Soil Organic Matter Fractions Research Articles

Effect of pH, surface charge and soil properties on the solid–solution partitioning of perfluoroalkyl substances (PFASs) in a wide range of temperate soils

The pH-dependent soil–water partitioning of six perfluoroalkyl substances (PFASs) of environmental concern (PFOA, PFDA, PFUnDA, PFHxS, PFOS and FOSA), was investigated for 11 temperate mineral soils and related to soil properties such as organic carbon content (0.2–3%), concentrations of Fe and Al (hydr)oxides, and texture. PFAS sorption was positively related to the perfluorocarbon chain length of the molecule, and inversely related to solution pH for all substances. The negative slope between log Kd and pH became steeper with increasing perfluorocarbon chain length of the PFAS (r2 = 0.75, p ≤ 0.05). Organic carbon (OC) alone was a poor predictor of the partitioning for all PFASs, except for FOSA (r2 = 0.71), and the OC-normalized PFAS partitioning, as derived from organic soil materials, underestimated PFAS sorption to the soils. Multiple linear regression suggested sorption contributions (p ≤ 0.05) from OC for perfluorooctane sulfonate (PFOS) and FOSA, and Fe/Al (hydr)oxides for PFOS, FOSA, and perfluorodecanoate (PFDA). FOSA was the only substance under study for which there was a statistically significant correlation between its binding and soil texture (silt + clay). To predict PFAS sorption, the surface net charge of the soil organic matter fraction of all soils was calculated using the Stockholm Humic Model. When calibrated against charge-dependent PFAS sorption to a peat (Oe) material, the derived model significantly underestimated the measured Kd values for 10 out of 11 soils. To conclude, additional sorbents, possibly including silicate minerals, contribute to the binding of PFASs in soil. More research is needed to develop geochemical models that can accurately predict PFAS sorption in soils.

Soil properties and crop yields as influenced by the frequency of straw incorporation in rape-wheat-triticale rotation

Straw, particularly cereal straw, is a valuable by-product of crop production, which can be used for various purposes, e.g. as livestock feed and bedding or for making fuels, however it should primarily be retained on farmland in order to prevent soil organic matter (SOM) losses and thus to maintain or improve soil quality. The aim of this study was to analyze effects of the frequency of crop residues (straw) incorporation into the soil on the content of soil organic matter and on crop yields. There were the following experimental treatments: SR – straw of all crop in the rotation removed, S1 – straw of one crop per rotation incorporated, S2 – straw of two crops in the rotation incorporated, and S3 – straw of three crops incorporated into the soil (loamy sand). After 21 years of crop rotation with straw removal (SR) the SOM level in the soil slightly decreased to 14.4 g∙kg –1 soil DM, compared to that in 1997 (14.6 g∙kg –1). However, when straw of one crop (rape) per rotation was incorporated (S1) the content of SOM increased to 15.0 g∙kg -1 soil DM, and to 15.6 and 16.0 g∙kg –1 in S2 and S3 treatments respectively. Straw retention had also a beneficial effect on the content of labile fractions of SOM (hot water extractable C and N). Grain yields and yield components of wheat and triticale, and seed yields of rape in the SR treatment were not significantly different from those obtained in S1, S2 and S3 treatments.

Effect of cover cropping on soil organic matter characteristics: Insights from a five-year field experiment in Nebraska

Cover crops are increasingly incorporated into crop rotations to improve soil health including the postulated positive impact on soil organic carbon (SOC) storage. However, a consistent positive response of SOC to cover cropping was not observed in several studies, necessitating further clarification of mechanisms by which cover crops impact soil organic matter (SOM) dynamics. In this study, we evaluated the effect of five years of winter cover cropping within a corn-soybean rotation in Nebraska under no-till on SOC and nitrogen (N) storage in the bulk topsoil (0–10 cm depth) as well as in particulate (free and occluded), water-extractable, and mineral-associated organic matter fractions. Furthermore, cover crop effects on potential C and N losses from the topsoil in form of CO2 and N2O emissions and leached organic C and N were evaluated. The different SOM fractions were analyzed for their functional group composition using diffused reflectance infrared Fourier transform spectroscopy to account for potential changes in the SOM composition due to cover crops. Our results indicated no effect of cover cropping on C and N losses (gaseous and leaching) and on bulk SOC and total N storage five years after implementation. Differences were detected for the SOM fractions, with a positive effect of cover crops on the amount of water-extractable organic matter. The spectroscopic results suggested a significant cover crop effect on the composition of free particulate organic matter fraction. Both fractions seem to be suitable for detecting early changes in SOM characteristics due to cover cropping under the studied conditions. A significantly measurable impact on bulk SOC and N storage in 0–10 cm depth for the studied corn-soybean systems might require more than five years of cover cropping, or management changes that target an increase in cover crop biomass production and input into the soil.

Biochemical composition of soil organic matter physical fractions under 32-year fertilization in Ferralic Cambisol

Biochemical properties of soil organic matter (SOM) are fundamental for soil fertility and health. However, it is unclear how fertilization regime influences the biochemical compositions and oxidation states of SOM and physical fractions. In this study, this issue was studied under four 32-year amendment regimes: unfertilized control, urea (N), N + calcium dihydrogen phosphate + potassium chloride (NPK), and NPK plus manure (NPKM). Three physical fractions: coarse particulate (> 250 μm, cPOM), fine particulate (53–250 μm, fPOM) and mineral-associated OM (< 53 μm, MAOM) were separated and measured by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). Compared with the background in 1986, the SOM increased by 10.6%, 14.2%, 23% and 52% in unfertilized control, N, NPK, and NPKM, respectively. The red soil here had not reached carbon saturation, because of the low conversion efficiency (6.8%) from input-carbon to soil organic carbon (SOC). Physical size but not amendment type primarily regulated the SOM molecular composition, with relative selective retention of aromatics and lignin in both the cPOM and fPOM, whereas N-containing compounds (particularly amino-N) were enriched in MAOM due to their high abilities to adsorb soil minerals. The C oxidation state was also mainly dependent on physical size, with the highest value in fPOM. The sources of SOM and its fractions, dominated by microbial-derived compounds (60–90%), were independent of physical size and fertilization. In conclusion, physical size arrangement (proxy of microbial decomposition degree) played a more important role in regulating the SOM biochemical features than initial quality of various amendments.Graphical

Soil organic matter fractions under different land uses and soil classes in the Brazilian semi-arid region

Context Land use change (LUC) is considered one of the main factors associated with soil carbon (C) loss worldwide. Aim Evaluate changes in labile and non-labile soil organic matter (SOM) fractions in different land use systems – native vegetation (NV), agriculture with conventional cultivation (CC) and pasture (PA) – in five soil classes in the Brazilian semi-arid region. Methods Soil samples were collected to a depth of 100 cm, and soil C and nitrogen (N) content and stocks were determined and also stocks of labile (LC) and non-labile (NlC) C fractions. In addition, the Carbon Management Index (CMI) was used to evaluate soil health changes. Key results SOC stocks significantly decreased after conversion from NV to CC (by 23%; 0–30 cm) and PA (by 22%; 0–100 cm). Losses due to LUC were greater in the LC than the NlC fraction, and this was reflected in the CMI reducing by 37% for PA and 57% for CC in the 0–100 cm layer. Regarding the different soil classes, LUC reduced SOC stocks only in the Luvisol, Planosol and Leptosol classes, while in the LC fraction, changes were observed in Acrisols, Cambisols and Planosols. Conclusions Clearing NV areas for CC systems and PA reduced the SOC stocks and SOM fractions and, consequently, decreased soil quality. Implications These findings underscore the importance of considering the quality of SOM when evaluating LUC impacts on SOC stocks in the different classes of soil in the Brazilian semi-arid region.

Spatial variability of soil organic carbon fractions in areas under cultivation of Amazonian species in the southern region of Amazonas state, Brazil

Soil organic carbon (OC) is heterogeneous and sensitive to agricultural management, so knowledge of its spatial variability can improve the monitoring of areas under anthropogenic influence, as OC can serve as a sensitive indicator of changes in the environment. The objective of this study was to evaluate the spatial variability of soil OC fractions in areas of cultivation with Amazonian species in the southern region of Amazonas state. A total of 256 georeferenced data points were collected in the 0.0–0.05 m and 0.05–0.10 m layers in the following agricultural systems: areas with cultivation of Guaraná, Annatto, Cupuaçu and forest. The OC contents were analyzed, and the chemical fractionation of soil organic matter was performed. The analytical results were evaluated through descriptive statistical analysis, and the spatial pattern was evaluated through geostatistical analysis. The conversion of natural ecosystems to agricultural systems affected the rates of addition and decomposition of soil organic matter. Changes in soil organic carbon stocks (SOC stock) due to the uses of different agricultural systems were determined by evaluating the free light fraction of soil organic matter. For the chemical fractions of organic matter, there was a predominance of the humin fraction (C-HU) in relation to the fractions of humic acid (C-FAH), fulvic acid (C-FAF) and OC associated with minerals (COAM) in the different land uses and soil layers analyzed. The geostatistical procedures proved to be important in determining the degree of carbon dependence and its fractionation in the context of spatial variability, and this information is useful in soil quality monitoring.

Deadwood affects the soil organic matter fractions and enzyme activity of soils in altitude gradient of temperate forests

The main objective of our study has been to determine the role of deadwood in the shaping of the amount of soil organic matter fractions in mountain forest soils. For this purpose, a climosequence approach comprising north (N) and south (S) exposure along the altitudinal gradient (600, 800, 1000 and 1200 ​m a.s.l.) was set up. By comparing the properties of decomposing deadwood and those of the soils located directly beneath the decaying wood we drew conclusions about the role of deadwood in the shaping of soil organic matter fractions and soil carbon storage in different climate conditions. The basic properties, enzymatic activity and fractions of soil organic matter (SOM) were determined in deadwood and affected directly by the components released from decaying wood. Heavily decomposed deadwood impacts soil organic matter stabilization more strongly than the less decayed deadwood and the light fraction of SOM is more sensitive to deadwood effects than the heavy fraction regardless of the location in the altitude gradient. Increase in SOM mineral-associated fraction C content is more pronounced in soils under the influence of deadwood located in lower locations of warmer exposure. Nutrients released from decaying wood stimulate the enzymatic activity of soils that are within the range of deadwood influence.

Превращения урацила , глицина и глюкозы, поступающих в составе опада в лесную подстилку дерново-подзолистой почвы

In model experiments under forest ecosystem conditions, samples of partially decomposed forest litter and soil from a 0–10 cm layer containing 14C-labeled organic matter were selected. The label was introduced in the form of low molecular weight water-soluble organic substances of individual nature – glycine, uracil and glucose together with unlabelled plant litter in nylon bags. The experiment was carried out in the late autumn period on the territory of the Malinsky forestry, 20 km south-west of Moscow, on a medium loamy sod-podzolic soil. For the study, samples were taken from two selection periods – 40 days after labelling and after two years. The selected samples were extracted and separated into soil organic matter fractions containing 14C in their composition, followed by radiometry. The results showed differences in the transformation and incorporation of radiocarbon of the studied products – glycine, uracil, and glucose into different fractions of soil organic matter of low molecular weight water-soluble substances, depending on their nature.

Pores distribution influences the soil microorganism's response to changes in temperature and moisture

Microorganisms are an essential fraction of soil organic matter, which presence and activity depend directly on soil physical conditions. This study aimed to address the effect of soil temperature and moisture under contrasting macroporosity conditions on soil biological properties. Soil physical-chemical characterization implicated the collection of composite samples and undisturbed surface soil samples (0 to 10 cm). Also, samples of undisturbed surface soil were extracted in 40 polyvinyl chloride cylinders of 18 cm diameter and 20 cm height for the arrangement of soil mesocosm as the experimental units of a completely randomized experiment with a 2x2x3 factorial arrangement. The experiment duration was 21 days, and the soil biological properties measured were microbial biomass (MB) and soil respiration (SR). Macroporosity showed a significant effect on MB, which indicates that aeration pore influences the number of microorganisms in the soil; for the SR, the macroporosity had a not significant effect. The temperature at the ranges established in the experiment did not significantly affect MB, whereas a highly significant effect of temperature over SR was observed. A highly significant effect of soil moisture was observed on MB and SR. Macroporosity, moisture, and temperature are determining factors in the presence of soil microorganisms, both directly and through the interaction between them. Herein the microorganisms have a wide range of thermal adaptation, and the effect of soil temperature can boost soil microorganisms. In turn, it was observed that the microorganisms present are significantly sensitive to the moisture deficit in soil.

Impact of deforestation on the soil physical and chemical attributes, and humic fraction of organic matter in dry environments in Brazil

Deforestation of Caatinga and inadequate land use of these dry environments have impacted soil quality in Northeastern Brazil. The objectives of this study were: (a) to evaluate the effect of deforestation and different agricultural uses on the physical and chemical properties of soil, and humic fractions of soil organic matter in dry environments; and (b) to detect the soil properties that were most affected by anthropic actions. We evaluated four dry areas in Chapada do Araripe, NE Brazil: preserved native vegetation; degraded native vegetation; cassava conventional cultivation; and eucalyptus agro-energy cultivation. Soil fertility, total organic carbon and humic fractions of soil organic matter were lower in the degraded native vegetation area. The best indicators for soil quality evaluation were: macroporosity; bulk density; soil resistance penetration; sum of bases (mainly Ca2+); available P; and saturation by Al3+. Total organic carbon and humic acid fractions of soil organic matter were important in improving soil quality. These properties were influenced by deforestation and agricultural uses, suggesting that the deforestation of native vegetation in dry environments has high capacity to degrade the soil, preventing its regeneration.

Soil aggregation and associated organic matter under management systems in sandy-textured soils, subtropical region of Brazil.

Increasing the diversity of plant species in agricultural production areas favors the maintenance or improvement of soil quality, particularly for soils with a sandy texture. This beneficial effect is related to the formation of aggregates of different origins. This study aimed to (i) verify whether soil use and management affect the proportion of biogenic (Bio) and physicogenic (Phy) aggregates and (ii) verify whether biogenic aggregation is more likely to lead to soil improvement than physicogenic aggregation. Three management systems were evaluated (permanent pasture, PP; no-tillage system, NT; and no-tillage + Brachiaria system, NT + B) as well as a reference area (Atlantic Forest biome vegetation, NF). According to their origin or formation pathway, the aggregates were separated, identified, and classified as Bio (formed by biological processes) and Phy (resulting from chemical and physical actions). The differentiation between Bio and Phy aggregates was performed based on the visualization of morphological features, such as shape, size, presence of roots, porosity, and subunit arrangements, and junctions. Only the PP area was able to promote greater aggregate formation of biological origin, with greater amounts of Bio aggregates. The highest total organic carbon (TOC) contents and the least negative δ13C values were also quantified in the aggregates of the PP area. The NT + B system provided an increase in the TOC content of its aggregates in comparison with aggregates in the NT and NF areas. Among the formation pathways, the Bio aggregates had the highest TOC and soil organic matter fractions contents and the most negative δ13C values. Perennial forage grasses vegetation was more important than the plant species diversity in favoring Bio aggregate formation. The beneficial effect of Brachiaria can be observed when incorporated as part of intercropping with corn in grain production systems. The biogenic aggregates favored the concentration of more labile soil organic matter fractions. The results of this study can provide important theoretical information for future studies focused on the combination of different plant species in agricultural food production areas on sandy-textured soils.

Soil Strength and Structural Stability Are Mediated by Soil Organic Matter Composition in Agricultural Expansion Areas of the Brazilian Cerrado Biome

A growing demand for resources has led to the expansion of agricultural areas worldwide. However, land conversion associated with poor soil management might lead to soil physical degradation. We investigated the effects of land conversion on soil physical properties in the Brazilian Cerrado region, under native Cerrado vegetation (NV)—pasture (PA) and NV—cropland (CL) conversion scenarios. Soil physical properties related to compaction, pore size distribution, and structure stability were assessed up to a 30 cm depth. Additionally, carbon levels of soil organic matter fractions (particulate and mineral-associated organic matter) were determined. Our results indicate that the compaction process equivalently reduced the soil porosity in PA and CL. However, soil penetration resistance was higher in PA (~2.5 MPa) than in CL (~1.5 MPa), as well as the stable mean weight diameter of soil aggregates. The highest total and labile organic carbon levels were observed in CL, while the lowest levels of total and labile organic carbon occurred in PA (smaller than in CL). These results suggest that the higher structural stability found in PA was mediated by the predominance of stabilized carbon (a decrease in the proportion of soil labile carbon), causing the gaining of soil strength under negligible soil volume variation (in comparison with CL). Our results suggest that the reduction in the soil porosity by compaction due to PA and CL uses can equivalently reduce macropore space and soil hydraulic functioning, and that soil carbon quality alterations (i.e., labile vs. stabilized fractions) are responsible for the gain in soil strength in long-term degraded PA areas. Future research should focus on understanding the magnitude in which soil organic matter controls soil physical attributes, such as soil strength in these expansion areas, and whether this gain in soil strength limits plant development and compromises productivity in the long term.

Variations in the concentration, composition, and sources of WEOM in erosion and deposition landscapes over an erosion intensity gradient on the Loess Plateau of China

Water-extractable organic matter (WEOM) is the most active and mobile fraction of soil organic matter and responds sensitively to soil erosion. However, previous studies on the chemical nature of WEOM in the erosion and deposition landscapes were controversial at site-level studies and mainly focused on surface soils, limiting our assessment of WEOM’s environmental fate at larger and deeper scales. Here, we collected eroded soils and deposited sediments from a depth of 0–200 cm in five eroding catchments with an erosion intensity gradient across the Loess Plateau. We examined the concentration, composition, and potential sources of WEOM using UV–visible absorption and fluorescence excitation-emission matrix-parallel factor analysis. Soil WEOM quantity and quality varied significantly in the erosion and deposition landscapes, with greater variations in soils below 20 cm, where WEOM in the deposition zone was characterized by higher concentration, aromaticity, hydrophobicity, and UVA and UVC humic-like components derived from terrestrial and soil organic matter. WEOM variations in deep soils increased with increasing erosion modulus among sites, and were mainly driven by changes in soil organic carbon concentration, aggregate stability, and carbon to nitrogen ratio. However, the pattern of WEOM parameters between erosion and deposition zones at 0–20 cm was opposite to that in soils below 20 cm. Vegetation inputs compensated the loss of organic matter in sites with suitable climatic status and weak erosion, and led to higher concentrations and exogenous features of WEOM in the surface erosion area. Concludingly, the WEOM concentration, components, and sources differed significantly between erosion and deposition zones, and these differences, as well as their response to erosion intensity, revealed an opposing pattern in surface and deep soils.

A rapid and sensitive assay to quantify amino sugars, neutral sugars and uronic acid necromass biomarkers using pre-column derivatization, ultra-high-performance liquid chromatography and high-resolution mass spectrometry

Microbial necromass comprises a large fraction of soil organic matter (SOM) due to the accumulation and stabilization of microbial residues from dead archaea, bacteria and fungi. Amino sugars, neutral sugars and uronic acids have been used as microbial necromass biomarkers to trace the origin and composition of microbial residues in the SOM pool. Due to the structural complexity of sugars, derivatization reactions and high-throughput analytical methods are required to separate and quantify these sugar-related compounds. Our aim was to develop a rapid and sensitive assay to measure amino sugar, neutral sugar and uronic acid compounds using pre-column 1-phenyl-3-methyl-5-pyrazolone (PMP) derivatization. PMP-derivatives were separated and quantified via reversed phase (RP) ultra-high-performance liquid chromatography (UPLC) coupled to high-resolution Orbitrap mass spectrometry (MS). The method was validated and applied on hydrolyzed peptidoglycans and the biomass of archaeal, bacterial, fungal and plant species, as well as with soils. This developed PMP method allowed the separation and quantification of 18 sugar-related compounds, including four amino sugars, three N-acetyl amino sugars, eight neutral sugars, and three uronic acids within 20 min. This PMP method showed a precision for isotope enrichment detection of 0.03–0.05 atom % 13C for D-glucose and D-glucosamine. This is the first time talosaminuronic acid (deriving from archaeal pseudopeptidoglycan) was identified and quantified using PMP derivatization. The application of this novel PMP method on pure hydrolyzed biomass and soils, showed the successful chromatographic and mass spectrometric separation and quantification of amino sugar, neutral sugar and uronic acid compounds. A multivariate analysis using these sugar-related PMP derivatives showed a clustering of the species according to their respective taxonomic group (archaea, gram-positive bacteria, gram-negative bacteria, fungi and plants). The modified PMP method can be applied to identify and quantify soil microbial necromass biomarkers, as well as their contribution to SOM. The sensitive isotope tracer detection allows tracing isotopically labeled materials into necromass biomarkers in SOM pools.

Agricultural practices influence biological soil quality indicators in an irrigated semiarid agro-ecosystem

A study was conducted to investigate how soil biological measurements can be used to indicate differences in soil quality in arid and semiarid agroecosystems. The objectives were to assess soil biological quality indicators under different land management practices and at different depths and to explore the relationships between soil microbial community and organic matter fractions. Soil samples were collected from three replicates of six land management systems at 0–5 cm and 5–15 cm soil depths. Four management systems had medium to fine soil texture including a permanent grass field with tall fescue (Lolium arundinaceum [Schreb]; TFC), peach orchard with clover understory (Prunus persica with Trifolium repens L.; PCC), alfalfa field (Medicago sativa; ALF) and conventionally tilled soil with an annual crop rotation (CTC). Two management systems, including a young cottonwood tree orchard (Populus fremontii and Populus deltoides; CWS) and conventionally tilled soil in annual crop rotation (CTS), had coarse soil texture. Management systems had significant impacts on most of the soil biological indicators assessed at the two depths with both depths responding similarly to management practices. For the soil biological indicators that showed differences, the TFC generally had the most favorable indicator measurements compared to other management systems. The principal component analysis showed a clear separation between the management systems and microbial communities at both soil depths. The ordination pattern was structured by soil textural differences between medium to fine-textured soils from TFC, PCC, ALF, and CTC fields and coarse-textured soils (CWS, CTS). Spearman’s correlation coefficients (rs) showed that the soil organic matter (rs = 0.85, P = 0.001) and the permanganate oxidizable carbon (rs = 0.93, P = 0.001) were significantly correlated with the total microbial biomass at 0–5 cm soil depth. Also, the soil organic matter (rs = 0.84, P = 0.001) was significantly correlated with the total microbial biomass at 5–15 cm soil depth. This study suggests that in semiarid agroecosystems, management practices with permanent covers have the potential to improve soil quality, and soil texture may also influence the directional changes in soil quality within this region.

Biological quality and organic matter dynamics in minesoil at 18 years reclamation with perennial grasses

Mining is one of the most destructive economic activities on natural ecosystems. There are around 750 ha impacted by open-pit coal mining in Candiota Mine, the largest in Brazil, with 1.2 billion tons subject to mining. In soils impacted by mining, litter decomposition, always linked to a more extended period of restoration and maintenance of the vegetation, presents positive correlations with the diversity of the edaphic fauna and the enzymatic activity of these areas, promoting improvements in the ecological functions of the new ecosystem. The objective of the present study was to analyze the impact of revegetation with different summer perennial grasses subjected to different mowing managements on the biological and chemical quality of soils after coal mining in an experimental area at the Candiota Mine. Analyzes were performed to identify edaphic organisms using modified Berlese-Tüllgren funnels and microbiological analyzes to quantify Microbial Biomass Carbon (MBC), Microbial Respiration (MR), and urease enzymatic activity. The different fractions of Soil Organic Matter (fulvic, humic acids, and humin) were also quantified to understand the degree of humification in the area. All results were compared in mean tests by Tuckey's test (p < 0.05) and by Principal Component Analysis. After 18 years of revegetation and 11.6 years of mowing management in the area, there was an improvement in MBC and MR compared to the Control, evidencing the positive effect of the perennial grasses used in the experimental area. However, mowing management cannot yet be considered the most promising management for improving edaphic conditions.

Soil organic matter fractions and carbon distribution under different management in Lesotho, southern Africa

AbstractSoil organic matter (SOM) is a complex mixture of multiple fractions of soil organic C (SOC) that can be influenced by management decisions. This study evaluate the effects of tillage (moldboard plow [MP] and no‐tillage [NT]) and commercial fertilizer as limestone ammonium nitrate (28–0–0) at three rates (0 [0‐N], 100 [100‐N], and 200 [200‐N] kg N ha–1) on SOC, soil total N (STN), and SOM fractions (total particulate organic matter‐C [POM‐C] and mineral‐associated organic matter‐C [MAOM‐C]). The study was established in 2008 on the National University of Lesotho Campus Farm, Roma Valley of the Maseru District in Lesotho, southern Africa. The soil classified as Berea series. Soil samples were collected from 0‐to‐5‐, 5‐to‐10‐, 10‐to‐15‐, and 15‐to‐30‐cm depths. Under NT, the SOC, STN, and POM at 0‐to‐15‐cm were 54, 47, and 40% higher than 15‐to‐30‐cm depth, respectively. Differently, 0‐to‐15‐cm the MP had 17, 17, 35% higher SOC, STN, and POM than 15‐to‐30‐cm depth. The SOC in the 0‐to‐15‐cm increased by 27% with MP and by 36% with NT at the 200‐N compared with the 100‐N rate. The highest N‐rate (200‐N) increased total POM by 28.8% for MP and 22.6% for NT than the100‐N rate. The greater increase in POM under MP with high N rate was probably related to the low initial total POM, where small changes in POM will be pronounced, whereas the high total POM in NT could mask the effect the high N rate addition. The coarse and fine POM accounted for 39 and 61%, respectively, of the changes in total POM at both NT and MP. The high percentage of SOC was observed within fine POM fraction (48.4%) in MP, which makes it susceptible to wind erosion, and within MAOM fraction (48.9%) in NT practice. The C/N ratio was highest with coarse POM (C/N ≈ 25) and lowest with MAOM (C/N ≈ 7.2) at both tillage practices. The low C/N ratio made MAOMC vulnerable to microbial decomposition. These findings suggest the need for conservation efforts where conservation tillage and N rates could be used to reduce SOC and POM losses and contribute to improve land sustainability and SOM conservation.

Seeing the forest for the fractions – Comparing soil organic matter fractionation methods using molecular features after forest stand conversion

The molecular composition of soil organic matter (SOM) contains key information on the persistence of soil carbon (C) in relation to changes in vegetation and environmental factors. Depending on the ecosystem, analytical method and specific objectives, many SOM fractions and numerous fractionation schemes have been proposed to study soil C. However, the molecular composition and environmental significance of those different SOM fractions have not yet been compared systematically. We use a reverse fit approach to fill this knowledge gap: i.e. we chose a study area with a well-known land history to assess which information is stored in the most frequently analysed SOM fractions. The Gaume forest (Belgium) is an ancient deciduous forest (>200 years) in which small stands were converted to Norway spruce (Picea abies) 40–50 years ago. Those stands are located along a lithological gradient in soil buffering capacity. We investigated the molecular composition of bulk mineral soil samples and five organic SOM fractions by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). The SOM fractions included particulate OM (POM), water extractable OM (WEOM), and fractions obtained by alkaline extraction, including the traditionally used humic acid (HA) and fulvic acid (FA) and the total base extract from which they are obtained (BE). Our results indicate that pyrolysates of bulk mineral soil did not prove useful to reflect environmental changes after forest stand conversion. Principal Component Analysis indicated that within each organic fraction similar changes occurred when comparing soil depths, degree of SOM decomposition, litter inputs, and soil buffering capacity. However, only the HA and BE appeared successful in capturing all these processes: the degree of SOM decomposition was not expressed in pyrolysates from the POM, differences in litter input between forest stand types were not evidenced in the WEOM, and effects of buffering capacity were not demonstrated in the WEOM and FA fractions. Thus, the molecular composition of different SOM fractions can be used complementary to each other to study environmental and ecological effects of forest stand conversion on soil C dynamics.

Assessing Soil Organic Carbon Stocks and Particle-Size Fractions across Cropping Systems in the Kiti Sub-Watershed in Central Benin

Soil organic carbon storage in agricultural soil constitutes a crucial potential for sustainable agricultural productivity and climate change mitigation. This paper aimed at assessing soil organic carbon stock and its distribution in three particle size fractions across five cropping systems located in Kiti sub-watershed in Benin. Soil samples were collected using a grid sampling method on four soil depth layers: 0–10, 10–20, 20–30 and 30–40 cm in five cropping systems maize–cotton relay cropping (MCRC), yam–maize intercropping (YMI), teak plantation (TP), 5-year fallow (5YF) and above 10-year fallow (Ab10YF) from July to August 2017. Soil organic carbon stock (C stock) was estimated for the different soil layers and particle-size fractionation of soil organic matter was performed considering three fractions. The fractions coarse particulate organic matter (cPOM: 250–2000 µm), fine particulate organic matter (fPOM: 53–250 µm) and non-particulate organic matter (NOM: &lt;53 µm) were separated from two soil depth layers: 0–10 and 10–20 cm. The results showed that fallow lands Ab10YF and 5YF exhibited the highest C stock, 22.20 and 17.74 Mg C·ha−1, while cultivated land under tillage MCRC depicted the lowest, C stock 11.48 Mg C·ha−1. The three organic carbon fractions showed a significant variation across the cropping systems with the NOM fraction holding the largest contribution to total soil organic carbon for all the cropping systems, ranging between 3.40 and 7.99 g/kg. The cPOM and fPOM were the most influenced by cropping systems with the highest concentration observed in Ab10YF and 5YF. The findings provide insights for upscaling farm management practices towards sustainable agricultural systems with substantial potential for carbon sequestration and climate change mitigation.

Soil Carbon and Nitrogen Stocks under Agrosilvopastoral Systems with Different Arrangements in a Transition Area between Cerrado and Caatinga Biomes in Brazil

Production systems that promote the accumulation of soil organic matter (SOM) must be implemented to maintain the sustainability of agriculture, livestock, and forestry. Since increases in MOS content contribute to improving the chemical, physical, and biological quality of the soil, as well as helping to reduce carbon emissions to mitigate climate change. Therefore, the objective of this study was to evaluate soil organic carbon (SOC) and nitrogen (N) stocks after the implementation of agrosilvopastoral (ASP) systems in a Cerrado-Caatinga transition zone in Brazil. Native vegetation of Cerrado-Caatinga (NV), regenerating stratum of Cerrado-Caatinga (RS), two arrangements of ASP systems cultivating Cenchrus ciliaris L. intercropped with Eucalyptus camaldulensis Dehnh. × Eucalyptus tereticornis Sm. hybrid (ASP1 and ASP2), and intercropped with Eucalyptus urophylla S.T. Blake × Eucalyptus grandis W. Mill ex Maiden hybrid (ASP3 and ASP4) were evaluated. Soil C and N stocks and the C content in the humic fractions of SOM were evaluated at 0–10, 10–20, and 20–30 cm soil depths. The introduction of ASP2, ASP3, and ASP4 systems in an area previously occupied by low productivity pasture increased and restored SOC stocks to levels found in NV, at a depth of 0–30 cm. N stocks were higher in ASP systems, regardless of the arrangement studied. As a result, the ASP systems provided accumulations that ranged from 1.0 to 4.31 Mg SOC ha−1 yr−1 and from 0.33 to 0.36 Mg N ha−1 yr−1. The carbon contents in humic fractions remained higher in NV. The hierarchical grouping and principal component analysis showed that the implementation of the ASP systems was efficient in increasing soil C and N stocks over time. In conclusion, the present study identified that integrated production systems can support land use intensification strategies based on sustainable and low-carbon agriculture in a transition area between the Cerrado and Caatinga biomes in Brazil.