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Research on microplastics (MP) in soils is much complicated due to the lack of dedicated (extraction) methodologies and the strong matrix interferences for MP detection, and there is almost no research on the dynamics of the smallest MP in soil. Here we compared the possible detection of the smallest MP fraction (1-2 µm) by µ-Raman spectroscopy and fluorescence microscopy in matrices of highly varying complexity. Samples of pure quartz sand, soil with removal of native soil organic matter (SOM), and soil with native SOM still present were amended with fluorescent polystyrene (PS) microparticles (diameter 1.65±0.04 µm) in different concentrations ranging from 0.1 to 0.001%, and after mixing and compaction both the Raman spectra and fluorescence microscopy images were obtained. Characteristic PS Raman peaks (main peak at 1009 cm-1) were visible in quartz sand (all concentrations) and soil without SOM (highest concentration only), but not in the other situations, whereas fluorescence microscopy clearly visualized the MP at all concentrations in all matrices. The possibility of direct and unambiguous fluorescent MP detection in real soil also circumvents the need for lengthy extraction procedures, and opens up new avenues for studying mechanistic aspects of the smallest MP fractions in soil.

Land use through its control on vegetation and fertilization can impact on soil geochemistry which in turn alsoinfluences the stabilization of soil organic carbon (SOC). Here, we assess soil organic carbon pools following afractionation method by Zimmermann et al. (2007), and analyse the fate of SOC with a process-based soil genesismodel, SoilGen2. We hypothesized that geochemical properties influenced the distribution of SOC and theseproperties can be applied in a model context to modify the decay rate of soil carbon pool. A set of volcanic soilsdata from Mt.Tangkuban Perahu and Mt. Burangrang in Indonesia covering different land uses (primary forest,pine forest, and agriculture) from Holocene age was used in this study. In the model, calibration was donesequentially including (i) weathering of amorphous and primary minerals, and (ii) decay of soil organic carbon.These processes are represented by various process parameters, and each simulation was run on a 8-10k yeartime scale. Our SOC fractionation study showed that the dominant SOC pool was located in sand-aggregatefractions and was higher with agricultural land use. This pool was positively correlated to pH, exchangeable Ca,aluminum-oxalate extraction (Alo), and amorphous materials. This result is also in line with a better performancein the SOC model by applying geochemically-modified rates. Our calibrated model shows the advantage ofincluding geochemical rate modifier in the volcanic soils. Further, the SOC levels will also be investigated underdifferent climate projection using SoilGen model.

Land use is recognized to impact soil geochemistry on the centennial to millennial time scale, with implications for the distribution and stability of soil organic carbon (SOC). Juvenile volcanic soils in tropical areas are subject to much faster pedogenesis, with then also possibly a significant mediation by land use on much shorter centennial or even decadal scale. Very scarce observational evidence exists and so such indirect implications of land use on SOC cycling are largely unknown. We here investigated SOC fractions, substrate specific mineralization (SOC or added plant residue), and net priming of SOC in function of forest or agricultural land use on Indonesian volcanic soils. The content of oxalate-extracted Al and exchangeable Ca correlated well with OC associated with sand–sized aggregates. The concomitant near doubling of the proportion of SOC in sand-sized aggregates compared to forest and likewise contrasts in Al and Ca suggest an enhanced formation of Al- (hydr)oxides and liming promoted aggregation and physical occlusion of OC. This was importantly also consistent with a relatively lesser degradability of SOC in the agricultural sites, though the net priming of SOC and degradability of added 13C-labelled ryegrass was not found to depend on land use. We expected that the formation of amorphous Al after conversion of native forest to agriculture would mainly have promoted mineral-association of SOC compared to under pine forest but found no indications for this. Enhanced weathering but improved small scale aggregation of tropical Andosols caused by conversion to agriculture may thus partially counter the otherwise expectable decline of SOC stocks following cultivation. Such indirect land use effects on the SOC balance appeared relevant for correct interpretation and prediction of the long-term C-balance of (agro)ecosystems with soil subject to intense development, like the here studied tropical Andosols.

We investigated changes in geochemical soil properties in response to deposition age and land use management over 30 - 50 years on tropical volcanic soils. Our purpose was to find out how weathering stage and land use interactively affect soil properties and organic carbon, and to check if phenoforms (management-related soil subtypes) exist within the genoforms (genetic soil types). Soil samples were taken at land uses that have been converted (pine forest and agricultural land) and a natural forest as the original land use. The results showed that pine forest soil displayed more intense weathering as indicated by higher values of three weathering indices. Intensive agricultural practices also improved soil chemical properties such as pH, exchangeable base cations, base saturation, and organic carbon stock leading to WRB-qualifier of "eutric" in cultivated soils, whereas the average of bulk density was relatively similar between forests and cultivated soils. Positive correlations were found between amorphous materials and Al-o, specific surface area, and micropore volume. Correlations between the content of short-range order Al (hydr-) oxides (indicated by Al-o) and organic carbon were found in pine forest and agricultural soils, particularly in subsoils. Our results clearly indicate the increase of base cations retention due to less acidification and an increase of organic carbon stock under agricultural land use, likely due to stabilization with non-crystalline materials.

The impact of soil age on geochemical properties and carbon cycling has been studied via chronosequences. However, only few studies have addressed how land-use and soil age might interactively shape properties of Andosols and in turn their capability to retain organic carbon (OC). Geochemical soil analyses and laboratory incubation experiments were carried out to assess soil characteristics and mineralization of soil organic carbon (SOC) in Indonesian soils with two contrasting land uses, viz. pine forest and horticulture. Both of these land uses are the results of conversion of primary forest which had similar parent materials, soil age, as well as weathering intensity. Results showed that intensive agricultural practices (+ 40-50 years) did not result in a significant loss of SOC or the increase of bulk density compared to forest. On the other hand, they were found to increase pH, exchangeable cations, base saturation, and most strikingly non-crystalline materials (i.e. Alo + ½ Feo) leading to phenotype formation in agricultural soils. Positive correlations were found between non-crystalline materials with properties such as soil specific surface area and micropores volume, and it was also positively correlated with SOC, particularly in the subsoil. This study highlighted the resilience of Andosols to soil degradation under agricultural practices and its ability to stabilize SOC.

Soil organic carbon (OC) levels generally increase with increasing clay and silt content under a similar climatic zone because of increased association of OC to clay minerals and stronger occlusion inside aggregates. Surprisingly though, in Western Europe many silt loam soils actually bear low topsoil OC levels compared to lighter textured soils. Soil texture obviously also strongly controls moisture availability with consequent indirect impact on heterotrophic activity. We hypothesized that with increasingly frequent summer drought: 1) soil microbial activity in sandy soils is more likely impeded due to their limited water holding capacity retention during droughts, while soil OC mineralization in silty soils remain be less drought-limited; 2) capillary rise from sufficiently shallow groundwater would, on the other hand, alleviate the water stress in lighter textures. To test these hypotheses, we established a one-year field trial with manipulation of soil texture, monitoring of soil moisture and maize-C decomposition via 13/12C-CO2 emissions. The upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low soil OC. Another sandy soil treatment with a gravel layer was also included beneath the sand layer to exclude capillary rise. Soil texture did not affect maize-C mineralization (Cmaize-min) until April 2019 and thereafter Cmaize-min rates were higher in the silt loam than in the sandy soils (P=0.01). θv correlated positively with the Cmaize-min rate for the sand-textured soils only but not for the finer textures. These results clearly highlight that soil texture controlled Cmaize-min indirectly through regulating moisture under the field conditions starting from about May, when soils faced a period of drought. By the end of the experiment, more added Cmaize was mineralized in the silt loam soil (81%) (Pmaize-min in the sandy soil, seemingly because the capillary fringe did not reach the sandy topsoil layer. These results imply that, under future climate scenarios the frequency of drought is expected to increase, the largely unimpeded microbial activity in silty soils might lead to a further stronger difference in soil OC with coarser textured soils under similar management.

Termites can play a localized prominent role in soil nutrient availability and cycling because mound materials are often enriched in nutrients relative to surrounding soil. Mound materials may thus prove to be useful amendments, though evidently mound spatial arrangement needs to be considered as well. Furthermore, it is not known if gradients of soil properties exist from termite mound to interspace sites. Studying both aspects would be required to decide whether spreading of mounds or spatially differentiated management of surrounding crop to accommodate soil fertility gradients would be valid nutrient-management strategies. Mound abundance and mass were estimated at 9 and 4 mounds ha−1, representing 38.9 and 6.3 t ha−1 on Nitisols and Vertisols, respectively. Soil physical and chemical properties were measured on samples collected from internal and external parts of mounds and adjacent soils at 0.5, 1 and 10 m away from mounds. In general, termite mounds were enriched in plant nutrients and SOC on Vertisols but not on Nitisols. Termite mounds constituted only 0.3 to 1.3% of the 0–15 cm SOM stock on a per ha basis but nevertheless the immediate vicinity of termite mounds was a relative fertile hotspot. Hence, under the studied condition, we suggest spatial arrangement of crop around termite mounds according to soil fertility gradient and spatially differentiated nutrient management strategies. Our result suggests recommendation of termite mound spreading for soil nutrient amendment has to consider plant nutrient stock in termite mounds on per ha basis besides their nutrient enrichment. Interesting topics for future investigation would be growth experiment for different crops with mound materials treatment. It would also be interesting to study the effect mound building termite on soil properties under different soil conditions, slope class and land use.

Agriculture is estimated to generate about 700 million tons of waste annually in the EU. Novel valorization technologies are developing continuously to recover and recycle valuable compounds and nutrients from waste materials. To close the nutrient loop, low-value agri-food wastes, co-products and by-products (AFWCBs) produced during the valorization process, need to be returned to the soil. However, knowledge on their reaction in soils that is needed to allow efficient and environmentally sound recycling is largely lacking. To this end, we set up a series of laboratory incubation experiments using 10 AFWCBs including insect frass residues made from three different feedstocks, anaerobic digestates from two feedstocks, potato-pulp, rice bran compost, duckweed and two reference crop residues (wheat straw and sugar beet) and measured net N release, C mineralization, dehydrogenase activity (DHA), microbial biomass C (MBC) and community structure. The suppressing potential of frasses and digestates against Rhizoctonia solani was determined using bean. The digestates released the highest net mineral N (50–70%) followed by rice bran compost (55%) and duckweed (30%), while frass made from general food waste and potato-pulp immobilized N like the reference straw for 91 days after incubation. All AFWCBs except digestates significantly increased MBC compared to the control while frasses, potato-pulp and duckweed increased DHA. Frasses and digestates significantly suppressed the development of Rhizoctonia solani in bean plants. AFWCBs from emerging valorizing technologies have the potential to improve microbial activities, C sequestration and may play a significant role in closing the nutrient loop.

Land use change can significantly influence both mineralogy and chemical soil properties. This conversion, particularly from forest to agricultural system occurs often in volcanic soils due to their favorable properties for food production. Under agriculture, minerals can weather faster than in natural vegetation and this also impacts soil functioning. We aim to assess the impact of land use on geochemical soil properties and soil organic carbon across soils of different age. This study was conducted in Mt. Tangkuban Perahu and Mt. Burangrang where the soils were derived from similar andesitic parent material and have different ages based on their lithology. Five sites were selected representing land uses that have been converted (pine forest and agricultural land) and one site of natural forest as the origin of land use. The results showed that land use management enhances the mineral transformation. Pine forest and agricultural sites displayed higher weathering degree than natural forests as indicated by higher clay content, iron crystallinity index and the presence of gibbsite. The weathering degree of soils in agricultural sites might result from the length of cultivation period and soil age. Land use conversion also altered chemical properties such as pH, CEC, basic cations, and the proportion of amorphous materials. Non-crystalline Al and Fe minerals as indicated by Alo+1/2Feo were highly correlated with organic carbon and specific surface area (SSA) in the subsoils of all land uses. However, we did not see the accumulation of organic carbon in subsoils compared to topsoils as the amount of non-crystalline Al and Fe minerals increases with depth, especially in agricultural lands where the organic fertilizer input is very high. In addition, a significant proportion of carbon was stored in sand aggregate fractions in agricultural land which have longer cultivation period, while it was more readily found in silt and clay fractions in the site with shorter period.

Soil organic carbon (SOC) levels generally increase with increasing clay and silt content under similar climatic conditions because of increased physicochemical protection. Surprisingly though, many silt loam soils in Western Europe have low top SOC levels compared to coarser textured soils. Soil texture also strongly controls soil moisture with consequent indirect impact on heterotrophic activity. Especially during periods of prolonged drought, which seem to be occurring more frequently throughout Europe, it could be expected that this soil textural control of moisture retention decisively affects added organic matter (OM) decomposition. We hypothesized that, contrary to expected textural effects on physicochemical protection of OM, the higher moisture availability in silty soils strongly favors added OM mineralization during periodic drought than sandy soils. Moreover, as a secondary objective, we investigated if during prolonged drought, capillary moisture supply could significantly alleviate the water stress on added OM decomposition in sand with rather shallow groundwater table. Therefore, we established a one-year field trial in which the upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low SOC. In a fourth treatment, a gravel layer was added beneath the sand layer to exclude capillary rise. Maize residues were mixed homogenously with soil in top 25 cm in all plots. Changes in soil moisture and maize-C mineralization (C-maize-min) were measured. We found that soil texture did not affect C-maize-min until after about five months and thereafter C-maize-min rates were higher in the silt loam than in the sandy soils. Moisture content correlated positively with the C-maize-min rate for the sand-textured soils only. Capillary rise did not result in a significant increase in C-maize-min in the sandy soil. After one year, a larger share of added C-maize was mineralized in the silt loam soil (81 +/-& nbsp; 6%) than in the sandy soil (56 +/- 7%). These results clearly highlight that soil texture controlled C-maize-min indirectly through regulating moisture under the field conditions when the study area faced a period of unusual drought. Moreover, our results imply that, under future climate scenarios, more frequent droughts might lead to a lesser SOC depletion in sandy soils compared to in finer textured soils under similar management.

Paddy soil indigenous N supply is often poorly related to N status and our aim was to assess its linkage to reduction of Fe3+ and Mn4+, primary terminal electron acceptors in submerged soils. Transplanted rice was grown in the greenhouse in four Bangladeshi paddy soils with distinct SOC to Feox ratio (1.6 to 4.9) for 72 days firstly under continuous and then intermittent flooding. Solution buildup rates of Fe2+ and Mn2+ in the first 2–3 weeks of flooding correlated negatively with soil organic carbon (SOC) to NH4-oxalate extractable Fe (Feox) and Mn (Mnox) ratios (p 50% of e− capture, though. Reduction of abundantly present octahedral Fe3+ in chlorites and vermiculite and their interstratified forms in these floodplain silty Inceptisols is hypothesized to also support microbial activity. Notwithstanding, a close temporal synergy existed between solution Fe and soil mineral N and their build-up rates were correlated (r: 0.77 to 0.90; p

In rice production, water-saving irrigation management is expanding and likely alters depth profiles of soil moisture, redox potential (Eh) and microbial activity. It is, however, unclear how such conditions then impact net soil N-release and availability to the rice crop, because we do not know well enough how water-saving irrigation management shapes depth-distribution of Eh and reductive processes, and microbial activity. A field experiment with rice was laid out on a typical young floodplain paddy soil of Bangladesh with three irrigation schemes, viz. continuous flooding (CF), safe alternate wetting and drying (AWD) and direct seeded rice (DSR), with 120 kg N ha−1 (N120) or without (N0) urea application. We evaluated changes in soil mineral N and plant N uptake, CH4 and CO2 emissions and soil pH, and at multiple depths soil Eh and temperature, dissolved C, Fe and Mn throughout 2015 dry (Boro) season (Jan–Apr). Eh stayed at or above ∼+300 mV except for sudden drops to ∼−200 mV with irrigation events in DSR. Eh quickly dropped to methanogenic conditions, under both AWD and CF; rises to ∼+200 mV were observed during AWD-drainage events but were restricted to upper 5.5 or 12.5 cm depths. Throughout the growing season there was a pronounced increase in reductive dissolution of Fe and Mn (hydro-) oxides, buildup of dissolved C, and CH4 effluxes under AWD and CF but not DSR, likely at least partially driven by the gradual soil warming from ∼20 °C till 28 °C. Predominant aerobic conditions under DSR lead to a nearly doubled C-emissions (CO2 + CH4) compared to AWD and CF, suggesting more soil organic matter (OM) degradation in the former case, while soil mineral N plus plant N build-up rate followed an opposite order. Urea application did not raise soil exchangeable N levels, even prior to significant plant uptake from 28 DAT (days after transplanting), and we forward temporal abiotic NH4+-fixation and N-removal processes as explanations. We conclude that regardless of some distinctions in temporal evolutions of puddle layer Eh, solution C, Fe and Mn, and CH4-emission, soil N-supply was quite comparable under AWD and CF, as was rice yield. In the context of N availability, AWD could be safely adopted for rice growth in the Bangladeshi Boro season. The eventual fertilizer N recovery efficiency was higher for CF (42%) than for AWD (32%), but AWD saved 12% irrigation water. While DSR saved 45% water there was a large yield penalty, likely due to drought stress but also by poor germination caused by cold night temperatures in mid-January, while seedling transplantation in CF and AWD plots was only later on 28 January. Further research should be conducted to investigate the fast and pronounced removal of exchangeable inorganic N after initial N buildup by soil OM mineralization, especially in CF and AWD. At this moment most likely candidate processes appear clay-NH4+ fixation and anaerobic NH4+-oxidation.

Although the amendment of various forms of exogenous organic matter (EOM) is a common practice in cropland production, it is to date not clear if its mineralisation in soil depends on application rate. Previous research suggested that spatial concentration of EOM in soil positively impacts its degradability. Here, we seek to test these reports and furthermore to investigate if an interactive effect exists with native soil organic carbon (SOC) concentration. We hypothesised that at a low EOM dose, more EOM remains undecomposed in soil and that this effect becomes stronger at lower SOC level. Moreover, as a secondary goal, we explored if priming of native SOC mineralisation depends on EOM dose. Therefore, we set up seventy-day soil incubation experiments with six varying C4-maize residue (δ13C = −12.7‰) doses (0–6 g kg−1) in soil mesocosms of loamy sand subsoils (δ13C = −25.7‰) with three different native SOC levels (i.e. 0.1, 0.5, and 1.0%). Soil CO2 emissions and isotopic signature measurements of CO2 allowed to discern EOM and SOC mineralisation. We found that EOM-derived C mineralisation increased proportionally to EOM dose, refuting the hypothesised relative undecomposed EOM at low concentration. Volumes of larger pore neck size diameter classes (60–100 and >300 μm) almost doubled at high EOM dose, demonstrating formation of macropores. Yet, this apparently did not impact EOM mineralisation, probably because O2 supply was always sufficient to allow unlimited activity of EOM degraders, even at higher EOM doses, as indicated by generally higher measured redox potential. With EOM dose, fungal marker PLFA abundances increased in the 1% SOC soil and protozoan abundances increased in all three soils, but apparently these shifts did not result in an enhanced relative degradation of the EOM. Increasing EOM doses induced negative priming, e.g. EOM ≥1 g kg−1 reduced SOC mineralisation by >43% and >24% compared to the control in the 0.1 and 0.5% SOC soils, respectively; whereas no priming occurred in soil with 1% SOC. These results were largely explained by the amount of added C relative to microbial biomass carbon, and the theorised switch of slow decomposers (so called K-strategists and involved in recalcitrant compound decomposition) from SOM to preferentially decompose EOM at higher doses. We also postulate that at low SOC %, the obvious increased O2 consumption with higher EOM dose more readily results in local anaerobic conditions in finer pores, i.e. where SOC is located and mineralised. We conclude that on the short term, agricultural management for SOM thus does not need to consider EOM doses but only the total amount of EOM.

The separate and collective role of soil nematodes and protists at community level in soil biogeochemical processes remains poorly quantified due to the lack of appropriate methodologies to study them independently from other soil biota under conditions emulating natural soil. Here we present a methodological exploration for the selective removal of nematodes and protists using gamma irradiation and the subsequent reinoculation with metazoan-free soil powder inoculum that allows reconstructing a complex microbial food web comprising the native microflora and soil protist communities, without other fauna. Nematodes, and culturable flagellates and ciliates were removed from 6 kGy onwards, while a small fraction of the amoebae survived (8% of the entire protist population) at 16 kGy but were totally removed at 27 kGy. Reinoculation with pulverized soil powder successfully established a protist community of similar size and composition as the control soil as assessed by both amplicon sequencing-based detection and microscopic observation after cultivation. A viable native microflora community remained from 6 to 16 kGy doses, but microbial activity was completely inhibited at 27 kGy. We suggest that, in ecological relevant experiments where selective removal of nematodes or other fauna is the purpose and protist community is meant to be kept intact, gamma irradiation needs to be followed routinely by soil powder inoculation in order to restore the suppressed protist community. This experiment also opens up new perspectives for studying the ecological roles of the entire protist community or specific groups thereof (flagellates and ciliates) in real soil and in the presence of a viable native microflora community.

Soil phosphorus (P) leaching is one of the major causes of diffuse P losses towards the environment in north-west Europe. In order to cut back these P losses over the long term, soil P stocks need to be drastically reduced. Greenhouse experiments have shown that a rapid reduction of the plant available P pool is attained by reducing the P fertilizer input to zero while cropping, but field trials confirming these findings are rare. Moreover, important additional questions arise in any P mining strategy: (i) to what extent will crop performance be affected in high P soils under 0 P fertilization, (ii) which crops reduce soil P stocks most efficiently and (iii) how can soil organic carbon (SOC) levels be maintained when organic fertilizers are no longer applied. In order to address these questions we set up two 0 P fertilization field trials at sites with a very high initial soil P status (i.e. a P saturation degree > 30%). On each site, we compared the effect of 0 P fertilization on crop yield, soil P status and SOC level to a business as usual scenario, for two crop rotations. At the first site, a comparison was made between a mixed crop rotation with or without the inclusion of a catch crop. At the second site, a comparison was made between an arable/grass and a vegetable crop rotation. Four consecutive years of 0 P fertilization had no effect on neither the crop yield nor the crop P uptake. Further, English ryegrass, silage maize, celeriac and Chinese cabbage made the largest contribution to P removal. Although 0 P fertilization reduced the P balance at field level with 102–121 kg P ha-1 over four years, this had no significant (measurable) effect yet on the soil P stocks. Continuation of these experiments over a longer time will thus be needed to allow to see any such effects. Simulations of the evolution of the SOC levels over a thirty-year period showed that the use of grass as green manure or as main crop only allows to partially maintain the SOC levels in the absence of any organic fertilizers. These results indicate that P mining is a challenging task, that will take a long time to reduce the soil P stocks and that will come at a cost for the farmer.

Long-term field experiments have confirmed belowground plant carbon (C) to be two to three times more efficient precursor of soil organic carbon (SOC) than aboveground plant residues. But it remains elusive just when this belowground biomass is relatively stabilized against mineralization: during the initial fast degradation, or on the longer term? A one-year field experiment was set up with biweekly follow-up of the in-situ mineralization of maize (Zea mays L.) C-inputs, either above- or belowground biomass based on soil δ13C resolved CO2 emissions. In addition, a treatment with maize roots and post-harvest remnant rhizodeposited-C left undisturbed in the field was included as well as a corresponding control that was physically disturbance as in the other treatments. We found that most maize-C was mineralized within a year with peaks in late fall and spring, i.e. 1 and 8 months after incorporation. Shoot-C decomposed only 1.4 times faster than root-C though statistically insignificant, which shows that on the short term (i.e. within a single year) the 2–3 fold stability of belowground biomass C is not manifested. Total maize-derived CO2 emissions of soil amended with only roots and with roots and post-harvest rhizodeposits were equal, likely because the rhizodeposited-C had already degraded quickly. A much smaller share of root- and rhizodeposited-C (43%) was mineralized when soil was not disturbed after harvest. We hypothesize that such a delayed physical disturbance, which in practice would only occur in early spring, could be key in stabilizing belowground biomass C in the field. Overall, our findings show a limited difference in degradability of roots and shoots under field conditions on the short term, and a large effect of tillage timing.

Nitrogen (N) depositions in Europe are decreasing, but this could not explain faster than expected improvement of N saturation indicators in temperate forests. Alongside there were local signs of initial recovery from acidification during the past three decades and enhanced leaching of dissolved organic carbon and nitrogen (DOC, DON). These two global change processes both affect total dissolved nitrogen (TDN) levels and often occur simultaneously, hence complicating mechanistic explanations for changing European forest N status. We aimed to test the hypothesis that forest N status in northwest Europe has started to improve. If this hypothesis is confirmed, we wanted to investigate to what extent such improvement is due to reduced N deposition. We evaluated the evolution of multiple N saturation indicators in five ICP Forests Level II plots in northern Belgium, using long-term soil solution and foliage datasets. The DON:TDN ratio (molar) in soil solution increased overall in the O horizon (mean 0.279–0.463, slope 0.023–0.037 yr −1 ) and in the mineral soil (mean 0.134–0.78, slope 0.007–0.051 yr −1 ) between 2005 and 2014. The DOC:NO 3 − ratio (molar) in soil solution increased in three plots in the O horizon (mean 6.84–22.15, slope 0.58–1.92 yr −1 ) and in four plots in the mineral soil (mean 2.07–25.32, slope −0.06–5.76 yr −1 ) between 2002 and 2014. The ratio of N and phosphorus (P) concentrations in foliage (mg g −1 ) and the ratio of base cations (Bc = Ca + K + Mg) and N concentrations in foliage (molar) remained unaltered between 1999 and 2013. Changes in the soil solution chemical composition thus confirmed an improvement in forest N status, despite sustained high NO 3 − concentrations, but biotic recovery appeared to be lagging behind. This demonstrates that insight in forest recovery from N saturation requires a multiple indicator approach, and further monitoring of tree nutritional status alongside soil processes is needed to monitor the evolution of European forest N status.

Drivers of nitrogen (N) mineralization in paddy soils, especially under anaerobic soil conditions, are elusive. The influences of exogenous organic matter (OM) and fertilizer application on the activities of five relevant enzymes (β-glucosaminidase, β-glucosidase, l -glutaminase, urease and arylamidase) were measured in two long-term field experiments. Of the two field experiments, the 18-year field experiment was established in a weathered terrace soil with rice-wheat crop rotation at the Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU) farm with five OM treatments and two levels of mineral N fertilizer. The 30-year experiment was established in a young floodplain soil with rice-rice crop rotation at the Bangladesh Agricultural University (BAU) farm with five mineral fertilizer treatments including one with farm yard manure. At BSMRAU, N fertilizer and OM amendments significantly increased all enzyme activities, suggesting the availability of primarily substrate for microbial activity. Whereas at BAU, non-responsiveness of β-glucosidase activity, suggesting that fertilizer and OM amendments had little effect on overall soil microbial activity. Nevertheless the microbial demand for N, β-glucosaminidase and l -glutaminase activities differed among the treatments (P l -glutaminase activity was the sole investigated variable that positively correlated to both the aerobic and anaerobic N mineralization rates in both field experiments. Combined with a negative correlation between β-glucosaminidase activity and N mineralization rate, it appears that terminal amino acid NH2 hydrolysis was a rate-limiting step for soil N mineralization at the BAU site. Future investigations with joint quantification of polyphenol accumulation and binding of N alongside an array of extracellular enzymes, including oxidases for phenols and hydrolases for N-compounds, would enable verification of the hypothesized binding and stabilization of N with accumulating polyphenols at BAU site under SOM storing management.

The availability of alternative electron acceptors like Fe3 + and Mn4 + may form a bottleneck to anaerobic SOM mineralization and thereby NH4+-release in flooded paddy soils. We assessed the influence of availability of soil Fe and Mn on anaerobic N mineralization in lab incubation experiments. Collected paddy soils from Bangladesh either untreated, amended with Fe2O3, or with Mn/Al mixed oxides were anaerobically incubated. In a first 8 weeks incubation with 5 treatments from a long-term field experiment (control, N, NP, NPK and N + FYM) we found no considerable differences in evolution of soil solution Fe and Mn between the control and Fe2O3 treated soils. Whereas, the soil solution contents of Fe were lower and Mn were higher in Mn/Al mixed oxide treated soils. Similar observations were made for dissolved Fe and Mn in a second 10-week incubation experiment with four farmers field soils. Evolution of KCl-extractable NH4+ was not affected by Mn4 + or Fe3 + application and we therefore conclude that availability of electron acceptors was not limiting release of NH4+ in the studied soils. The large and rapid increase of exchangeable-NH4+ at the onset of the incubations provoked the question whether part of it derived from release of fixed-NH4+. A third experiment revealed, however, instead a small significant increase of the fixed-NH4+ within four weeks. In addition, the microbial biomass carbon already plateaued after two weeks. Both results suggest that released mineral N was mainly derived from biotic anaerobic N mineralization and not from defixation of NH4+. Finally, while not directly dependent on Fe and Mn application, there was a remarkable convergence in the buildup of soil exchangeable-NH4+ and soil solution Fe concentrations. This warrants further investigation and still suggests involvement of reductive Fe and Mn-oxide dissolution in NH4+-release, e.g. through release of bound organic N after reduction of these oxides.

There is growing interest in the application of the natural fallout radionuclide 7 Be as a soil erosion and sediment tracer. Development of robust datasets is, however, hampered by unquantified variability in its vertical distribution within surface soil. Models that convert 7 Be inventory measurements to soil erosion estimates are all based on the observed depth distribution of 7 Be, described by the relaxation mass depth (h 0 ) parameter. Previous work, however, has not considered potential spatial variation in h 0 linked to natural variability in soil physical properties, which could have major implications for the reliability of soil erosion estimates. Two complementary experiments were designed to study the variability in depth distribution within and between potential reference sites. First, a field sampling programme was carried out whereby two reference sites with variable degree of compaction were sampled using two different sectioning techniques, i.e. by use of a fine increment soil collector (FISC) and the scraping methodology. During a laboratory rainfall simulation experiment, water spiked with stable 9 Be was used to study the variability in 9 Be soil depth distribution within and between the two reference sites. In the field experiment, variations in the 7 Be depth distribution, and thus in h 0 , were limited between both reference sites (13 to 16%). In contrast, the impact of the sectioning technique was remarkable, with scraping resulting in a higher h 0 (up to 60%) compared to the estimates based on the use of a FISC. The rainfall simulation experiment offered the opportunity to study the variation in 9 Be depth distribution in more detail. With an average h 0 of 4.66 kg m − 2 , 9 Be penetrated deeper in the non-compacted (NC) reference site cores, while the compacted (C) cores showed an average h 0 of 2.42 kg m − 2 . The reported h 0 values at the former site were also characterized by a larger coefficient of variation (24%) than those at the latter site (11%). Lower bulk density, higher infiltration rates and a pore network characterized by a higher macroporosity and connectivity, as revealed by the X-ray Computed Tomography (CT) scans, explained the deeper penetration of Be into the topsoil of reference site NC. The results indicate the importance of selecting appropriate reference sites and for ensuring an adequate sampling strategy to encompass local variability in soil physical properties. Hydraulic conductivity assessment could be a useful tool to properly assess suitable reference sites and the number of samples needed to assess the reference inventory.

Biochar addition to soils is heralded to reduce N2O emissions, but still, the explanatory mechanisms have not been resolved. Moreover, it is uncertain whether N2O emission reductions would persist after prolonged biochar incorporation in the field. In this study, we incorporated four biochar types in a loam textured cropland field and intact soil cores were sampled to investigate the physical control of biochar on denitrification after 7 months. During a first incubation experiment, we measured N2O emissions from undisturbed and disturbed (i.e. sieved (2 mm) and grounded) soil cores. Both in the disturbed and undisturbed soil cores biochar at water filled pore space (WFPS) of 80% reduced the N2O emissions by 50–90%, refuting the hypothesis that biochar exerts an indirect physical control over soil denitrification several months after incorporation. Secondly, we hypothesized that biochar creates denitrification ‘hotspots’ in soil, where complete reduction of N2O to N2 is promoted compared to non-amended soil. In these hotspots biochar particles could act as microlocations with local anaerobic conditions and local higher pH, stimulating in this way complete denitrification. Via the acetylene inhibition method we did not observe a reduction in the N2O/(N2O + N2) ratio, which could suggest that biochar did not promote the reduction of N2O to N2. Manipulations likely to promote labile C bioavailability, here either by glucose addition or by soil particulate OM disclosure after disruption of soil aggregates, resulted in the most prominent biochar-induced N2O emission reductions.

The few available studies on maize biomass in soil of field experiments have shown that belowground maize biomass resides more than twice as long as aboveground residues. Our principal objective was to investigate the effect of maize crop residue incorporation on the long-term dynamics of soil organic carbon (SOC) for a selection of croplands with sandy to sandy loam texture in Flanders, Belgium. After a period of more than 20 years, we resampled 31 and 29 parcels that had either silage maize (MS, only belowground biomass input) or grain maize (MG, both above- and belowground biomass input) dominated crop rotations. Changes in maize-derived SOC (Cmaize) of MS and MG systems between 1993 and 2018 were evaluated by detecting SOC content and its δ13C value. In addition, a 140-day incubation experiment (at 20.5 °C) was carried out to compare the stability of Cmaize and the related nitrogen (N) mineralization between the MG and MS fields. After two decades, δ13C raised 2.4 and 2.6‰, while Cmaize increased 1.6 and 2.2 g kg−1 in MS and MG systems, respectively. The estimated average humification coefficient over this period of maize-C was 0.11 for the MS system, about twice as that of the MG system (0.06). This reconfirms findings of stronger contribution of belowground biomass versus aboveground residues to stabilized SOC pools over the medium to long term. By the end of the incubation, on average 10.4% and 11.6% of Cmaize had been decomposed in the MS and MG soils, respectively. These Cmaize mineralization rates were statistically equal for the two systems, which indicates that mainly belowground biomass remained in the soils. Also, net N mineralization was indifferent to the crop rotations (MS: 58.8 mg N kg−1 and MG: 52.6 mg N kg−1). To reconcile the two main observations in this study, we hypothesize that the lower preservation of shoot-C could be manifested on a shorter term. Detailed research is needed to determine the time scale of increased stabilization of root-C versus shoot-C under field conditions.

Soil texture is well known to directly affect bioavailability of organic matter to heterotrophs, but it also steers their activity by moderating soil moisture fluctuation. Disentangling these direct and indirect textural controls is, however, not trivial and attempts to do so are very scarce. Most attention has just gone to the stimulation of soil carbon (C) mineralization by soil moisture fluctuation per se. To quantify the indirect moisture-mediation control of soil texture on C mineralization, we monitored maize straw degradation in various soil texture/moisture regime combinations. Moisture levels were firstly kept fixed at 32% WFPS (experiment Fixed32) in a sand, sandy loam and silt loam soil or allowed to fluctuate between 20% and 50% water-filled pore space (WFPS, Dry-wet20-50). Total maize-C (Cmaize) mineralized was highly similar between these three textures and thus the direct textural control was minor. On the contrary with the fluctuating moisture level, around threefold more added Cmaize was mineralized (P

Evidence is accumulating that belowground crop residues contribute more efficiently to the build-up and maintenance of native soil organic carbon (SOC) than aboveground plant parts. We corroborated previous testing of the hypothesis that root-C would preferentially accumulate in soil microaggregates, where it is physically protected against microbially mediated decomposition. In three European field trials with C-3 to C-4 crop transitions, we compared the content of maize-C (Zea mays L.) in soil from rotations with grain maize (MG) or silage maize (MS) (i.e. with incorporation of roots and shoots or roots only). After decades of maize cultivation, SOC content did not differ within three out of four MG-MS pairs, although obviously larger amounts of shoot biomass were added to soil in the case of MG. We found that relative contribution of roots was on average 3.5 times more than shoots to the build-up of SOC per equivalent mass of residue C added to soils. Preferential occlusion of root-C as silt-sized intra-microaggregate particulate organic matter (iPOM) was not observed. There were much larger effects from shoot incorporation on maize-C in the >53-mu m fraction and free silt and clay. Storage of root-C as sand-sized iPOM was not quantified here, but first estimates suggested that physical entrapment at this level could only partly explain the longevity of root-C in soil. We reconfirm the relative stability of root-C in soil, but do not conclude that this stems from preferential physical entrapment over shoot-C. Future work should investigate the cause of preferential root-C association with the clay-sized fraction and if this occurs before or after microbial processing. Highlights The hypothesized preferential physical stabilization of root-C was assessed. Aboveground biomass had a minor effect on SOC storage in European long-term trials. Contribution of maize root-C to maintenance of SOC was three-fold that of shoot-C inputs. There was no preferential occlusion of root-C in silt-sized microaggregates.

Soil organic N is largely composed of inherently biologically labile proteinaceous N and its persistence in soil is mainly explained by stabilization through binding to minerals and other soil organic matter (SOM) components at varying strengths. In order to separate kinetically different soil N fractions we hypothesize that an approach which isolates soil N fractions on the basis of bonding strength is required, rather than employing chemical agents or physical methods. We developed a sequential subcritical water extraction (SCWE) procedure at 100, 150 and 200 °C to isolate SOM fractions. We assessed these SCWE N fractions as predictors for aerobic and anaerobic N mineralization measured from 25 paddy soil cores in incubations. SCWE organic carbon (SCWE OC) and N (SCWE N) increased exponentially with the increase of temperature and N was extracted preferentially over OC. The efficiency of SCWE and the selectivity towards N were both lower in soils with increasingly reactive clay mineralogy. Stepwise linear regression found no relations between the SCWE fractions and the anaerobic N mineralization rate but instead with pH and a model parameter describing the temperature dependency of SCWE extraction. Both were linked to texture, mineralogy and content of pedogenic oxides, which suggests an indirect relation between anaerobic NH4+ release and these edaphic soil factors. N mineralization appeared to be largely decoupled from SOM quantity and quality. From the present study on young paddy soils low in pedogenic oxides and with high fixed NH4+ content we cannot infer the performance of SCWE to isolate bio-available N in more developed upland soils. There may be potential to separate kinetically different SOM pools from upland soils because 1° for aerobic N mineralization at 100–150 °C SCWE N was the best predictor; and 2° SCWE selectively extracted N over C and this preference depended on the mineralogical composition. Hence N fractions differing in bonding strength with minerals or SOM might be isolated at different temperatures, and specifically this association has frequently been found a prominent stabilization mechanism of N in temperate region cropland soils.

Soil moisture strongly affects the balance between nitrification, denitrification and N2O reduction and therefore the nitrogen (N) efficiency and N losses in agricultural systems. In rice systems, there is a need to improve alternative water management practices, which are designed to save water and reduce methane emissions, but may increase N2O and decrease nitrogen use efficiency. In a field experiment with three water management treatments, we measured N2O isotopocule signatures (δ15N, δ18O and site preference, SP) of emitted and pore air N2O over the course of six weeks in the early rice growing season. Isotopocule measurements were coupled with simultaneous measurements of pore water NO3−, NH4+, dissolved organic carbon (DOC), water filled pore space (WFPS) and soil redox potential (Eh) at three soil depths. We then used the relationship between SP × δ18O-N2O and SP × δ15N-N2O in simple two endmember mixing models to evaluate the contribution of nitrification, denitrification, fungal denitrification to total N2O emissions and to estimate N2O reduction rates. N2O emissions were higher in a dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to water-seeded + alternate wetting and drying (WS-AWD) and water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD treatment the highest emissions were associated with a high contribution from denitrification and a decrease in N2O reduction; while in the WS treatments, the highest emissions occurred when contributions from denitrification/nitrifier-denitrification and nitrification/fungal denitrification were more equal. Modeled denitrification rates appeared to be tightly linked to nitrification and NO3− availability in all treatments, thus water management affected the rate of denitrification and N2O reduction by controlling the substrate availability for each process (NO3− and N2O), likely through changes in mineralization and nitrification rates. Our model estimates of mean N2O reduction rates match well those observed in 15N fertilizer labeling studies in rice systems and show promise for the use of dual isotopocule mixing models to estimate N2 losses.

There is increasing pressure to reduce water use in irrigated rice production to save water, reduce methane emissions and reduce grain arsenic uptake arising from anaerobic conditions. However, under such practices emissions of nitrous oxide (N2O) often increase. Rice systems generally exhibit strong stratification of environmental conditions that drive the balance between N2O production and consumption, and ultimately the emissions of N2O. We investigated how the introduction of alternate wetting and drying (AWD) relative to conventional flood (FLD) irrigation modifies the depth distribution of environmental conditions and nutrients (NO3−, NH4+, dissolved organic carbon, soil redox (Eh) and water filled pore space, (WFPS)). We then examined how these variables related to N2O production and consumption via the measurement of δ15N-N2Oemitted/poreair, δ15N-NO3−, N2Oturnover and subsurface N2O fluxes at five depths (5, 12.5, 25, 50 and 80 cm). These measurements, together with N2O surface emissions were taken on six days surrounding a broadcast urea fertilizer application and for six days surrounding the onset of final drainage. The highest emissions were observed in the AWD treatment at the onset of measurements. These emissions were driven by high NH4+ availability and could mainly be attributed to nitrification directly or indirectly via coupled nitrification-denitrification in the upper depths. In both irrigation treatments, an increase in NO3− and dissolved N2O concentrations and a drop in δ15N-NO3− values indicated rapid and ephemeral nitrification following the fertilization, but without significant effects on N2O surface emissions. At 50 and 80 cm, δ15N-N2Oporeair was enriched relative to upper depths, pointing to N2O reduction at these depths in both treatments. We conclude that the increased N2O emissions under AWD compared to FLD management were associated with enhanced nitrification in the upper soil layers during plant establishment and thus related to basal N fertilization and mineralization of native soil N rather than in-season fertilization.

Biochar composition and stability is under intense research. Yet the question remains to what extent the current state-of-the-art applies to artisanally charred biomass in tropical regions. We compared kiln and drum based biochars with their counterpart controlled (at 400 °C) slow pyrolysis biochars from coconut shells, rice husks and Palmyra nutshell for their biochemical composition, thermal stability and biodegradability in soil. Thermal behavior of individual organic constituents was quantified by pyrolysis-field ionization mass spectroscopy (Py-FIMS). Comparison of the mass spectra demonstrated higher abundances of either phenols, lignin and carbohydrate monomers or of lipids in the artisanally produced biochars. Hence, relatively more untransformed plant matter was preserved by artisanal charring and also the thermal stability of carbohydrates, alkylaromatics and N-containing compounds was lower for all three feedstocks. This indicates lower prevailing temperatures compared to controlled pyrolysis biochar, at least in parts of the biomass charring in the kilns or drum. Nine-weeks biochar derived C mineralization upon soil incorporation revealed a relatively lower biological stability of the controlled pyrolysis biochars. The proportion of detected ion intensity from thermolabile lower mass signals ( m/z

Continuous intensive monocultures of rice can lead to subsoil compaction, reduced topsoil quality and decline in rice yield. The objectives of this study were to evaluate the effect of rotating rice with upland crops on properties of an alluvial paddy clay soil, rice yield components, and economic profitability. A field experiment was established in the Vietnamese Mekong Delta for 10 years with a randomized complete block design including four rice based crop rotations and four replications: (i) rice–rice–rice (control, i.e., farmers’ practice), (ii) rice–maize–rice, (iii) rice–mung bean–rice, and (iv) rice–mung bean–maize. Rice alternated with upland crops significantly improved physical quality of soil in terms of bulk density, soil porosity, soil aggregate stability, and soil penetration resistance compared to the traditional rice monoculture practice, especially in the 10–20 and 20–30 cm depth layers. As a consequence, also rice rooting depth and root mass density was strongly increased in all three rice upland crop rotations. This resulted in a higher plant height, total number of tillers and panicles, filled grain percentage and a rice yield that was 32–36% higher compared to the control, and farmer’s profitability even increased 2.5–2.9 times. The reason for improved rice growth upon deeper root development should be investigated further, with specific attention to micronutrient availability.

The hazard for recurring food insecurity in the Sahel is largely affected by insufficient biomass productivity of degraded, marginal lands. In general, water and soil conservation (WSC) techniques are believed to tackle the deterioration of soil quality by enhancing soil hydraulic properties, soil life, and soil organic carbon (SOC) content, but this beneficial effect on soil quality is hardly ever quantified in WSC research. This study therefore evaluates the effect of WSC on soil chemical, physical, and biological quality indicators by means of an in situ experiment, which was installed in 2011 nearby Sadore, Niger. The treatments include the following: zai + manure (Z), demi-lunes + manure, scarification + manure, control + manure, and control. WSC increases grain yields up to 0·7 ± 0·2 Mg ha−1 on degraded, marginal lands, which is similar to yields produced on fertile lands in the area. Nevertheless, a trade-off between biomass production and SOC accumulation was observed. After three growing seasons, SOC was found to rise significantly for the treatments provided with manure, from ±2·5 to ±5 g kg−1, but the increase was lowest for Z, which produced the highest yields. WSC also showed significantly elevated nematode counts but did not considerably affect other soil chemical and soil physical properties. Hence, on the short term, WSC does not improve soil quality, as was widely proclaimed. Therefore, future research should include the assessment of long-term WSC effects and the evaluation of integrated management combining WSC with alternative, yield-enhancing strategies. Copyright © 2015 John Wiley & Sons, Ltd.