Search

Although organic carbon sequestration in agricultural soils has been recommended as a 'win-win strategy' for mitigating climate change and ensuring food security, great uncertainty still remains in identifying the relationships between soil organic carbon (SOC) sequestration and crop productivity. Using data from 17 long-term experiments in China we determined the effects of fertilization strategies on SOC stocks at 0-20cm depth in the North, North East, North West and South. The impacts of changes in topsoil SOC stocks on the yield and yield stability of winter wheat (Triticum aestivum L.) and maize (Zea mays L.) were determined. Results showed that application of inorganic fertilizers (NPK) plus animal manure over 20-30years significantly increased SOC stocks to 20-cm depth by 32-87% whilst NPK plus wheat/maize straw application increased it by 26-38% compared to controls. The efficiency of SOC sequestration differed between regions with 7.4-13.1% of annual C input into the topsoil being retained as SOC over the study periods. In the northern regions, application of manure had little additional effect on yield compared to NPK over a wide range of topsoil SOC stocks (18->50MgCha(-1)). In the South, average yield from manure applied treatments was 2.5 times greater than that from NPK treatments. Moreover, the yield with NPK plus manure increased until SOC stocks (20-cm depth) increased to ~35MgCha(-1). In the northern regions, yield stability was not increased by application of NPK plus manure compared to NPK, whereas in the South there was a significant improvement. We conclude that manure application and straw incorporation could potentially lead to SOC sequestration in topsoil in China, but beneficial effects of this increase in SOC stocks to 20-cm depth on crop yield and yield stability may only be achieved in the South., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Mineral-associated organic carbon (MOC), that is stabilized by fine soil particles (i.e., silt plus clay, <53 μm), is important for soil organic carbon (SOC) persistence and sequestration, due to its large contribution to total SOC (TSOC) and long turnover time. Our objectives were to investigate how climate, soil type, soil texture, and agricultural managements affect MOC contributions to TSOC in China. We created a dataset from 103 published papers, including 1106 data points pairing MOC and TSOC across three major land use types: cropland, grassland, and forest. Overall, the MOC/TSOC ratio ranged from 0.27 to 0.80 and varied significantly among soil groups in cropland, grassland, and forest. Croplands and forest exhibited significantly higher median MOC/TSOC ratios than in grassland. Moreover, forest and grassland soils in temperate regions had higher MOC/TSOC ratios than in subtropical regions. Furthermore, the MOC/TSOC ratio was much higher in ultisol, compared with the other soil types. Both the MOC content and MOC/TSOC ratio were positively correlated with the amount of fine fraction (silt plus clay) in soil, highlighting the importance of soil texture in stabilizing organic carbon across various climate zones. In cropland, different fertilization practices and land uses (e.g., upland, paddy, and upland-paddy rotation) significantly altered MOC/TSOC ratios, but not in cropping systems (e.g., mono- and double-cropping) characterized by climatic differences. This study demonstrates that the MOC/TSOC ratio is mainly driven by soil texture, soil types, and related climate and land uses, and thus the variations in MOC/TSOC ratios should be taken into account when quantitatively estimating soil C sequestration potential of silt plus clay particles on a large scale., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

We aimed to quantify the relative contributions of plant residue and organic manure to soil carbon sequestration. Using a 27-year-long inorganic fertilizer and manure amendment experiment in a maize (Zea mays L.) double-cropping system, we quantified changes in harvestable maize biomass and soil organic carbon stocks (0-20 cm depth) between 1986-2012. By employing natural (13)C tracing techniques, we derived the proportional contributions of below-ground crop biomass return (maize-derived carbon) and external manure amendment (manure-derived carbon) to the total soil organic carbon stock. The average retention of maize-derived carbon plus manure-derived carbon during the early period of the trial (up to 11 years) was relatively high (10%) compared to the later period (22 to 27 years, 5.1-6.3%). About 11% of maize-derived carbon was converted to soil organic carbon, which was double the retention of manure-derived carbon (4.4-5.1%). This result emphasized that organic amendments were necessary to a win-win strategy for both SOC sequestration and maize production.

Partial substitution of synthetic nitrogen fertilizer with manure (organic substitution) is highly recommended to minimize environmental risks without compromising crop productivity in intensive agricultural systems. However, our understanding of the effect of organic substitution on soil organic carbon (OC) and total nitrogen (TN) in deep soil and its impact on crop productivity remains limited. Here we investigated OC and TN changes in soil profile down to 100 cm, crop yield, and sustainable yield index under synthetic nitrogen, phosphate, and potassium fertilizers (NPK), NPK plus straw (NPKS), and organic substitution (NPKM) treatments over two decades in four upland fields across different climate zones. Compared with the initial values, two decades of NPKM treatment significantly (p < 0.05) increased OC and TN stocks in either topsoil (by 25.6–103.8 and 15.8–89.8%) or deep soil (by 2.9–71.3 and 5.7–36.9%), respectively, across all sites. The increases in OC and TN stocks in 0–100 cm soil receiving NPKM were significantly higher than those receiving NPK at all sites and NPKS at three high-evaporation sites. Compared with NPKS and NPK treatments, crop yield and N uptake were significantly increased under NPKM treatment only at the Qiyang site. Furthermore, OC sequestration in the entire soil profile down to 100 cm and TN accumulation in topsoil exhibited significant positive correlations with crop N uptake, relative crop yield, and sustainable yield index. In conclusion, long-term partial substitution of synthetic N fertilizer with manure facilitates soil OC and TN sequestration in the entire 100 cm profile and thus maintains high crop productivity in upland areas. [ABSTRACT FROM AUTHOR]

Purpose : Soil enzymes are the main drivers of microbial catabolic and anabolic processes. However, the response of functional enzymes strategies and microbial metabolic efficiency (MME) to nutrient stoichiometry remains unclear in straw-amended black soil. Methods: Four nutrients gradients of N0, N1, N2, and N3 were set, and N0–N3 mean straw-amended soil with increasing addition of nitrogen (N), phosphorus (P), and sulfur (S). The activities of five enzymes involved in carbon (C), N, and P acquisition were investigated under different nutrient treatments in two soils during an 84-day incubation study. Microbial nutrient availability and C use efficiency (CUE) were assessed based on enzymatic stoichiometry (ES), and their potential relationship to change net soil-C (Net-C) was then established. Results: Compared to soil with straw alone, the Net-C in Gongzhuling and Hailun increased by 1.2–2.3 and 1.3–1.9 times with stoichiometric nutrient management, respectively. ES revealed that microbial metabolism was successively limited by N and P in Gongzhuling soil and was mainly limited by P in Hailun soil. Nutrient supplementation improved the MME, supported by a decrease of − 9.2% (− 3.4 to − 17.8%) in qCO2 and an increase of 10.1% (2.9–23.6%) in CUE. Path analysis suggested that nutrient supplementation influenced Net-C by directly and positively regulating C and P acquisition EEAs and CUE. The Net-C in Gongzhuling was more sensitive to nutrient supplementation, while that in Hailun was more limited by P, indicating that microbial metabolism might be related to soil pH or clay content. Conclusions: Combined addition of straw and N, P, and S improved Net-C by alleviating microbial nutrient limitation and improving CUE, in which process P acquisition enzyme, P availability, and soil properties (pH or clay content) played a key role. [ABSTRACT FROM AUTHOR]

Manure application to soil has declined globally due to increased availability of mineral fertilizers. However, mineral fertilizer overuse has caused serious consequences for soil quality and the environment. We analyzed the results of 20 long‐term field trials (22–32 years, start year [ranged from 1980 to 1990] to 2012) and combined this with a climate change model and soil organic carbon (SOC) model to quantify the importance of manure and mineral fertilizers for grain yield and SOC sequestration across croplands in China. During the past three decades mineral fertilizers have increased grain yield for 91–184% but had minor impact on SOC sequestration (4–16%). In contrast, manure applied with mineral fertilizer increased grain yield by only 6–19% but strongly raised the SOC content (9–39%) compared with mineral fertilizer. Modeling (to the year 2099) indicated that manure used in combination with mineral fertilizers will increase future C sequestration in soils across China by 2,086 Tg C and by 2,482 Tg C based on current net primary productivity with no climate change and on increased net primary productivity with climate change scenarios, respectively. This corresponds to an additional 43–58% increase in C sequestration compared to mineral fertilizers only. The manure efficiency for C sequestration in soil was about 9.6% of C input and decreased with increasing SOC content. To maintain the current SOC content (i.e., 2010), 11 t·ha−1·year−1 fresh manure or 4.8 t·ha−1·year−1 dry maize straw would be required. We conclude that the regular use of manure with mineral fertilizers is essential for the long‐term dual functions of soil for food production and SOC sequestration. Key Points: The increase of SOC by manure is much higher than that of crop yield; manure C efficiency is 9.6% of C input and decreased with SOC contentMineral fertilizers just maintained the SOC, while combined with manure they sequester more C under current climate and increase crop yieldWe modeled the minimal level of manure application to maintain SOC on the current management practices [ABSTRACT FROM AUTHOR]

Long-term manure application is recognized as an efficient management practice to enhance soil organic carbon (SOC) accumulation and nitrogen (N) mineralization capacity. A field study was established in 1979 to understand the impact of long-term manure and/or chemical fertilizer application on soil fertility in a continuous maize cropping system. Soil samples were collected from field plots in 2012 from 9 fertilization treatments (M0CK, M0N, M0NPK, M30CK, M30N, M30NPK, M60CK, M60N, and M60NPK) where M0, M30, and M60 refer to manure applied at rates of 0, 30, and 60 t ha−1 yr−1, respectively; CK indicates no fertilizer; N and NPK refer to chemical fertilizer in the forms of either N or N plus phosphorus (P) and potassium (K). Soils were separated into three particle-size fractions (2000–250, 250–53, and <53 μm) by dry- and wet-sieving. A laboratory incubation study of these separated particle-size fractions was used to evaluate the effect of long-term manure, in combination with/without chemical fertilization application, on the accumulation and mineralization of SOC and total N in each fraction. Results showed that long-term manure application significantly increased SOC and total N content and enhanced C and N mineralization in the three particle-size fractions. The content of SOC and total N followed the order 2000–250 μm > 250–53μm > 53 μm fraction, whereas the amount of C and N mineralization followed the reverse order. In the <53 μm fraction, the M60NPK treatment significantly increased the amount of C and N mineralized (7.0 and 10.1 times, respectively) compared to the M0CK treatment. Long-term manure application, especially when combined with chemical fertilizers, resulted in increased soil microbial biomass C and N, and a decreased microbial metabolic quotient. Consequently, long-term manure fertilization was beneficial to both soil C and N turnover and microbial activity, and had significant effect on the microbial metabolic quotient. [ABSTRACT FROM AUTHOR]

Fertilizer application can potentially change soil organic C (SOC) sequestration of croplands. However, how fertilization affects SOC sequestration and soil C fractions has not been sufficiently investigated. Here, we studied this issue in a rice-barley system of eastern China after 23-year repeated organic and chemical fertilizer amendments. The fertilization treatments were as follows: unfertilized control (CK), pig manure (M), chemical fertilizer NPK (NPK), and pig manure combined with NPK (MNPK). Our results showed that M, NPK, and MNPK significantly increased 25, 13, and 30 % of SOC content as compared with CK. Compared with CK, the three fertilizations, especially manure addition, significantly increased the content of SOC fractions such as microbial biomass C, dissolved organic C, labile organic C, and particulate organic C. Compared with the initial SOC stock at 0-20-cm depth (41.2 Mg C ha), M, NPK, and MNPK increased 11, 1, and 16 % of SOC storage, respectively, whereas CK decreased 9 % of SOC pool. The SOC sequestration was positively correlated with the cumulative C input (root residue-C + manure-C) (SOC sequestration = 0.21 × C input − 5.67, R = 0.88, P < 0.001). This linear relationship suggested that (i) the soil had not reached a SOC saturation, (ii) the conversion rate from input-C to SOC was 21 %, and (iii) a C input threshold of 1.17 Mg C ha year was needed for maintaining the initial SOC level. Therefore, manure application is recommended for promoting SOC sequestration in the rice-barley cropping system of eastern China. [ABSTRACT FROM AUTHOR]

We determined the historical change in soil organic carbon (SOC) stocks from long-term field trials that represent major soil types and climatic conditions of northern China. Soil carbon and general circulation models were validated using these field trial data sets. We then applied these models to predict future change in SOC stocks to 2100 using two net primary production (NPP) scenarios (i.e., current NPP or 1% year−1 NPP increase). The conversion rate of plant residues to SOC was higher in single-cropping sites than in double-cropping sites. The prediction of future SOC sequestration potential indicated that these soils will be a net source of carbon dioxide (CO2) under no fertilizer inputs. Even when inorganic nutrients were applied, the additional carbon input from increased plant residues could not meet the depletion of SOC in parts of northern China. Manure or straw application could however improve the SOC sequestration potential at all sites. The SOC sequestration potential in northern China was estimated to be −4.3 to 18.2 t C ha−1 by 2100. The effect of projected climate change on the annual rate of SOC change did not differ significantly between climate scenarios. The average annual rate of SOC change under current and increased NPP scenarios (at 850 ppm CO2) was approximately 0.136 t C ha−1 yr−1 in northern China. These findings highlight the need to maintain, and where possible increase, organic carbon inputs into these farming systems which are rapidly becoming inorganic fertilizer intensive. [ABSTRACT FROM AUTHOR]

Abstract: Long-term fertilization has a significant impact on total soil organic carbon (SOC) stock. However, fertilization impact on physical fractions of SOC is still poorly understood for red soils in southern China. This study assessed the impact of 17years (1990–2007) of long-term fertilization on the changes in different SOC fractions under an intensive maize (Zea mays L)–wheat (Triticum Aestivium L) cropping system in a red soil of southern China through various treatments: the unfertilized control (CK), the recommended applied rates of N (N), NP (NP), NPK (NPK), NPK+manure (NPKM), NPK+straw (NPKS) and manure only (M), and a 150% recommended applied rate of NPK+manure (1.5NPKM). Soil samples from 0 to 20cm soil layer taken in September, 2007, were separated into free particulate organic C (fPOC), intra-microaggregate particulate organic C (iPOC), and mineral associated organic C (MOC) with physical fractionation. In comparison with CK, all the C fractions and maize and wheat yields were significantly increased, except for N and NP treatments. The treatments with manure (M, NPKM, and 1.5NPKM) showed higher C sequestration rates in MOC (323–515kgha−1 yr−1), fPOC (291–408kgha−1 yr−1) and iPOC (162–179kgha−1 yr−1). It was estimated that 8.0 to 35.7% of the gross C input from manure and crop residues over a period of seventeen years contributed to the increase of total SOC stock. Both MOC C sequestration efficiency (CES) and C sequestration distribution (CSD) were the highest among the C fractions for all the treatments. Significantly positive linear correlations were observed between accumulated C sequestrations in all fractions with gross C input and both maize and wheat yields. Our result indicated that MOC was the primary fraction of C sequestration in the red soils. The most efficient fertilization practice for sequestering C in each fraction in the red soils was continuous applications of either manure or manure plus mineral fertilizers. [Copyright &y& Elsevier]

Abstract: The effects of conservation tillage on profile distributions of soil organic carbon (SOC), total N (TN), and C:N ratios were not well documented in China. The work was conducted at two field experimental sites in the northeast of China. Soils were Cambisols and continuously cultivated with maize (Zea mays L.) for 12years (Jianping site) and 5years (Changtu site). Two treatments were conducted at each site and were traditional tillage (TT, i.e., mouldboard plow with residue incorporation) and conservation tillage (CT, i.e., no-tillage with residues left on the soil surface). The responses of SOC and TN stocks to CT differed between the sites and among the soil depths. In the surface soil (0–20cm), SOC and TN concentrations and C:N ratio were uniformly-distributed under TT. These same response variables decreased with increasing depth under CT. The stratification ratios of SOC for surface to deeper depth (i.e., 0–5cm:20–40cm) ranged from 1.5 to 1.8 under CT but only from 1.2 to 1.3 under TT. CT also increased stratification of soil TN and the C:N ratio compared to TT. [Copyright &y& Elsevier]

Purpose: Although organic amendments have been recommended as one of the practices for crop production and soil carbon sequestration, little has been done to evaluate soil organic carbon (SOC) dynamics following long-term application of organic amendments. The objective of this research were to (1) assess the effect of long-term organic amendments on SOC dynamics in rice-based systems; (2) evaluate the relationship between soil carbon sequestration and carbon input based on various mineral and organic fertilization treatments. Materials and methods: A multi-sites analysis was conducted on four long-term experiments with double-rice (three sites) and rice-wheat (one site) cropping systems which started in the 1980s in Southern China. We selected three groups of treatments in common at each site: (1) control (no fertilizer), (2) mineral nitrogen-phosphorus with and without potassium (NPK/NP), and (3) the combined treatments of mineral NP/NPK with pig manure (M), green manure (G, Astragalus sinicus L.), rice straw (S), and/or their combinations. Harvestable crop biomass was annually recorded for all plots. SOC in topsoil was determined in 1-5 yearly intervals after rice harvest. Results and discussion: Analysis showed that organic amendments sustained or significantly increased carbon biomass, but had little effects on the coefficient of variance (CV) of the carbon biomass production compared with the mineral NPK/NP treatments. With additional carbon input, organic amendments increased SOC significantly by 7-45% after 25-28 years of fertilization compared with the mineral treatments. These combined treatments sequestered carbon at a rate from 0.20 to 0.48 t ha year under the double-rice and 0.70 to 0.88 t ha year under rice-wheat cropping system. The estimated annual SOC decomposition rate ranged from 0.15 to 0.82 t ha at these studied sites. Our analyses revealed strong positive correlations between soil carbon sequestration and carbon input for all sites. Conclusions: We concluded that organic amendments applied as substitution and extra nutrients had significant effect on soil carbon sequestration and served as a carbon sink for the duration of the experiments. Paddy soil high in clay content had the potential to sequester more carbon. Soil carbon sequestration efficiency-declined with carbon input at some sites with loam soil texture, suggesting applying a large amount of organic amendments is not recommended as a sustainable agricultural management practice because of the high risk of non-point environment pollution. [ABSTRACT FROM AUTHOR]

Abstract: A 12-year field experiment was used to examine effects of return rates of crop straw residue on organic C sequestration by chemical fertilization in surface (0–20cm) soil. The tested soil was a Hapli-Ustic Cambosol (FAO taxonomy) and the used crop was maize (Zea mays L.). The four main treatments were consisted of a gradient of return rate of straw residue, namely 0%, 25%, 50% and 100% of shattered straw residue (referred to as 0%S, 25%S, 50%S and 100%S, respectively). The three sub-treatments contained unfertilized control (CK), unbalanced N fertilization (N) and balanced NPK fertilization (NPK). Along the gradient of straw return rate, topsoil C storage significantly increased. The balanced NPK fertilization was more effective in sequestrating topsoil C than the unbalanced N fertilization. Responses of topsoil C storage to the straw return and fertilization significantly increased with the experimental duration. With increasing straw return rates, C-sequestration effects of the N, and NPK treatments in topsoil changed from not significant to significant or from significant to more significant in statistics. Our findings demonstrate that C-sequestration effects of chemical fertilization in topsoil strongly depend on return regime of straw residue. [Copyright &y& Elsevier]

Long-term use of artificial fertiliser has a significant impact on soil organic carbon (SOC). We used physical–chemical fractionation methods to assess the impact of long-term (26 years) fertilisation in a maize cropping system developed on Brown Earth in Northeast China. Plot treatments consisted of control (CK); nitrogen (N) fertiliser (N2); low-level organic manure combined with inorganic N and phosphorus (P) fertiliser (M1N1P1); medium-level organic manure combined with inorganic N fertiliser (M2N2); and high-level organic manure combined with inorganic N and P fertiliser (M4N2P1). Our objectives were to (1) determine the contents of and variations in the SOC fractions; (2) explore the relationship between total SOC and its fractions. In treatments involving organic manure (M1N1P1, M2N2, and M4N2P1), total SOC and physically protected microaggregate (μagg) and μagg occluded particulate organic carbon (iPOC) contents increased by 9.9–58.9%, 1.3–34.7%, 29.5–127.9% relative to control, respectively. But there no significant differences ( P > 0.05) were detected for the chemically, physically–chemically, and physically–biochemically protected fractions among the M1N1P1, M2N2, and M4N2P1 treatments. Regression analysis revealed that there was a linear positive correlation between SOC and the unprotected coarse particulate organic carbon (cPOC), physically protected μagg, and iPOC fractions ( P < 0.05). However, physically–chemically, and physically–biochemically protected fractions responded negatively to SOC content. The highest rate of C accumulation among the SOC fractions occurred in the cPOC fraction, which accounted for as much as 32% of C accumulation as total SOC increased, suggesting that cPOC may be the most sensitive fraction to fertiliser application. We found that treatments had no effect on C levels in H-μsilt and NH-μsilt, indicating that the microaggregated silt C-fractions may have reached a steady state in terms of C saturation in the Brown Earth of Northeast China. [ABSTRACT FROM AUTHOR]

Straw retention has been widely implemented to increase soil organic carbon (SOC) sequestration and greenhouse gas (GHG) mitigation for global agriculture. However, the combined effects of long-term straw retention on crop production, SOC sequestration and GHG emissions response to climate change remain unknown. Two nearby wheat–maize rotation field experiments in the North China Plain, combined with local weather, soil and agronomic measurements, were used to evaluate the applicability of the SPACSYS model. The model was then applied to assess the response of crop yield, SOC storage and nitrous oxide (N 2 O) emissions to three nitrogen fertilizer application levels (100, 200, 400 kg N ha−1) and three representative concentration pathway scenarios (RCP2.6, RCP4.5 and RCP8.5) under straw retention for the wheat–maize rotation systems during 2021–2100. In general, the model was reasonably accurate with R 2 and model efficiency (EF) ranging from 0.25 to 0.96 and 0.32–0.94, respectively. Climate change decreased wheat yield by 4–39%, while maize showed a slight increase (3%) under RCP2.6 and a decrease of 1–15% under RCP4.5 and RCP8.5. The SOC storage in the 0–20 cm soil increased at a rate of 73–195 kg C ha−1 yr−1 but decreased within the upper 1 m soil at 280–390 kg C ha−1 yr−1. Climate change reduced the positive effect of SOC sequestration except in RCP2.6 and stimulated substantial N 2 O emissions ranging from 1–7.5 to 2.9–23.2 kg N ha−1 yr−1, consequently, the global warming potential increased from –79–2555 to 548–9357 kg CO 2 -eq ha−1 yr−1 under various N fertilizer application levels. This study reveals that the negative feedback under climate change with modelling approach, and extensive efforts are needed to make adaptations to ensure food production and reduce GHG emissions in the future. [Display omitted] • Climate warming led to the decline of wheat and maize yield except under RCP2.6. • Negative feedback on SOC storage was found under RCP4.5 and RCP8.5. • The SOC storage within 0–1 m soil depth declined under climate warming. • The N 2 O emissions were strongly intensified by climate warming. [ABSTRACT FROM AUTHOR]

The carbon sequestration efficiency of manure (CSE-manure) is critical for the accurate estimation of manuring-induced changes in soil organic carbon (SOC). However, the mechanisms of large-scale spatiotemporal changes in CSE-manure and their crucial drivers are unclear. To investigate these under various manure regimes, we performed field observation and collected relevant data using 1023 fields across China's main cropland, as well as collected data on high-resolution climate and soil properties. The experimental durations significantly (P < 0.001) affected CSE-manure. The CSE-manure values were 31.4% ± 13.4 (mean ± SD), 19.2% ± 10.3%, and 16.1% ± 9.4 at the experimental durations of < 10, 10–20, and > 20 years, respectively. In addition, the manure sources significantly affected CSE-manure. The CSE-manure values of the manures from horse (27.9% ± 19.0) and sheep (26.9% ± 11.0) and farmyard manure (25.7% ± 14.8) were higher than those of the manures from pig (19.8% ± 15.4), poultry (19.1% ± 10.58), and cattle (23.1% ± 13.1). The CSE-manure values varied as per the region; the values were higher in North China than in South China. Of all the investigated variables, carbon and nitrogen input from manure, soil bulk density, annual average air temperature and precipitation, and initial SOC and soil clay content were the most important drivers of CSE-manure. The results suggested that carbon sequestration potential was higher in the northeast region. The spatiotemporal CSE-manure values under various climate conditions were predicted using trained machine learning models. Simulation results suggested that by 2100, the rate of decrease in CSE-manure will be higher at RCP (Representative Concentration Pathways) 8.5, and climate change will have a great impact on CSE-manure due to high warming effects. • The CSE-manure was decreased with the increase of experimental durations. • The CSE-manure values varied in different regions, with higher values in North than in South China. • Manure-C and N, soil bulk density, climate were the important drivers of CSE-manure. [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]

Understanding drivers of straw decomposition is essential for adopting appropriate management practice to improve soil fertility and promote carbon (C) sequestration in agricultural systems. However, predicting straw decomposition and characteristics is difficult because of the interactions between many factors related to straw properties, soil properties, and climate, especially under future climate change conditions. This study investigated the driving factors of straw decomposition of six types of crop straw including wheat, maize, rice, soybean, rape, and other straw by synthesizing 1642 paired data from 98 published papers at spatial and temporal scales across China. All the data derived from the field experiments using little bags over twelve years. Overall, despite large differences in climatic and soil properties, the remaining straw carbon (C, %) could be accurately represented by a three-exponent equation with thermal time (accumulative temperature). The lignin/nitrogen and lignin/phosphorus ratios of straw can be used to define the size of labile, intermediate, and recalcitrant C pool. The remaining C for an individual type of straw in the mild-temperature zone was higher than that in the warm-temperature and subtropical zone within one calendar year. The remaining straw C after one thermal year was 40.28%, 37.97%, 37.77%, 34.71%, 30.87%, and 27.99% for rice, soybean, rape, wheat, maize, and other straw, respectively. Soil available nitrogen and phosphorus influenced the remaining straw C at different decomposition stages. For one calendar year, the total amount of remaining straw C was estimated to be 29.41 Tg and future temperature increase of 2 °C could reduce the remaining straw C by 1.78 Tg. These findings confirmed the long-term straw decomposition could be mainly driven by temperature and straw quality, and quantitatively predicted by thermal time with the three-exponent equation for a wide array of straw types at spatial and temporal scales in agro-ecosystems of China. [ABSTRACT FROM AUTHOR]

• Biochar plus chemical fertilizer (BF) improved crop yield more than biochar alone (B). • B and BF reduced global warming potential by 27.1 % and 14.3 %, respectively. • Effect sizes of B and BF on crop yield coupled with these on soil organic carbon. • Effect sizes of B & BF on crop yield were controlled by biochar C:N ratio & soil pH. Biochar application has been widely recommended as a potential solution to tackle the challenges of food security and climate change in agroecosystems, but the effect sizes of biochar application on crop yield, soil carbon sequestration, and global warming potential (GWP) shows great uncertainties. To explore the effect variation of biochar application alone (B) and biochar combined with chemical fertilizers (BF) on crop yield, soil organic carbon (SOC), and GWP, this study reviewed updated datasets with 455, 131, and 95 independent experiments globally to identify the key factors influencing the responses of crop yield, SOC, and GWP to B and BF, respectively. Overall, the effect sizes were different between B and BF in both improving crop yield (15.1 % and 48.4 %, respectively) and decreasing GWP (27.1 % and 14.3 %, respectively), whereas there were almost no differences in terms of increasing SOC (32.9 % and 34.8 %, respectively). In addition, the effect sizes of B and BF on crop yield were coupled with these on SOC. Increased biochar carbon to nitrogen ratio (C:N ratio) and soil pH decreased the impact of B and BF on crop yield, while increased SOC promoted the impact of BF on crop yield. The effect size on GWP increased with biochar pH increasing but soil pH decreasing under B and BF. The wetness index, soil properties (SOC, pH, and clay), and biochar properties (type, pH, C:N ratio, and application rate) jointly explained 70 %–79 %, 90 %–93 %, and 70 %–97 % of the effect variations in crop yield, SOC, and GWP, respectively. The biochar C:N ratio and soil pH were the most important factors determining the effect size of biochar application on crop yield, SOC, and GWP. Taken together, biochar application is a tripartite win-win solution for improving crop yield, increasing SOC, and decreasing GWP mainly depending on biochar properties and soil pH. [ABSTRACT FROM AUTHOR]