Soil Organic Matter Fractions Research Articles

The pyrolytical fingerprint of nitrogen compounds reflects the content and quality of soil organic carbon

The increasing land degradation is a problem that affects many soils in countries with a Mediterranean climate. In this aspect the soil organic matter (SOM) plays an important role, due to its progressive biodegradation parallels to desertification and the concomitant emissions of CO2 to the atmosphere. These facts make basic research on the structure and composition of SOM important for soil conservation. Organic N-compounds in soil are of particular interest due to their chemical structure and speciation status in the SOM which can play an important role in soil N bioavailability and in the whole biogeochemical activity of the soil. For this reason, studying the possible relationships between the different N-compounds and soil properties, such as SOM content and its chemical characteristics, can provide new information on the stabilization and storage of organic C in soil. For this research, 30 soils from Spanish ecosystems with a wide range of SOM content were selected. The molecular composition of SOM in whole soil samples including N-compounds, was analyzed by analytical pyrolysis (Py-GC/MS). A parallel characterization of SOM quality was carried out using solid state 13C NMR and UV–vis spectroscopy. Based on their chemical structure, the N-compounds identified by Py-GC/MS were classified into seven main groups: indoles, pyridines, pyrazoles, benzonitriles, imidazoles, pyrroles and quinolines. Multivariate statistical analyses were used to explore the relationship between the distribution of the above compounds and the SOM content. A significant predictive model was obtained for the SOM using partial least squares (PLS) regression, which was used to predict SOM content using the pyrolytic N-compounds as descriptors. This would show that there is a relationship between the patterns of N-compounds and the biogeochemical mechanisms involved in the different C storage levels the soils. Also, multidimensional scaling (MDS) and principal components analysis (PCA) showed to what extent the individual N-compounds are informative of status and quality of the humic acid fraction of SOM. As a whole, the results obtained by Py-GC/MS suggest that indoles, alkylindoles, alkylbenzimidazoles and alkylpyridines could be indicators of SOM accumulation while unsubstituted benzonitrile and pyridine are related to SOM quality.

Shrub willow chips incorporated after potato harvest enhance soil properties in Prince Edward Island, Canada

Willow (Salix viminalis spp.) shrubs are being planted along riverbanks, on erodible and marginal farmland. Wood chips made from the woody biomass could improve the properties of light-textured soils with low organic matter content in potato-based systems. Willow chips were applied at 0, 20, 40, and 60 Mg ha−1 (fresh weight) as a soil amendment. Soil health parameters were evaluated after 12 months, followed by C and N contents in whole soil, particulate, and mineral-associated organic matter fractions after 18 months. Willow chip application increased soil aggregation, respiration, C and N contents in whole soil, and plant-available K.

Identifying rotation and tillage practices that maintain or enhance soil carbon and its relation to soil health

Physical fractions of soil organic matter (SOM) are established indicators of management-induced change and have been used to estimate the soil carbon storage capacity and storage potential. Here, we use SOM physical fractions and soil textures to identify management practices that maintain or enhance soil health and carbon storage in agricultural soils in Ontario. Metadata from the National Soil Database were used to estimate carbon storage potentials and calculate carbon deficits. A map was created showing carbon deficits in Ontario's agricultural soils and indicates that these soils have the potential to store an additional 0 to 2 kg m−2 in the top 20 cm of the soil. Tillage system generally had no effect on the size of the carbon deficit at four long-term agricultural experiments (Delhi, Elora, Ottawa, and Ridgetown). There was only a significant tillage effect at Ridgetown and only in the maize–soybean crop rotation, where the carbon deficit was 2.95 g C kg soil−1 under conventional tillage compared to 8.97 g C kg soil−1 with no tillage. A statistically significant effect of crop rotation was detected in Elora and Ridgetown. In Elora, continuous alfalfa had the smallest carbon deficit (7.25 g C kg soil−1) and maize–soybean rotation had the largest deficit (12.07 g C kg soil−1). In Ridgetown, the maize–soybean rotation had the smallest carbon deficit (2.95 g C kg soil−1). Regression analysis showed a weak negative relationship ( R2 = 0.11; P < 0.001) between carbon storage deficits and soil health scores. This suggests that increasing SOM levels alone may not improve soil health.

Organic matter pools in a fluvisol after 29 years under different land uses in an irrigation region in northeast Brazil

Soil organic matter (SOM) is a critical component of the soil system and a sensitive indicator of soil quality. We aimed to investigate the response of SOM pools to different land uses and management in irrigated croplands of the semi-arid region of northeast Brazil. Four sites irrigated under long-term (29 years) land uses were selected: banana (B1 and B2); elephant grass (EG) - perennial systems; maize and cowpea succession (MCP) - annual system; B1 was cropped for 17 years +2 years of maize and cowpea +17 years banana, while B2 was continuously cropped. A native forest (NF) site was selected as reference of steady state. Soil samples were collected at four depths in the rows and inter-rows of each management system. The higher decline in SOM fractions was perceived at 0–0.05 m depth and microbial indices (microbial biomass C and N, and basal respiration and metabolic quotient) were very sensitive indicators of soil quality. Perennial systems (B1 and EG) maintained the C and N stocks at higher levels equivalent to NF at the 0–0.40 m layer, while MCP and B2 had lower total organic carbon (TOC) (15 and 17%, respectively) and total nitrogen stocks (4.27 and 8.67%, respectively). Perennial cultivation with B1 and EG showed also the highest soil C and N microbial biomass and stratification ratio and, the lowest respiratory quotients, all indications of elevated soil C sequestration. In general, land use change from native forest to annual crops reduced plant residue input and increased soil disturbance, favoring higher SOM mineralization, while perennial crops and grasses were the most effective in increasing C and N stocks due to the maintenance of crop residues on the soil surface and the absence of soil tillage.

The stability of carbon from a maize-derived hydrochar as a function of fractionation and hydrothermal carbonization temperature in a Podzol

Hydrochar (HC) produced by the hydrothermal carbonization (HTC) of typically wet biomass is generally considered to be less effective for carbon (C) sequestration in soils compared to biochar (BC) by pyrolysis, due to a higher content of more easily decomposable C. Although the recalcitrance of HC is suggested to improve with increasing HTC production temperature, the way it interacts and becomes associated with soil organic matter (SOM) fractions of different stabilities against decomposition, may also influence its effectiveness for C sequestration in soils. In that respect, this study aimed to verify the potential of HCs from maize silage produced at different HTC temperatures (190, 210 and 230 °C) for C sequestration in a HC-amended sandy loam Podzol. To do this, we conducted a pot trial experiment and traced the fate of HC-derived C (HC-C) within different SOM fractions, namely the free- and occluded particulate organic matter (POMF and POMO, respectively) fractions and that comprising organic matter (OM) bound to clays (OMCl). Approx. 1 year after applying 5% of the different HTC temperature HCs to the soil, the SOM fractions were isolated by density fractionation for each HC treatment (HC190, HC210 and HC230) and the control (absent of HC). All fractions and the HCs were analyzed for organic C (OC) content and isotopic signatures (δ 13C). From the δ 13C signatures, the amount of HC-C and native soil organic carbon (SOC) within each fraction was calculated. Increased C contents and decreased H/C and O/C ratios were observed with increasing HTC production temperatures, which suggests a lower stability for the low temperature HC. After ca. 1 year, a loss of ~ 20–23% of the bulk soil TOC was found in the HC-amended soils. The POMF fraction of the HC-amended soils showed losses of 68–81% HC-C and 52–72% native SOC, which may be due to a positive priming effect caused by HC addition. The POMO and OMCl fractions of the HC-amended soils contained more OC than the control, indicating the integration of HC-C together with SOM within these more stable fractions, while the effect of HTC production temperature on the level of decomposition of the resultant HCs was negligible. In all HC treatments, the OMCl fraction comprised the least amount of HC-C, thus showing the weakest response to C amendment. In conclusion, long(er)-term research on the C net balance that accounts for the observed priming-induced TOC losses and the HC-C enrichment in more stable fractions is required to verify the potential of the different HCs for the purpose of C sequestration in soils.Graphical

Soil Organic Matter, Aggregates, and Microbial Characteristics of Intercropping Soybean under Straw Incorporation and N Input

Soil organic matter (SOM), soil aggregates, and soil microbes play key roles in agriculture soil fertility. In intercropping systems, the influences of straw incorporation and N input on the dynamics of soil physicochemical and microbial properties and their relationships are still unclear. We explore the changes in soil physicochemical and microbial properties with two straw managements, i.e., wheat straw incorporation (SI) and straw removal (SR), and four N supply rates for intercropped soybean, i.e., 60 (N60), 30 (N30), 15 (N15), and 0 (N0) kg N ha−1, in the wheat–maize–soybean relay strip intercropping systems. The results showed that SOM and SOM fractions contents, soil macroaggregate stability, and microbial and fungal α-diversity, e.g., Chao1 and Shannon indices, increased through straw incorporation and N input. The α-diversity was significantly positively correlated with soil physicochemical characteristics. Compared with SR, the relative abundance of ActinobacteriaandMortierellomycota in SI increased, but the relative abundance of Proteobacteria, Acidobacteria, and Ascomycota in SI decreased. In SI treatment, soil physicochemical characteristics and microbial diversity improved through N input, but that difference was not significant between N60 and N30. In conclusion, SI+N30 was the most effective way to maintain soil fertility and reduce the N fertilizer input in the wheat–maize–soybean relay strip intercropping.

ESTOQUES DE CARBONO DO SOLO E COMPARTIMENTOS DA MATÉRIA ORGÂNICA SOB SISTEMAS CONVENCIONAIS NO SEMIÁRIDO BRASILEIRO

ABSTRACT The objective of this study was to evaluate the effect of the conversion of secondary native vegetation (NV) to conventional systems (agriculture and pasture) in soil organic carbon (SOC) and carbon of fractions particulate organic matter (POM) and mineral-associated organic matter (MAOM) in the Brazilian semi-arid region. The study was carried out in the municipalities of Delmiro Gouveia, Inhapi and Pariconha, in Alagoas, Brazil. Soils were collected in the layers of 0-0.1, 0.1-0.2 and 0.2-0.3 m. The treatments analyzed were: agricultural crops with 4, 15 and 30 years and pasture with 10 years. As a reference, the secondary Caatinga was used. The results show that in soils with sandy texture (Neossolos Quartzarênico and Regolítico – Arenosols and Regosols, respectively), there were reductions in SOC levels and carbon in the compartment associated with minerals. The inverse can be observed in the clay-textured Argissolo (Acrisols), with 30 years of cultivation, in which there was an increase in SOC and C in the quantitative fractions of soil organic matter. In addition, despite the sandy texture of the Neossolo Regolítico, POM levels were increased in the pasture system in comparison to native vegetation, but it was not enough to recover the original SOC content of this system.

Carbon saturation deficit and litter quality drive the stabilization of litter-derived C in mineral-associated organic matter in long-term no-till soil

Long-term no-till cropping systems can induce significant differences in the mineral associated organic matter (MAOM) saturation levels but little is known on the effect of MAOM saturation on “new” C stabilization from added litter in different fractions of soil organic matter (SOM). We assessed the effect of C saturation deficit (Csd) in the MAOM on C stabilization in different SOM fractions in the surface layers of a sandy clay loam Acrisol under five no-till cropping systems adopted over 36 years in a field experiment. The cropping systems with varying C inputs led to a range of C content and Csd in the MAOM (<20 µm) in a thin soil layer (0–5 cm). In each field plot with different Csd levels, 13C-labeled litter from shoot biomass of black oat (grass) and vetch (legume) was added at a rate equivalent to 4.5 Mg ha-1C in PVC collars. After 15-month field incubation, soil was sampled and physically fractionated. Higher C stabilization in MAOM was observed for legume than grass-derived C in the top 0–2.5 cm layer, but only for soils with higher C stabilization capacity. When litter-derived C stabilization in MAOM was limited by its previous C level close to saturation, C incorporation was greater in the intra- and inter-aggregate SOM fractions. Our findings revealed that Csd and litter quality affect C stabilization in surface soil layers of no-till soils, and when C stabilization in MAOM is low due to saturation of the MAOM fraction, the C accrual occurs preferentially in labile and intra-aggregate fractions in long-term no-till soils. Therefore, sustainable management practices that promote continuous and diversified C inputs involving legume cover crops are crucial to sustain C incorporation in relatively stable forms in long-term no-till soils.

Fertilizer quality and labile soil organic matter fractions are vital for organic carbon sequestration in temperate arable soils within a long-term trial in Switzerland

Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0–20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state 13C CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (<6.3 µm) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned.

Comparison of Sample Preparation Techniques for the (−)ESI-FT-ICR-MS Analysis of Humic and Fulvic Acids

Soil organic matter (SOM) plays a key role in the global carbon and nitrogen cycles. Soil biogeochemistry is regularly studied by extracting the base-soluble fractions of SOM: acid-insoluble humic acid (HA) and acid-soluble fulvic acid (FA). Electrospray ionization-Fourier transform-ion cyclotron resonance-mass spectrometry (ESI-FT-ICR-MS) is commonly utilized for molecularly characterizing these fractions. Different sample preparation techniques exist for the analysis of HA and FA though questions remain regarding data comparability following different preparations. Comparisons of different sample preparation techniques here revealed that the negative-mode ESI-FT-ICR-MS analytical window can be skewed to detect different groups of molecules, with primary differences in oxygenation, aromaticity, and molecular weight. It was also observed that HA and FA from soils versus an aquatic matrix behaved very differently. Thus, we conclude that sample preparation techniques determined to be "most optimal" in our study are in no way universal. We recommend that future studies of HA and FA involve similar comparative studies for determining the most suitable sample preparation technique for their particular type of HA or FA matrices. This will enhance data comparability among different studies and environmental systems and ultimately allow us to better understand the complex composition of environmental matrices.

Thermal-gradient analysis of soil organic matter using an elemental analyser – A tool for qualitative characterization?

Analysing the thermal stability of soil organic matter (SOM) by heating soil material up to 900 °C and recording the time/temperature-dependent evolution of CO2, using an elemental analyser, may be an easily available tool to obtain information on the composition and stabilization of SOM. We studied four sample sets, including size fractions of mineral horizons, particulate organic matter (POM), soil aggregates, and thermally treated materials. All analyses were conducted with a thermal gradient of 70 K min−1, with i) stages of constant temperature at 400, 600 and 900 °C (‘DIN 19359′), ii) stages at 450, 600 and 900 °C (modified DIN method), or iii) continual heating. With both stage methods, the large majority of organic carbon (OC) was present in the least thermally stable fraction, thermally decomposing at T ≤400 or 450 °C, respectively. However, neither did these methods result in the same relative proportions of fractions of similar thermal stability, nor did they result in time/temperature-dependent CO2-evolution patterns characteristically reflecting the different sample sets. Thus, results of the stage-heating methods were operationally defined, as different SOM fractions in terms of composition or stabilization mechanism were combusted, but showing similar patterns. Nonetheless, most POM samples revealed less thermal stability than the other fractions with any method. Evaluation of CO2 development during continual heating provided more options, including T of maximum CO2 evolution, T50, that is, T at which 50% of CO2 has evolved, and peak deconvolution using Gauss functions. The mineral samples showed at least two maxima and variable T50 values up to 550 °C. Most POM fractions had lower T50, but not in general, possibly caused by the presence of black carbon or strongly complexing cations. The aggregate samples were mostly in between the mineral and POM samples. Irrespective of the method applied, direct unambiguous identification of samples in terms of composition or stabilization mechanisms was not possible. Nonetheless, we consider an elemental analyser a valuable tool to evaluate the thermal stability of large samples sets at relatively low cost and time effort. Particularly peak deconvolution from continual heating offers the option of quantifying OC fractions, differing in thermal stability.

Faster carbon turnover in topsoil with straw addition is less beneficial to carbon sequestration than subsoil and mixed soil

AbstractIn arable soils, the topsoil and subsoil are commonly mixed in the operation of deep tillage. It is not yet clear whether the influence of crop straw on soil organic matter (SOM) mineralization of the mixed soil (from the topsoil and subsoil) is different from that of solely the topsoil or subsoil. We conducted a 120‐d incubation experiment with the addition of 13C‐labeled maize (Zea mays L.) straw to study SOM mineralization and straw decomposition in the topsoil (0–15 cm), subsoil (15–30 cm), and their mixture (0–30 cm). Straw addition promoted SOM mineralization and thus induced a positive priming effect. The mineralization of SOM in topsoil with straw addition was 122% higher than in subsoil, and 45% higher than in mixed soil. After the 120‐d incubation, 12–16% of straw C was mineralized as CO2, contributing approximately 50–70% to the total CO2 production. Furthermore, straw C was also incorporated into SOM fractions, with 34, 7, and 59% in the light fraction, particulate organic C (coarse and fine), and mineral‐associated organic C (MAOC) in topsoil, respectively. The SOM mineralization and priming were greater in the topsoil with higher dissolved organic C and NO3–N throughout the incubation. Despite more straw C retained in MAOC in topsoil, the significant decrease in total MAOC with straw addition indicated that SOM turnover induced by fresh C application was probably faster in topsoil than the subsoil and mixed soil. Our findings indicated that both subsoil and mixed soil tended to sequester C with straw addition, whereas topsoil C turnover was faster than subsoil in the presence of straw addition.

Examining the contributions of maize shoots, roots, and manure to stable soil organic carbon pools in tropical smallholder farming soils

Continuous inputs of organic matter are vital for sustaining soil organic carbon (SOC) and productivity of soils in smallholder crop-livestock systems. However, the dynamics of the different inputs i.e. maize shoots, roots and manure used are poorly understood. Along with organic inputs, use of mineral fertilizers can alter the nutrient stoichiometry of organic matter inputs and have implications for SOC turnover. This study sought to understand how maize-based inputs and alterations to nutrient stoichiometry contribute to stable SOC pools. We hypothesized that higher quality litter (i.e., manure) contributes more than maize residues to stable SOC pools and that N, P and S additions, designed to balance the stoichiometry of inputs to reflect the stable fine fraction of soil organic matter (C:N:P:S-10,000:833:200:143) results in greater SOC stabilization. We used a 13C natural abundance approach, where the C4 maize residues were incubated for 11 months to trace C stabilization into different SOC pools within a C3 soil. Contrary to our expectations, we observed greater recovery and stabilization of shoot-derived C (2 X more than manure and 1.63 X more than roots) in the mineral-associated organic matter (MAOM) fraction. Mineral N, P and S additions reduced new C recovery in MAOM by 40 % compared to no mineral nutrient’s additions. Our study highlights the importance of residue retention as a strategy to maintain SOC and soil health in smallholder systems, and our results challenge the idea that nutrient additions increase C stabilization of added residues.

Improvements in soil properties under adaptive multipaddock grazing relative to conventional grazing

AbstractWithin managed ecosystems, such as some livestock grazed grasslands, soil physical, chemical, and biological properties may be severely compromised relative to native grasslands. Conventional grazing (CG) management, commonly referred to as continuous grazing, can affect soil properties and health by reducing soil C stocks and other available nutrients, while creating bare patches in vegetation that may enhance erosion and runoff. In contrast, adaptive multipaddock (AMP) grazing, an intensive form of rotational grazing that moves dense cattle herds quickly over the land followed by rest periods for the regrowth of plants, has been proposed as a regenerative grassland management tool that can improve soil properties such as soil C stocks, soil structure, as well as nutrient and water retention. Our research analyzed soils from 10 grasslands in the southeast United States representing either CG or AMP grazing management. We analyzed the A‐horizons of these soils for physical, chemical, and biological properties considered indicators of soil health across each management type. Chemical soil properties (e.g., cation exchange capacity [CEC], base saturation [BS], electrical conductivity [EC]) were improved where AMP grazing management was implemented. Additionally, farms using AMP grazing management had greater A‐horizon C and N stocks in bulk soils and across multiple soil organic matter (SOM) fractions. No biological indicators measured were affected by the grassland management except potential N mineralization rate, which was lower under AMP. Taken together, these results provide evidence that AMP grazing management could be implemented to regenerate several grassland soil properties across land currently under conventional grazing management.

Physical fractions of organic matter and mineralizable soil carbon as quality indicators in areas under different forms of use in the Cerrado-Pantanal Ecotone

Understanding soil organic matter (SOM) dynamics is essential to employ management that contribute to the improvement of soil quality (SQ). The aim of this study was to characterize the SOM and evaluate the emission of mineralizable C (C-CO2) in different management systems. The soil was collected in five managed areas: exposed soil (ES), conventional tillage system (CTS), no-tillage system (NTS), permanent pasture (PP) and sugarcane (SC), in addition to a forest area (NF), in the layers of 0-5, 5-10, and 10-20cm. Total organic carbon (TOC), physical-granulometric fractionation of SOM were performed, determining the contents and stocks of particulate organic matter (C-POM; StockPOM) and mineral organic matter (C-MOM; StockMOM), in addition to calculating SQ indices. In addition to C-CO2 emissions from the soil. The areas of PP and NTS presented the highest levels of TOC in the surface layer. The highest levels of C-MOM and StockMOM were observed in the PP area, besides higher CSI (carbon stock index), reaching 1.67 in the 10-20cm layer. The areas of PP and SC were similar to the NF in all layers regarding CMI (carbon management index). In CTS, there were higher peaks in emissions and accumulation of C-CO2. It is evident that the improvements in the SQ in the areas of PP, SC, and NTS caused mainly by the deposition of plant material and by soil revolving not being performed. In the CTS, high emission peaks of C-CO2 show that the lack of conservation management practices contributes to the emission of greenhouse gases.

Land-use change reduces soil nitrogen retention of both particulate and mineral-associated organic matter in a temperate grassland

Soil organic matter (SOM) fractions vary in formation and microbial activities, thus playing different roles in exogenous nitrogen (N) retention in terrestrial ecosystems. However, it remains unclear how land-use and environmental changes affect the behavior of SOM fractions in retaining exogenous N. Here, we investigated N distribution among four SOM fractions and how soil N retention capacity responds to tillage and increased snowfall. We monitored N retention in SOM fractions by adding 15NH415NO3 isotope in the field in a temperate grassland in Inner Mongolia. Our results showed that the fine mineral-associated organic matter (MOM < 20 μm) had the largest N pool with a lower mass. The free particulate organic matter (fPOM) accounted for only 0.8% of total SOM mass, representing the second-largest N pool. The coarse mineral-associated organic matter (MOM > 20 μm) represented the fewer N pool with the largest mass. MOM < 20 μm and fPOM retained >90% of the 15N tracer in soil. Deepened snow did not affect 15N retention in SOM fractions, while tillage decreased 15N retention in MOM < 20 μm, fPOM, and occluded particulate organic matter within aggregates (oPOM). We suggested that the reduction in soil total N retention under tillage conditions was mainly due to the reduced N retention in fPOM and MOM < 20 μm. Structural equation modeling analysis revealed that tillage-induced decrease in 15N retention of MOM < 20 μm was regulated by both decreased microbial 15N retention and reduced clay and silt contents. The decrease in 15N retention of fPOM was probably due to the decreased microbial 15N retention along with the increased plant 15N uptake. This research reveals divergent pathways of 15N retention among different SOM fractions in response to land-use change and provides novel insights into the estimation of soil N retention capacity with SOM fractions taken into consideration.

Impacts of nutrient addition on soil carbon and nitrogen stoichiometry and stability in globally-distributed grasslands

Global changes will modify future nutrient availability with implications for grassland biogeochemistry. Soil organic matter (SOM) is central to grasslands for both provision of nutrients and climate mitigation through carbon (C) storage. While we know that C and nitrogen (N) in SOM can be influenced by greater nutrient availability, we lack understanding of nutrient effects on C and N coupling and stability in soil. Different SOM fractions have different functional relevance and mean residence times, i.e., mineral-associated organic matter (MAOM) has a higher mean residence time than particulate organic matter (POM). By separating effects of nutrient supply on the different SOM fractions, we can better evaluate changes in soil C and N coupling and stability and associated mechanisms. To this end, we studied responses of C and N ratios and distributions across POM and MAOM to 6–10 years of N, phosphorus (P), potassium and micronutrients (K+µ), and combined NPK+µ additions at 11 grassland sites spanning 3 continents and globally relevant environmental gradients in climate, plant growth, soil texture, and nutrient availability. We found addition of N and NPK+µ generally reduced C:N in MAOM and POM. However, at low fertility and at warm, sandy sites, nutrient addition promoted higher MAOM and POM C:N, respectively. Addition of NPK+µ also promoted C storage in POM relative to MAOM, and this was consistent across sites. Our results suggest that addition of macro- and micronutrients consistently decrease SOM stabilization, whereas responses of soil C:N stoichiometry were contingent on SOM fraction and environmental conditions.

Органическое вещество и агрегатное состояние чернозема выщелоченного и его полугидроморфного аналога

A comparative study of indicators of the state of organic matter and the structural state of two arable soils of the Tambov region - leached chernozem (Voronic Chernozems Pachic) and meadow-chernozem soil (Voronic Chernozems Pachic), cultivated with the same agricultural technology in one field, was carried out. The meadow chernozem soil was characterized by a higher content of humus and humic acids in its composition compared to the automorphic analog, apparently due to additional soil moisture and an increased supply of post-harvest residues. This is confirmed by the fact that the content of easily degradable organic matter (light fraction) and the level of its humification in meadow-chernozem soil were significantly higher than in leached chernozem. The degradation level of the meadow-chernozem soil, calculated by the ratio of the content of the light fraction was lower to the total content of humus, was lower, and the indicators of the aggregate and microaggregate state were more favorable than those of the leached chernozem. Keywords: SOIL ORGANIC MATTER, LIGHT FRACTION OF SOIL ORGANIC MATTER, AGGREGATE COMPOSITION OF SOILS, MICROAGGREGATE COMPOSITION OF SOILS

Carbon pathways in aggregates and density fractions in Mollisols under water and straw management: Evidence from 13C natural abundance

Mollisols have a high potential to mitigate climate change and play an important role in the global carbon (C) cycle due to their inherently high soil organic matter (SOM). However, little is known about the mechanism of C stabilization in Mollisols, especially subjected to different management effects. To trace C stabilization between aggregates and SOM density fractions in Mollisols, soil samples were collected from different experimental plots: controlled irrigation + rice (Oryza sativa L.) straw removal (CI), flooded irrigation + rice straw removal (FI), controlled irrigation + rice straw return (CI-SR), and flooded irrigation + rice straw return (FI-SR). Each soil sample was separated into three aggregate size classes (>250 μm, 53–250 μm, and <53 μm), then each class individually subjected to density fractionation to obtain free and occluded light fractions (fLF and oLF), as well as dense and mineral-heavy fractions (DF and MF). The overall C content and 13C abundance of fractions were measured to interpret C transfer and accumulation among aggregate and density fractions. The highest increase in the soil organic C (9.27–24.88%) was observed in CI-SR compared with the other treatments. Irrigation and straw return primarily affected C accumulation within macroaggregates and the MF, which were the dominant forms in the aggregates and density fractions, respectively. An enrichment in δ13C was found from macroaggregates to silt + clay, and from light to heavy fractions, indicating that Mollisols macroaggregates and light fractions acted as the source or initial store of plant residues, and that the C in the silt + clay class and heaviest fraction contained more microbially-transformed C than did the macroaggregates and light fractions. A detailed scheme of C transfer within aggregates and SOM fractions based on the δ13C natural abundance revealed the following general sequence: free light → occluded light → dense → mineral fractions, concurring with results reported at upland sites. There is a greater probability of C transfer between SOM density fractions under CI. This contrasted with the results that CI decreases the possibility of C exchange between aggregates than FI, which indicates differences in the C stabilization processes between no-flooded and flooded conditions of Mollisols. In addition, straw returning can reduce the possibility of C exchange both between and within aggregates, which plays a significant role in maintaining the stability of the Mollisols carbon cycle. The present study provides further detailed insights into the C stabilizing mechanisms in Mollisols which depend on management (straw, water status). This finding is conducive to the sustainable use and management of Mollisols toward maintaining or increasing C stock and in realizing the objective of C-neutral agriculture.

Latitudinal patterns of light and heavy organic matter fractions in arid and semi-arid soils

Semi-arid and arid ecosystems are important for the global C cycle. Despite this, it remains unclear how organic matter fractions vary across latitudinal gradients, and what drives this variation, in dry ecosystems. In this study, we sampled soils from 100 sites across a latitudinal gradient in the dry valleys of southwestern China to explore the latitudinal patterns of light fraction organic matter (LFOM) and heavy fraction organic matter (HFOM) at two soil depths (0–10 cm and 10–20 cm). Across the studied gradient, HFOM accounted for a larger fraction of soil organic matter than LFOM. LFOM increased exponentially with increasing latitude at both 0–10 cm and 10–20 cm depths. Heavy fraction organic C increased linearly with increasing latitude at both depths, while heavy fraction organic N only increased with latitude in soils from 10 to 20 cm depth. Latitudinal patterns of LFOM were mainly explained by climate, with the most important driver being mean annual temperature, followed by mean annual precipitation. Soil physicochemical factors – in particular cation exchange capacity and silt content – explained the most variation in HFOM. Total microbial biomass was also important in explaining variation in HFOM, especially in the 10–20 cm soil layer. Overall, our results shed light on the spatial distribution of organic matter fractions in arid and semi-arid regions. We also identify candidate drivers of the variation in LFOM and HFOM in arid and semi-arid regions, finding that climate primarily explains variation in LFOM while soil physiochemistry primarily explains variation in HFOM.