict BSP with fair accuracy for the range of agricultural soils examined. As such, further study and enhancement of the approach outlined herein on a wider array of soils is warranted. Highlights • Soil base saturation percentage was predicted via PXRF. • Na was negligible in agricultural soils. • Regression tree was optimal for BSP prediction. [ABSTRACT FROM AUTHOR]

Abstract It has long been established that earthworms beneficially affect plant growth. This is to a large extent due to the high fertility of their casts. However, it is not clear how fertile casts are compared to bulk soil, and how their fertility varies between earthworm feeding guilds and with physico-chemical soil properties. Using meta-analysis, we quantified the fertility of earthworm casts and identified its controlling factors. Our analysis included 405 observations from 81 articles, originating from all continents except Antarctica. We quantified cast fertility by determining the enrichment of earthworm casts relative to the bulk soil ("relative cast fertility"; RCF) for total organic carbon (TOC), total phosphorus (P) and total nitrogen (N) concentrations, as well as for plant available pools of N (total mineral N) and P (available P: P-Olsen, P-Bray or comparable metrics), C-to-N ratio and microbial biomass C. In addition to these response variables, we studied four additional ones closely related to soil fertility: pH-H 2 O, clay content, cation exchange capacity (CEC), and base saturation. With the exception of C-to-N ratio, microbial C and clay content, all studied response variables were significantly increased in casts compared to the bulk soil. Increases in total elemental concentrations (TOC, total P and total N), which are the result of preferential feeding or concentration processes , were comparable and ranged between 40 and 48%. Nutrient availability, which is to a large extent the result of (bio)chemical transformation processes in the earthworm gut, was increased more strongly than total elemental concentrations (241% and 84% for mineral N and available P, respectively). Increases in pH (0.5 pH units), cation exchange capacity (40%), and base saturation (27%) were also large and significant. None of the soil-related possible controlling factors could satisfactorily explain the variation in RCF; plant presence (or other sources of organic C input such as residue application) was the only controlling factor that consistently increased RCF across soil properties. With the exception of available P, none of the studied response variables could be linked to earthworm feeding guild. Our results show that earthworm casts are much more fertile than bulk soil for almost all analysed cast fertility properties. However, these positive RCFs are to a large extent dependent upon the presence of plants. In general, earthworm feeding guild or specific physico-chemical soil properties could not explain the large variability in RCF for the various response variables. Therefore, we hypothesize that RCF effects depend on intricate interactions between earthworm species traits and specific soil properties. Understanding these interactions requires trait-based approaches combined with mechanistic modelling of biochemical processes in the earthworm gut and casts. Highlights • We analysed fertility of earthworm casts using a meta-analysis of 81 articles. • Casts contain on average 40–48% more total P, total N and organic C than bulk soil. • Available N and available P are even more increased (241% and 84%, respectively). • Relative Cast Fertility (RCF) is strongly dependent on presence of plants or residues. • A combination of trait-based analysis and modelling is needed to better predict RCF. [ABSTRACT FROM AUTHOR]

Abstract Sampling density is a parameter that allows to identify the spatial variability of properties of interest in soils and is essential for precision agriculture practices, such as fertilizer application in variable rate. Soil sensors and spectroscopic techniques have been investigated and reported as promising tools, although showing some limitation in characterizing the chemical fertility of soils. One of the causes of the limitations is the way samples are prepared. Therefore, the aim of this work was to evaluate the effect of sieving and of drying temperature, on the determination of total nitrogen and phosphorus and potassium contents, by using Vis-NIR and Mid-Infrared spectroscopies. We found that Diffuse Reflectance Spectroscopy in the Visible and Near-Infrared region of the spectra shows better performance of the prediction models than of the Attenuated Total Reflectance Spectroscopy in the Mid-Infrared region. We did not identify significant influence of drying temperature neither of soil particle size on the predictive quality of the models generated via Vis-NIR spectroscopy; thus, the extra expenditure of time and cost for a more intense preparation of the soil samples would not be justifiable. In addition, we infer that the high prediction error may limit the applicability of spectroscopy for guiding the variable-rate application of fertilizers within the scope of precision agriculture. Highlights • Vis-NIR spectroscopy shows better performance than MIR spectroscopy. • Drying temperature does not influence the predictive quality of spectroscopic models. • Soil particles size shows minimal influence in the Vis-NIR model performance. • Prediction errors limit the applicability of spectroscopy on precision agriculture. [ABSTRACT FROM AUTHOR]

Abstract Rapid industrial development and human activities have caused a degradation of soil quality and fertility. There is increasing interest in rehabilitating low fertility soils to improve crop yield and sustainability. Biochar, a carbonaceous material intentionally produced from biomass, is widely used as an amendment to improve soil fertility by retaining nutrients and, potentially, enhancing nutrient bioavailability. But, biochar is not a simple carbon material with uniform properties, so appropriate biochar selection must consider soil type and target crop. In this respect, many recent studies have evaluated several modification methods to maximize the effectiveness of biochar such as optimizing the pyrolysis process, mixing with other soil amendments, composting with other additives, activating by physicochemical processes, and coating with other organic materials. However, the economic feasibility of biochar application cannot be neglected. Strategies for reducing biochar losses and its application costs, and increasing its use efficiency need to be developed. This review synthesized current understanding and introduces holistic and practical approaches for biochar application to low fertility soils, with consideration of economic aspects. Highlights • Biochar has potential to be the best management practice for low fertility soils. • Biochar coating with organic materials can result in enhanced crop nutrient supply. • Biochar may accelerate the composting process and improve the end-product quality. • The influence of biochar varies strongly according to the types of feedstock/soil. [ABSTRACT FROM AUTHOR]

Abstract Long-term fertilization has a considerable effect on the dynamics of soil organic carbon (SOC). However, quantifying the contribution of fertilization practices to SOC is still a challenge. In this study, we selected a 30-yr fertilization experiment planted with a shift from C 3 to C 4 crops. Our objectives were (i) to determine temporal dynamics of SOC and newly-derived C (f new) with the cultivation time and their vertical distribution along with soil profile; (ii) to assess the effect of the fertilization on SOC and f new. Three treatments were involved: no fertilization (Control), NPK application (NPK), and continuously planted with C 3 crops adjacent to the experiment as a reference (NAT). The SOC was physically separated into cPOM, fPOM, iPOM, s + c_m and s + c_f fractions, and their δ13C values were determined. Our results show the f new in the bulk soil and C fractions presented an exponential increase over the cultivation time in the NPK treatment but this was only observed for the bulk soil and the s + c_f fraction in the Control treatment. The f new had a priority to be stored within microaggregates, and then enriched in the silt and clay sized fraction over time. The SOC and f new in the bulk soil and C fractions decreased with soil depth so that the difference between the Control and NPK treatments was observed only in the 0–20 cm depth. Our results demonstrate that the long-term fertilization increases the new C proportion in the bulk soil and C fractions but this contribution was limited to the plough layer. Graphical abstract Unlabelled Image Highlights • The proportion of f new in the bulk soil and C fractions showed an exponential increase over time. • The f new had a priority to be stored within microaggregates, and then was enriched in the silt and clay sized fraction. • The increase of the f new was limited to soil plough layer. [ABSTRACT FROM AUTHOR]

Abstract Soil fertility is typically assessed by chemical analysis, which is expensive and time-consuming, and hence impractical for site-specific fertilizer management. Visible and near infrared (Vis–NIR) spectroscopy has been used for determining soil properties and chemically extractable plant nutrients. However, the suitability of Vis–NIR for accurate assessment of nutrient availability to plants has not yet been fully explored. In this work, we examined the accuracy of this technique as a new nutrient availability index, and in the case of P as a proxy for plant-available P. To this end, total plant-available P in soil was quantified in a P-depletion experiment with crops, and the availability of Ca, Mg, K, and Fe was assessed by chemical extraction. Vis–NIR spectroscopy allowed us to accurately estimate plant-available P, which depends not only on soil factors but also on the crop performance to take up P. Vis–NIR spectroscopy proved effective in identifying P, Ca, Mg, K, and Fe responsive sites. Precise estimation of plant-available P was a result of accurately predicting soil properties governing P availability to plants by Vis–NIR spectroscopy. In addition, this technique provided accurate predictions of soil properties influencing the dynamics of applied P and K fertilizer, which can be useful to adapt fertilization practices to soil properties. Vis–NIR spectroscopy can therefore enable a qualitative leap to cost-effective integral assessment of soil fertility by providing accurate predictions of soil functioning rather than mere estimates of availability indices, thereby facilitating more sustainable use of resources in agriculture. Highlights • Visible–near infrared spectroscopy (Vis–NIR) allows one to predict P, Ca, Mg, K and Fe responsive sites. • Available P to plants as assessed with crops was also accurately estimated with Vis-NIR. • Vis–NIR affords prediction of soil properties governing the dynamics of applied P and K fertilizer. • Vis-NIR enables prediction of soil functioning rather than mere estimation of availability indices. [ABSTRACT FROM AUTHOR]

Abstract Soil organic carbon (SOC) sequestration at subsurface layers (i.e. >20 cm) remains unclear under long-term fertilization practices. Based on long-term datasets of fertilization experiments in four typical Chinese croplands, representing soils with high fertility at Gongzhuling (GZL, black soil) and Chongqing (CQ, purple soil), and low fertility at Zhengzhou (ZZ, aquatic Chao soil) and Qiyang (QY, red soil), we calculated SOC storage, its change relative to initial condition (ΔSOC) in 0–20, 20–40 and 40–60 cm. We also obtained annual organic C inputs (OCI; stubble, roots and manure amendment) and derived soil C sequestration efficiency (CSE : the ratio of ΔSOC over OCI) in 0–20 cm and 0–60 cm. The fertilization treatments include cropping with no fertilization (CK), chemical nitrogen, phosphorus and potassium fertilizers (NPK) and combined chemical fertilizers and manure (NPKM). Results showed SOC stock significantly increased with fertilizations (i.e. initial, CK < NPK < NPKM). Relative to initial condition, surface (0–20 cm) and subsurface (20–60 cm) SOC stocks significantly decreased under CK at all sites except GZL, a site with elevated SOC stocks under all fertilizations and depths. Subsurface SOC stocks significantly increased at high fertility soils (i.e., GZL and CQ) but remained no change or significantly decreased at low fertility soils (i.e., ZZ and QY) under NPK and NPKM. Accordingly, CSE derived in 0–60 cm was consistently higher than that in 0–20 cm in high fertility soils but lower in low fertility soils. These results demonstrated that subsurface soils (20–60 cm) remained as C sinks in indigenously high fertility sites but experienced substantial C depletions in low fertility sites. This study informed the need to account for subsurface soil carbon changes for accurate estimates of soil C sequestration capacity under long-term fertilization. Highlights • Relative to initial condition, CK reduced SOC stocks in 0–60 cm in low fertility soils. • Relative to initial condition, NPK and NPKM enhanced SOC stocks in 0–60 cm in high fertility soils. • CSE is consistently greater at 0–60 cm than 0–20 cm in high fertility soils. • CSE is consistently lower at 0–60 cm than 0–20 cm in low fertility soils. [ABSTRACT FROM AUTHOR]

Abstract Soil organic carbon (SOC) in agricultural soils is vital for soil fertility for sustainable agricultural production and climate change resilience. Process-based farming system models are widely used to predict SOC dynamics in agricultural soils, but their application at regional scales is largely limited by computational requirements, data availability, and uncertainties in model predictions. Here we present an approach of combining a farming system model and a simplified surrogate model that emulates and mimics the behaviour of complex process-based models to predict SOC change (∆SOC) and its uncertainty in Australian dryland cropping regions under anticipated climate change. We first calibrated and validated the farming system model APSIM for simulating ∆SOC (0–30 cm soil) using data from 90 farming-system trials at 28 sites across the study regions. Next we conducted a comprehensive simulation across the region using the validated APSIM model to predict ∆SOC over the period 2009–2070. Then simple surrogate models were developed based on the APSIM outputs. The surrogate models were able to explain >96% of the variation in APSIM-predicted ∆SOC. Last the surrogate models were applied across the regions at the resolution of 1 km. In our simulations, Australian dryland cropping soils under farmers' common management practices and future climate conditions were a net carbon source (0.66 Mg C ha−1 with the 95% confidence interval ranging from −5.79 to 8.38 Mg C ha−1) during the 62-year period. Across the regions, simulated ∆SOC exhibited great spatial variability ranging from −108.8 to 9.89 Mg C ha−1 at the resolution of 1 km, showing significant (P < 0.05) negative correlation with baseline SOC level, temperature and rainfall, and positive correlation with pasture frequency (the duration of pasture in the rotation divided by the whole duration of the rotation) and nitrogen application rate. The uncertainty in ∆SOC and the underlying drivers were also assessed. This study presented a novel approach to efficiently predict future SOC dynamics and their uncertainty at fine resolutions, facilitating the development of site-specific management strategies for soil carbon sequestration. Highlights • We predict SOC dynamics from 2009 to 2070 in Australian dryland cropping soils. • Australian cropping soils are a net carbon source under business-as-usual management. • Simple surrogate models accurately reproduce APSIM-predicted SOC dynamics. • Surrogate modelling can quickly predict future SOC dynamics at large scales at high resolution. [ABSTRACT FROM AUTHOR]

Soil organic carbon (C) is an important indicator of agricultural and environmental quality. It improves soil fertility and helps to mitigate greenhouse gas emissions. Soil spectroscopy with either vis–NIR (350–2500 nm) or mid-IR (4000–400 cm −1 ) spectra have been used successfully to predict organic C concentrations in soil. However, research to improve predictions of soil organic C by simply combining vis–NIR and mid-IR spectra to model them together has been unsuccessful. Here we use the Outer Product Analysis (OPA) to fuse vis–NIR and mid-IR spectra by bringing them into a common spectral domain. Using the fused data, we derived models to predict soil organic C and compared its predictions to those derived with vis–NIR and mid-IR models separately. We analyzed 1259 tropical soil samples from surface and subsurface layers across agricultural areas in Central Brazil. Soil organic C contents were determined by a modified Walkley-Black method, and vis–NIR and mid-IR reflectance spectra were obtained with a FieldSpec Pro and a Nicolet 6700 Fourier Transformed Infrared (FT-IR), respectively. Reflectances were log-transformed into absorbances. The mean content of soil organic C was 9.14 g kg −1 (SD = 5.64 g kg −1 ). The OPA algorithm was used to emphasize co-evolutions of each spectral domain into the same one by multiplying the absorbances from both sets of spectra to produce a matrix with all possible products between them. Support Vector Machine with linear kernel function was used for the spectroscopic modeling. Predictions of soil organic C using vis–NIR, mid-IR, and fused spectra were statistically compared by the Tukey's test using the coefficient of determination (R 2 ), root mean squared error (RMSE), and ratio of performance to interquartile distance (RPIQ). Absorbances in vis–NIR and mid-IR were emphasized in the common spectral domain presenting stronger correlations with soil organic C than individual ranges. Soil organic C predictions with the OPA fused spectra were significantly better (R 2  = 0.81, RMSE = 2.42 g kg −1 , and RPIQ = 2.87) than those with vis–NIR (R 2  = 0.69, RMSE = 3.38 g kg −1 , and RPIQ = 2.08) or mid-IR spectra (R 2  = 0.77, RMSE = 2.90 g kg −1 , and RPIQ = 2.43). Fusing vis–NIR and mid-IR spectra by OPA improves predictions of soil organic C. [ABSTRACT FROM AUTHOR]

Accurate digital soil maps of soil organic matter (SOM) are needed to evaluate soil fertility, to estimate stocks, and for ecological and environment modeling. We used 5982 soil profiles collected during the second national soil survey of China, along with 19 environment predictors, to derive a spatial model of SOM concentration in the topsoil (0–20 cm layer). The environmental predictors relate to the soil forming factors, climate, vegetation, relief and parent material. We developed the model using the Cubist machine-learning algorithm combined with a non-parametric bootstrap to derive estimates of model uncertainty. We optimized the Cubist model using a 10-fold cross-validation and the best model used 17 rules. The correlation coefficient between the observed and predicted values was 0.65, and the root mean squared error was 0.28 g/kg. We then applied the model over China and mapped the SOM distribution at a resolution of 90 × 90 m. Our predictions show that there is more SOM in the eastern Tibetan Plateau, northern Heilongjiang province, northeast Mongolia, and a small area of Tianshan Mountain in Xinjiang. There is less SOM in the Loess Plateau and most of the desert areas in northwest China. The average topsoil SOM content is 24.82 g/kg. The study provides a map that can be used for decision-making and contribute towards a baseline assessment for inventory and monitoring. The map could also aid the design of future soil surveys and help with the development of a SOM monitoring network in China. [ABSTRACT FROM AUTHOR]

Land-use change is widespread throughout the tropics where large areas of natural forests being converted to secondary forests and tree plantations. In order to search for appropriate forest management, it is necessary to evaluate litter production, its temporal variation, and nutrient return of these ecosystems. In this study therefore, we investigated litterfall production, standing litter and litter quality for 5 years in a tropical seasonal rainforest (TSR), tropical secondary forest (TSF), tropical anthropogenic forest (TAF), and a rubber monoculture (RM) in Xishuangbanna, SW China. Results showed that the annual mean litterfall was significantly higher in TSF (14.83 Mg ha −1  year −1 ), followed by TAF (12.69 Mg ha −1  year −1 ), TSR (11.20 Mg ha −1  year −1 ) and RM (9.21 Mg ha −1  year −1 ). Both precipitation and temperature exerted greater impacts on litterfall amount in TSR and TSF than that in TAF and RM. The peak litterfall occurred in the cool and dry season for all four forest sites. Average standing litter for 5 years followed the order of TAF (3.08 Mg ha −1 ) > RM (2.91 Mg ha −1 ) > TSR (2.52 Mg ha −1 ) > TSF (2.04 Mg ha −1 ). Total returns of C, N, P, and K to mineral soil showed a decreasing trend of TSF > TAF > TSR > RM, which had a variation pattern similar to that for litter production. Nutrient-use efficiency varied with the mineral elements involved, and they differed significantly across the four forest types. Overall, litter production and nutrient returns increased after conversion of tropical primary forest to secondary forest and anthropogenic forest, but both responses decreased after conversion to the rubber monoculture. Therefore, establishing a multilayered community on degraded tropical land or introducing native species into ageing rubber forest offers a promising way to maintain the sustainable use and management of local forest resources. [ABSTRACT FROM AUTHOR]

The impact of biochar (BC) application on soil varies with BC feedstock and soil type. The objective of this study was to investigate the linkage between the properties and surface functionality of various BCs and their role in the rehabilitation of two infertile soils. Sandy loam (SL) and sandy (S) soils were collected from agricultural areas in Korea and Vietnam, respectively. The BCs of amur silvergrass residue (AB), paddy straw (PB), and umbrella tree (UB) were applied to the soils at a rate of 30 t ha −1 and incubated at 25 °C for 90 d. Soil carbon (C) mineralization was investigated by a periodic measurement of CO 2 efflux. Soil texture strongly influenced the CO 2 efflux more than the BC type as indicated by 2–4 folds increase in cumulative CO 2 -C efflux from the SL soil compared to that from the S soil. For the PB-, AB-, and UB-treated S soils, the values of cation exchange capacity (CEC) were increased by 906%, 180%, and 130%, respectively, compared to that of the control; however, for the PB-treated SL soil, only a 13% increase in CEC was found. The pH in the PB-treated S soil sharply increased by 4.5 units compared to that in the control, due to a high concentration of readily soluble compounds in the PB and the low buffering capacity of the S soil. The S soil was more sensitive to the addition of BCs than the SL soil. A more prominent improvement in soil fertility can be achieved by BC application to the sandy soil having low clay, nutrient, and organic matter contents. [ABSTRACT FROM AUTHOR]

Addition of organic amendments is a common practice to restore fertility and to increase productivity of degraded soils. Long-term effects of this practice on Mediterranean soils are controversial, with previous works showing contrasting results about the durability of the organic material added and its effects on the structure of microbial communities. This article presents results from a long-term soil remediation experiment, where a range of soil chemical and biochemical indicators, as well as indices of microbial diversity and community structure, were analysed 13 years after the first application of two organic amendments (leonardite and biosolid compost) at different doses, in an area contaminated by trace elements. In general, differences in chemical and biochemical properties and trace element availability between control and treated soils were still very evident, mainly in those soils treated with the highest amendment dose. The structure and composition of the soil microbial community was significantly affected by the type of management. The addition of both amendments favoured the increase of the fungal/bacterial ratio in the soil community, although a correlation with the C/N ratio of amendments was not found. The abiotic factors that acted as main drivers of the belowground communities differed between bacteria (more sensitive to Zn and Cd contamination) and fungi (soil pH and nitrogen content). Organic amendments had a direct positive effect on these abiotic factors, especially on the soil pH, a key factor in achieving long-term remediation. The results revealed that the effect of both amendments on the soil is maintained years after their application, although it is necessary to repeat their application to maintain soil pH within appropriate ranges and achieve a long-lasting recovery of soil functions. [ABSTRACT FROM AUTHOR]

Temperature sensitivity of soil respiration ( Q 10 ) is an important parameter when modeling the effects of global warming on terrestrial ecosystem carbon release. Widely applied chemical fertilizers can significantly affect soil productivity and carbon cycling in agroecosystems. However, little is known about how Q 10 responds to chemical fertilization under different levels of initial soil fertility. On the Chinese Loess Plateau, changes in soil respiration rates and Q 10 were investigated in soils of two fertility levels: low fertility (L) and high fertility (H). For each soil fertility level, there was one control plot and one chemical fertilized plot (+NP), which in total formed four treatments: L, L + NP, H and H + NP. All the treatments were replicated for three times on a continuous winter wheat cropping system. Respiration rates of surface soil in each treatment were in situ monitored from October 2010 through September 2015. Our results showed that after NP fertilization, soil respiration rates were increased by 46% in low fertility soil, yet only by 14% in high fertility soil ( P  < 0.05). The Q 10 after NP fertilization was significantly decreased by 6.9% in low fertility soil, but was unchanged in the high fertility soil. The Q 10 variation might be attributed to the different response of microbial respiration Q 10 in the two soils. The decreased Q 10 with NP fertilization in the low fertility soil was possibly due to N-induced increase of substrate quality for soil microbes and increased activities of both cellobiohydrolase and polyphenol oxidase. In the high fertility soil, the unchanged Q 10 with NP fertilization may be the integrated result of less affected substrate quality and neutral response of polyphenol oxidase activity. Overall, our results suggested that the effects of NP fertilization on soil respiration and its temperature sensitivity varied with soil initial fertility levels, and therefore must be properly accounted for when estimating potential effects of local agricultural management to regional agroecosystems under future climate conditions. [ABSTRACT FROM AUTHOR]

In 1978, after >70 years of continuous management of the long-term fertilization experiment in Bad Lauchstädt, a general change of soil management was implemented for several treatments. Experimental plots which had previously received a high input of organic matter (OM) now received only low OM input and vice versa. The resulting changes of SOC concentrations on different treatments vary from −0.1 to +0.1 g kg −1  yr −1 . These trends were analyzed in order to explain the observed variations with the mean carbon input together with the quality of the different sources to build up new SOC. The results showed that this quality related carbon flux from fresh organic matter (FOM) into SOC, here defined as “carbon reproduction flux” (C rep ), is a useful indicator to explain changes in SOC trends. Moreover, it can be used to predict the amount of FOM carbon input that is needed to achieve a given sequestration rate. The results revealed that carbon sequestration requires less effort on plots that had already higher carbon input rates in the past because the SOC storage, still far from the equilibrium state, had already a positive trend. Therefore, it is important to consider not only SOC stock but also the direction of the current trend when selecting measures that increase SOC to achieve the goal of the “4‰ Initiative: soils for food security and climate” which was launched during the COP21 conference. [ABSTRACT FROM AUTHOR]

Phenol oxidases (POs) are a group of soil extracellular oxidoreductase enzymes, which are involved in oxidative processes related to nutrient cycling. This class of enzymes has multiple functions at both the organism and ecosystem level and can trigger either positive or negative feedback loops between soil organisms and soil organic matter. The purpose of this study was to evaluate: (i) whether PO activities have a trend different from those of microbial biomass and activity; and (ii) whether PO activities are enhanced or reduced by fertilizer application. Soil samples were taken from plots in a 14-year-old experimental walnut orchard, subjected, since planting, to three fertilizer treatments: organic (compost) or mineral (urea) fertilization at the same rate of nitrogen application (100 kg N ha − 1 ), or left unfertilized. Soil samples were divided according to sampling depth (0–20 cm and 20–40 cm). Results showed that the application of compost significantly increased C and N pools. qCO 2 and DHD/C ext data indicated that the biota at 20–40 cm soil depth was more stressed or mainly composed of microorganisms with low substrate utilization efficiency. Phenol oxidase, tyrosinase, and catechol oxidase activities were significantly promoted in the surface layer by compost addition. In contrast, laccase activity showed a large increase in the deeper soil layer when supplied with mineral N, whereas compost addition led to increased activity in the surface layer. These findings suggest that soil phenol oxidases play a part in the determination of soil functionality, but they need to be investigated in greater depth in order to understand the mechanisms regulating their activities. [ABSTRACT FROM AUTHOR]

Published laboratory studies suggest minimum solubility of soil phosphorus (P) in the pH range traditionally considered as the target for lime applications on agricultural land. However, the potential significance of these results for the timescale relevant in practical agriculture remains to demonstrate. We revisited four long-term (50–76 years) agricultural lime experiments located on Quaternary deposits with topsoil texture ranging from silt loam to silty clay and initial pH between 5.0 and 6.0. Crop uptake data and desorption experiments in the laboratory indicated that liming enhanced the solubility of P added with fertiliser in the decades following the application of lime. An empirical mass-balance model demonstrated that changes in acetate-lactate extractable P (‘P-AL’), previously shown to be related to reactive phosphate adsorbed on soil mineral surfaces, was proportional to field P balances (fertiliser P – harvested P) and inversely proportional to the net proton load to the soil. Extraction data suggest that liming enhanced the incorporation of silica in non-crystalline sesquioxides, which in turn inhibited the conversion of fertiliser P into less extractable forms by migration of P to inside the oxides. Liming induced a lowering of the C/N ratio in one of the experiments, suggesting stimulated decomposition of soil organic matter. However, lime applications did not lead to a decline in the pool of soil organic P, as measured by the combustion method. Mass-balance considerations showed that changes in organic-matter input due to liming were unlikely to result in observable changes in the pool of soil organic P. [ABSTRACT FROM AUTHOR]

• Phosphorus budgets for fertility experiments were studied with a mass balance model. • P K-edge XANES confirmed organic P to be unresponsive to fertilization. • In 3 of 5 fertilized soils, only 70 to 85 % of the applied P could be accounted for. • In unfertilized soils, deep P uptake could explain the optimised model parameters. To gain insights into phosphorus (P) dynamics in soils and the ability to predict soil responses to varying fertilizer inputs, mass balance models prove to be valuable tools. In this study, a new dynamic mass balance model, PBalD8, was used to describe the change in extracted P in the A horizon of soils subjected to diverse fertilizer treatments over a period of 50 to 60 years in five soil fertility experiments. The model employed a Freundlich equation to describe soil-solution partitioning of P and assumed that acid-lactate-extractable P represented a labile pool of P in instant equilibrium with soil solution P. Additionally, oxalate-extractable inorganic P was presumed to comprise the sum of the labile and stable pools of P, with mass flux to and from the latter described by Fick's first law. The model was evaluated using results from extractions and P K-edge XANES spectroscopy. Notably, organic P, as revealed by P K-edge XANES, did not substantially contribute to long-term changes in soil P content and was therefore excluded from consideration. In general, the model offered reasonable fits to the extracted P concentrations. However, for the P-depleted treatments, a prerequisite was that the P removal through harvest was lower compared to measurements. Conversely, in three of the soils, the modelled fertilizer inputs needed to be reduced to 70 % to 85 % of the known additions. These discrepancies may be attributed to the involvement of deeper soil horizons, including deep crop uptake and mixing with lower soil layers, although other factors such as lateral dispersion and inaccuracies in estimating applied fertilizers cannot be discounted. These results underscore the necessity of gaining a more comprehensive understanding of how deeper soil horizons influence P mass balances in agricultural soils. In one of the soils, Fjärdingslöv, P K-edge XANES results demonstrated the formation of calcium phosphate over time in the highest fertilization treatment, consistent with the model. Additionally, in two soils, Kungsängen and the P-depleted Vreta Kloster soil, the model predicted a significant contribution from mineral weathering. However, the PBalD8 model also projected higher P leaching rates than those observed, suggesting that the model may not fully capture this P output term. [ABSTRACT FROM AUTHOR]

• The dilution method generated microcosms with different levels of soil microbial diversity. • Microbial diversity loss causes less stability of AOB. • Nitrosospira briensis was the key microbe contributing to the resilience of the AOB. • Repeated application of N fertilizer had a negative impact on AOA abundance. Diversity of the soil microbial community is an important factor affecting its stability against disturbance. However, the impact of the decline in soil microbial diversity on the stability of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) is not known, particularly considering the repeated soil nutrient disturbances occurring in modern agricultural systems. Here, we conducted a microcosm experiment and modified the soil microbial diversity using the dilution-to-extinction approach to determine the stability and population dynamics of AOB and AOA communities with repeated nitrogen (N) fertilizer application. Our results demonstrated that the AOB community became more abundant and stable against repeated disturbances by N in the treatments with the highest microbial diversity. In contrast, the abundance of AOA decreased following repeated N fertilizer application, regardless of the microbial diversity. Notably, during the initial application phase, AOA displayed a potential for increased abundance in treatments with high soil microbial diversity. These findings highlight that the soil microbial diversity controls the stability of ammonia oxidizers during short-interval repeated N disturbances. [ABSTRACT FROM AUTHOR]

• Straw-induced priming effect (PE) in two arable soils subjected to different fertilization regimes were investigated. • PE in soils under different fertilization regimes was negitively linked to the MAOC/SOC ratio. • The fungal abundances and ratio of fungal/bacterial growth correlated positively with SOM-derived CO 2. • The PE intensities were significantly lower in GZL soil compared to UQ soil. • Manure application greatly decreased PE via increased mineral protection and N availability. Rational chemical fertilizer application, especially combined with manure or straw amendment, serves as a key strategy for facilitating soil organic carbon (SOC) sequestration and soil fertility improvement. However, the effects of fertilization managements on the SOC priming induced by straw return remains unclear. Here, a microcosm was conducted to clarify the mechanisms of 13C-labeled maize straw addition effects on priming effect (PE) in two 29-year fertilized soils (Urumqi, UQ; Gongzhuling, GZL) subjected to four fertilization regimes (no fertilizer, CK; chemical fertilizers, NPK; combined application of NPK with straw, NPKS; combined application of NPK with manure, NPKM). Results showed that straw addition strongly increased native SOC decomposition by 15–44%, inducing positive PEs across sites and fertilization treatments. Compared to CK treatment, the NPK and NPKS treatments significantly increased PE by 26% and 98%, respectively, in the UQ soil, but had little effect on the PE in the GZL soil. The NPKM treatment strongly decreased PE by 51–55%, mainly due to the increased proportion of mineral-associated organic carbon (MAOC) and N availability, and decreased fungal abundances, as compared to the CK treatment in UQ and GZL soils. The PE intensities were significantly lower in GZL soil (5.1–11.7 mg CO 2 -C/g SOC) relative to UQ soil (13.7–54.8 mg CO 2 -C/g SOC), primarily due to the higher MAOC/SOC ratio and N availability in the former across fertilization regimes. Overall, manure application has the potential to mitigate soil C loss via priming, mainly resulting from enhanced SOC stabilization via mineral protection and increased N availability, thus facilitating SOC sequestration in agroecosystems. [ABSTRACT FROM AUTHOR]

Land use and land management have a recognized impact on the cycle of elements such as carbon and nitrogen and are thus expected to play an important role in the mitigation against human-induced environmental changes such as global warming. Brazil is the world's largest producer of sugarcane and has in recent years, experienced a shift from traditional cane preharvest burning (B) to mechanized green-trashing (GT), whose consequences on soil organic carbon and nutrients are still uncertain. This study performed in the São-Paulo state, Southeast Brazil, investigated the impact of shifting from B to GT on soil organic carbon and nitrogen stocks, and chemical elements at three 4-year and one 12-year old trial sites. Soil samples were collected from four sites representing different soil types (clayey to sandy-clay Hapludox; Hapudult and sandy Quartzpsamment). Sampling was up to 0.4 m (0–0.05; 0.05–0.1; 0.1–0.2; 0.2–0.4 m) depths at six randomly selected pits from six plots (of 0.2 ha size each) per treatment arranged in randomized block design. The samples were analysed for carbon and nitrogen content and other soil properties such as texture, bulk density, aggregate stability, pH and chemical concentrations. Overall, conversion from B to GT increased fresh organic matter retained on soil surface by 12.8–13.0 Mg ha − 1 y − 1 , and soil organic carbon and nitrogen stocks in the top 0.4 m soil layer by 1.06 Mg C ha − 1 y − 1 and 0.11 Mg N ha − 1 y − 1 , respectively. The highest carbon sequestration rate (108‰ y − 1 ) occurred in the most carbon depleted soils, which were sandy soils with lowest proportion of macroaggregates and chemical elements such as calcium, magnesium and potassium. All the chemical elements also showed a general increase in concentration, especially within the topmost soil layer, because increased soil organic matter had greater ability to tighten them together. Considering the average sequestration rate of 1.06 ton ha − 1 y − 1 , the resulting soil carbon sequestration rate for Brazil where 8.8 million hectares are under green-trashing of sugarcane residues would be 9.3 million ton C y − 1 . However, intermittent tillage every 4–5 years for replanting poses limitations to carbon sequestration because a 12 year experiment which was sampled 1 year after replanting did not show any carbon sequestration, suggesting that all carbon stored between planting and replanting was lost at tillage. There is need for further research to quantify the dynamics of carbon stocks during and immediately after tillage and to find sugarcane replanting techniques which minimize soil disturbance and associated carbon losses. [ABSTRACT FROM AUTHOR]

Growing cover crops to recycle soil nutrients in no-tillage systems provide nutrients accumulation in more labile forms in the soil surface reducing the demand for fertilizers. This study aimed to evaluate the long-term (23-yrs) effect of soil tillage systems and winter cover crops on ( i ) soil acidity, ( ii ) nutrient availability, and ( iii ) P and K budget in a subtropical Oxisol from Southern Brazil. The experiment was established in 1986 with six winter treatments (blue lupine, hairy vetch, oat, radish, wheat, and fallow) and two tillage systems (conventional tillage - CT and no-tillage - NT) in a very clayey Rhodic Hapludox in Southern Brazil. Nutrient availability (P, K, Ca, and Mg) and soil acidity (pH, potential acidity, base and Al saturation) were evaluated in five soil depths (0–5, 5–10, 10–20, 20–30, and 30–40 cm). Nutrient budget was calculated considering the inputs (amount of P and K applied via fertilizer) and outputs of P and K from the system (exported by the grains), and the soil available P and K before and after 23-years of experiment. Continuous NT system for 23-years resulted in higher soil fertility in the topsoil (0–10 cm) compared to CT, but with some limitations of nutrient availability and soil acidity below 10 cm depth. Long-term NT builds up a strong gradient of nutrient availability, with higher concentration of nutrients on the soil surface layers that abruptly decrease with soil depth, unlike CT. Surface application of lime in NT reduced soil acidity up to 20 cm compared to CT. The budget of P was negative for all treatments, highlighting the P-sink behavior of this strongly weathered subtropical Oxisol. However, NT system resulted in less negative budget compared to CT and, therefore, higher efficiency of use of P. Growing cover crops in the winter is effective to increase P and K availability through plant cycling, but the plants grown in winter did not affect soil acidity. Nutrient cycling by winter cover crops reduced P and K losses, especially when the soil is not plowed. Fallow in the winter decreases the use efficiency of P and K. Among the cover crops tested, black oat stood out by its greater production of biomass, resulting in higher P and K availability in the soil surface. Lupine resulted in a greater cycling of P possibly due to its ability to absorb P from less labile forms in the soil. [ABSTRACT FROM AUTHOR]

Phosphorus (P) plays a key role in the global biogeochemical cycle. The dynamics of P in soils is of considerable importance to the management of soil fertility. However, little is known about P dynamics in reclaimed land at the centennial scale during the initial stage of soil development, when the ambient conditions undergo substantial changes. In this study, conducted in Eastern China, we used a sequential extraction method to analyze the changes in P species in sediments and soils in a coastal reclaimed area representing a sequence of soil ages. Significant accumulation of total P accompanied by organic matter enrichment, desalination, and decalcification were observed in the initial pedogenesis of reclaimed tidal flat sediments. A rapid change in top soil P species within a century of pedogenesis was revealed, which was characterized by a marked decline in P contained within primary minerals (DeP) and an increase in loosely absorbed P, Fe-bound P, authigenic P (CaP), and organic P. DeP decreased from 67.3% to 25.3% of total P, whereas CaP increased from 8.4% to 32.9% along the reclamation gradient. Dissolution of primary minerals and a high rate of fertilization were identified as the major causes of this P trajectory. A key factor controlling the evolution of soil P speciation is the marked change in ambient conditions following land conversion, resulting in a shift in anaerobic conditions, a decrease in pH, accumulation of organic matter, and depletion of carbonates. Under the strong interference of human activities, the P dynamics in soils derived from tidal flat sediments differ from those predicted by the Walker and Syers model. Our results indicate that current nutrient management practices in the studied area can not only avert soil P depletion but also increase P availability. [ABSTRACT FROM AUTHOR]

Tree plantations contribute towards balancing global carbon (C) and nitrogen (N) cycles, with the C:N ratio being a key factor determining soil fertility in plantations. In the present study, we investigated how the management practices of Chinese fir ( Cunninghamia lanceolata ) plantations affect soil organic carbon (SOC), C:N ratio and soil quality. We assessed how these soil properties vary for stands of (1) different ages (up to 97 years) within the same rotation and (2) similar ages but in different rotations (up to four). Soil samples were collected and analysed from incremental depths (0–20, 20–40, 40–60, 60–80, and 80–100 cm). Continuous replanting of Chinese fir at the same site caused SOC stock and C:N ratio to decline after the second rotation. SOC stock (0–100 cm) decreased by 3, 3.6, and 14.3% between the first and second, second and third, and third and fourth rotations, respectively. The SOC concentration and C:N ratio declined from 21- to 40-year-old stands, and then increased in the 97-year-old stand throughout all soil depths. The stratification ratio (SR) index of SOC stock showed that continuous cultivation causes soil quality to decrease with increasing rotation cycle. Approximately 35–45% of equivalent soil mass SOC stocks were distributed in the upper soil layer (0–20 cm) in stands of all ages, indicating more organic C accumulation in the surface layer compared to subsurface layers (> 20 cm). In conclusion, we recommend that (1) cutting cycles of the stands should be increased from 20 to 25 years (current practice) to ~ 30 years of age and (2) plantations should only be cultivated to the second rotation to maintain site productivity, which would maximise both the ecological and economic value of this practice to the environment. [ABSTRACT FROM AUTHOR]

Acidification of cropland soils is a serious problem in China that may cause long term pH decline, which threatens the sustainability of soil fertility and crop yields. The objective of this research was to investigate those rates and mechanisms for a slightly calcareous soil. The field data were based on selected treatments from a long-term (1991–2015) N fertilization experiment at a rice-wheat rotation on a slightly calcareous (pH 7.7), young and hardly weathered clay soil (Regosol), including two plots with urea and NH 4 Cl fertilization and a control plot (no N fertilization). Urea and NH 4 Cl were applied at annual rates of 300 kg (1991–1996) and 285 kg (1997–2015) N ha − 1 . Soil pH and exchangeable Ca 2 + and Mg 2 + declined in the first 12 years for the urea treatments, but remained constant thereafter. For the NH 4 Cl and control treatment, the exchangeable base cations showed an oscillating decrease over the 25 years. Soil pH and exchangeable Ca 2 + , and Mg 2 + levels were significantly ( P < 0.05) lower in the NH 4 Cl treatment plot than the control. After 25 years, soil pH in the plots receiving urea and NH 4 Cl were 0.9 and 2.0 units lower than the control, respectively, while the total exchangeable base cations were about 10% and 16% lower, respectively. Carbonate dissolution, base cation exchange and mineral weathering were the main H + buffer pathways of the Regosol. Long-term N fertilization favored the formation of smectite and vermiculite and accelerated the weathering of mica, potassium feldspar, and illite. This was more evident in the NH 4 Cl treatment than in the urea treatment. Consequently, the soil acid-buffering capacity was slightly enhanced after year 13 in the order of urea > control > NH 4 Cl. The increased weathering of soil primary minerals and clay minerals was the main reason that soil pH and exchangeable base cations stayed constant after about 12 years. [ABSTRACT FROM AUTHOR]

Biochar may increase soil fertility and crop yield by enhancing soil organic carbon (SOC), nutrient availability and water retention. However, the mechanisms through which biochar effects crop yield at field scale and the time span over which biochar can affect soil properties and crop yield are still little known. The aim of this study was to evaluate the effect of wood biochar as soil amendment, combined with the application of synthetic fertilizer in a zero-tillage system, on soil physical-chemical properties and soybean grain yield in a 5-year field trial settled on a sandy clay loam Haplic Ferralsol of the Brazilian savannah. Immediately after biochar application (year 0), soil pH and plant available water (PAW) increased and Al 3+ and bulk density (BD) decreased linearly with biochar application rate. Later, in year 5, biochar had no effect for PAW and BD increased with biochar rates. The SOC was not affected until the third year after application, but increased linearly with biochar rates at 4 and 5 years after its application. Macro-porosity (MAC) was affected in a quadratic manner reaching a maximum at 8 Mg ha −1 biochar rate in years 0 and 2, however in year 5 it decreased with increasing biochar rates. Soybean grain yield increased with biochar rates until the third year after its application. There was almost no interaction effect of biochar and synthetic fertilizer; except for potassium (K + ) availability and grain yield in year 3. Possible control mechanisms of biochar effect on soybean grain yield through its effect on soil fertility and physical-hydraulic properties are discussed. The effect of biochar on soybean yield depended on multiple independent factors that included soil and environmental variables. Because of this its effect on yield was variable through the years, which makes it rather unpredictable. Biochar, however, may have an important contribution to sustainable production through enhancing the effect of conservation soil management, like the use of green manure or mulching, on SOC accumulation. [ABSTRACT FROM AUTHOR]

As many soils are unable to supply sufficient amounts of phosphorus (P) to plants, P availability is often a growth-limiting factor. Literature shows that levels of readily available P can be considerably higher in earthworm casts than in the surrounding bulk soil, possibly resulting in increased plant P uptake. However, the underlying mechanisms through which this increase of readily available P occurs are unclear. In a greenhouse pot experiment with annual ryegrass ( Lolium multiflorum ) on a soil with low P availability, we tested whether the presence of the anecic earthworm Lumbricus terrestris resulted in increased grass growth and P uptake and explored the possible mechanisms behind such an effect. During the pot experiment, earthworm casts were collected and analysed for water-extractable P as well as for most other relevant ions. The earthworm's presence significantly increased grass yield from 9.80 to 10.80 g dry matter per pot (p = 0.044) and P uptake from 6.32 to 8.04 mg per pot (p = 0.002). Due to increased microbial activity, the solution chemistry in the water extracts of the casts was dramatically changed compared to the bulk soil samples. The concentrations of dissolved inorganic P in cast were enhanced by a factor ~ 30–1000 compared to the levels found in bulk soil. The pH in earthworm casts increased from 5.9 to values between 7.4 and 9.0 and the Ca concentrations decreased by about a factor ~ 2-3 compared to bulk soil samples. In addition, there was an increase in dissolved organic carbon (35 mg L − 1 in casts versus 3 mg L − 1 in soil). Surface complexation modelling, using the Charge Distribution model, suggests that, besides an increase in pH, particularly an enhanced interaction of dissolved organic matter with reactive surface of metal (hydr)oxides is likely to be a major driving force for the augmented release of orthophosphate (PO 4 ) via competitive adsorption and desorption. This competition for adsorption sites is an alternative pathway, next to mineralisation of organic matter, through which earthworms can increase soil fertility. [ABSTRACT FROM AUTHOR]