Soil Organic Matter Fractions Research Articles

Land-Use Change Depletes Quantity and Quality of Soil Organic Matter Fractions in Ethiopian Highlands

The depletion of soil organic matter (SOM) reserve after deforestation and subsequent management practices are well documented, but the impacts of land-use change on the persistence and vulnerability of storage C and N remain uncertain. We investigated soil organic C (SOC) and N stocks in a landscape of chrono-sequence natural forest, grazing/crop lands and plantation forest in the highlands of North-West Ethiopia. We hypothesized that in addition to depleting total C and N pools, multiple conversions of natural forest significantly change the relative proportion of labile and recalcitrant C and N fractions in soils, and thus affect SOM quality. To examine this hypothesis, we estimated depletion of SOC and N stocks and labile (1 & 2) and recalcitrant (fraction 3) C and N pools in soil organic matter following the acid hydrolysis technique. Our studies showed the highest loss of C stock was in grazing land (58%) followed by cropland (50%) and eucalyptus plantation (47%), while on average ca. 57% N stock was depleted. Eucalyptus plantation exhibited potential for soil C recovery, although not for N, after 30 years. The fractionation of SOM revealed that depletions of labile 1 C stocks were similar in grazing and crop lands (36%), and loss of recalcitrant C was highest in grazing soil (56%). However, increases in relative concentrations of labile fraction 1 in grazing land and recalcitrant C and N in cropland suggest the quality of these pools might be influenced by management activities. Also, the C:N ratio of C fractions and recalcitrant indices (RIC and RIN) clearly demonstrated that land conversion from natural forest to managed systems changes the inherent quality of the fractions, which was obscured in whole soil analysis. These findings underscore the importance of considering the quality of SOM when evaluating disturbance impacts on SOC and N stocks.

How we used APSIM to simulate conservation agriculture practices in the rice-wheat system of the Eastern Gangetic Plains

Examples of how to simulate performance of conservation agriculture (CA) and conventional tillage (CT) practices using cropping systems models are rare in the literature, and from the Eastern Gangetic Plains (EGP). Here we report a comprehensive evaluation of the capacity of APSIM for simulating the performance of CA and CT cropping practices under a diverse range of tillage (CT vs zero tillage (ZT)), crop establishment options (puddled transplanted rice vs unpuddled transplanted rice), residue, N rates, and irrigation practices from two sites in the EGP that differed in soil type, water table dynamics, and agro-climatic conditions. We followed a robust procedure of model parameterisation, calibration, and validation, then undertook statistical analyses to evaluate model performance. We have demonstrated that when different values for key model input parameters are employed (i.e. change in soil properties (Ks, BD)), crop rooting parameters (xf- root hospitality, kl- root extraction efficiency) and soil microorganism activity (F biom - fraction of soil organic matter present as microbial biomass and F inert - the inert fraction of soil organic matter), the model performed well in simulating the different performances of CA and CT management practices across the environments in the EGP. Model performance was markedly better in the full-N than in zero-N, but both are still considered acceptable. In addition to well-watered and fertilised treatments, the model was able to capture an observed crop failure in rainfed unpuddled transplanted rice accurately, illustrating an ability to capture crop response under a wide range of water stress environments. As demonstrated by robust statistical criteria, APSIM was able to capture the effect of cropping system, irrigation, tillage, residue, and N-application rate within the bounds of experimental uncertainty, hence is now deemed a suitable tool for scenario analyses around the relevant practices. • APSIM was calibrated and validated under CA vs CT, N rates, and irrigation practices in two diverse environments. • Modifying several key model input parameters (Ks, BD, xf, kl, Fbiom, and Finert) by generic factors captures CA vs CT performance across treatments and environments. • APSIM's ability to simulate zero-N gives confidence that the indigenous soil supply of N is being accurately simulated. • APSIM successfully captured the observed crop failure in rainfed unpuddled transplanted rice. • APSIM simulated the observed soil NO 3 - -N and water dynamics within the bounds of experimental error.

Organic Matter in Soils with Anthropic Horizons in The Eastern Amazon, Pará (Brazil)

The aim of this study was to evaluate the distribution of chemical and physical fractions of soil organic matter (SOM) in anthropic horizons of soil profiles in the west of Pará. The highest total organic carbon values were observed in the superficial horizons, especially in the antrópico LAd and antrópico CHd (45.0g kg-1). Were observed predominance of recalcitrant organic material. It appears that most of the C found in the SOM fractions is associated with C stabilization mechanisms such as the recalcitrance provided by the presence of pyrogenic coal, in addition to its ability to present carboxylic groups that increase its interaction with the mineral fraction of the soil, characterizing the mechanism of chemical protection. It is observed that the SOM fractions can function as indicators that contribute to better understanding of the soil carbon dynamics in soils with antrópico horizons.

Innovative Technology of Accelerated Composting of Chicken Manure to Obtain an Organic Fertilizer with a High Content of Humic Acids

Abstract One of the most important fractions of soil organic matter which has significant environmental and agricultural importance is humus. The specific humus compounds of soil include the complex of amorphous organic humic substances which include humic acids, fulvic acids and humins. The effectiveness of the natural formation of active humic substances during composting processes depends on the chemical composition of organic residues and the environmental conditions influencing the development and activity of native microorganisms. The aim of the project was to build innovative composting boxes for chicken manure that allow to effectively obtain a fertilizer with the highest possible content of humic acids, by creating perfect and controlled conditions for the rapid development of indigenous thermophilic microorganisms and by combining it with the owned bio-acceleration technology. For perfect conditions (temperature, humidity, oxygen, pH) for the controlled growth of microorganisms, an algorithm of the dependence of the box operation parameters on the course of the biological process, was built. The product obtained after composting is an easily digestible complex organic fertilizer of brown-black colour, pH 7.5-8.0 and NPK 5:3:4 and a high content of native humic acids. Its use increases the growth of green mass of plants and improves soil fertility, which has been proved by the conducted pot research.

No-tillage promotes C accumulation in soil and a slight increase in yield stability and profitability of rice in subtropical lowland ecosystems

Context The effects of no-tillage (NT) on soil organic C (SOC) and rice yield in lowland soils are poorly understood in subtropical ecosystems. Aims In a long-term (24 years) field experiment, we assessed the effect of NT on SOC stocks in labile (>53 μm, particulate) and stable (<53 μm, mineral associated) fractions of soil organic matter (SOM), grain yield, between-season stability, and gross margin compared to conventional tillage (CT) and pre-germinated tillage (PG) systems in a lowland Gleysol in southern Brazil. Methods Soil from eight different layers down to 40 cm under each tillage system was sampled in a field experiment, and additional soil samples were obtained from an adjacent area under native grassland (NG) as reference for SOC stocks. Key results While the PG and CT systems maintained similar SOC stocks as the NG soil, NT increased SOC stocks at an annual rate of 0.41 Mg ha−1 in relation to the traditional CT soil. Rice grain yield increased twice over the 24-year period, amounting to 12 Mg ha−1 in the last crop season. Despite lower yield in NT system in most crop seasons, a slight effect on yield stability and profitability (<10%) in favour of NT, compared with CT and PG systems, was observed. Conclusions Based on our findings, NT promotes C accumulation in subtropical paddy rice soils. Implications There is a beneficial effect of NT on rice yield stability and profitability in the long term.

Soil organic matter fractions in an Oxisol under tillage systems and winter cover crops for 26 years in the Brazilian subtropics

The improvement of carbon (C) accumulation in soils has been one of the main purposes of the conservation systems in agricultural production. This study aimed to assess the long-term effect of conventional tillage (CT) and no-tillage (NT) combined with winter cover crops, black oat and oilseed radish, and fallow on C accumulation and stabilization in a very clayey Oxisol in Southern Brazil. Soil samples were collected in the 0-0.05, 0.05-0.10 and 0.10-0.20 m layers of a 26-year-old experiment. Distribution of size-class aggregates, C stock in aggregates, total C stock, and C stocks in the physical fractions, free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM) and mineral-associated organic matter (min-OM) were assessed. NT had a higher percentage of macroaggregates and C stock in this size-class, and also higher C stock in bulk soil, free-POM and occluded-POM fractions than CT in 0-0.05 m (Tukey’s test p < 0.05), due to higher input of biomass and minimum soil mobilization in NT. Oat and radish had higher C stock in macroaggregates than fallow in 0.05-0.10 m (Tukey’s test p < 0.05). Radish had the highest C stock in the free-POM (0-0.05 m). Fallow decreased the stabilization of macroaggregates and C accumulation in free-POM, due to the lower C input from aboveground biomass over the years. In conclusion, NT after 26 years improved C accumulation and stabilization, mainly in the superficial layer and in POM fractions, and winter cover crops favored the formation and stability of macroaggregates.

SOIL ORGANIC MATTER FRACTIONS AND MULTIVARIATE ANALYSIS IN THE DEFINITION OF PASTURE MANAGEMENT ZONES

The use of Precision Agriculture tools and soil attributes has contributed to local agricultural management in the identification of different productive potentials and quality of pastures. The present study aimed to use Precision Agriculture tools to investigate the spatial variability of soil organic matter fractions and soil chemical and physical attributes and to delineate management zones in pasture soils cultivated under tropical conditions. The study was conducted on a hay production farm located in Seropédica, Brazil. Fifty points were collected on a irregular grid and soil samples were taken at depths of 0-0.20 and 0.20-0.40 m and crop productivity was evaluated. Chemical and physical soil organic matter fractionation was performed to obtain data on fulvic acid, humic and humin fractions, mineral-associated and particulate organic carbon, and light organic matter. Geostatistics and multivariate analysis (principal component analysis and k-means clustering) were performed to define the management zones. The results obtained contributed to the division of the pasture area into two regions that can be managed in different ways aiming to increase soil organic matter in a localized manner with the possibility of reducing the use of inputs and directed management that respects the productive potential of the pasture on the farm.

Long‐term agricultural practice effects on carbon and nitrogen isotopes of soil organic matter fractions

AbstractUnderstanding the effects of long‐term traditional and alternative agricultural management practice effects on carbon (C) and nitrogen (N) cycling and storage within particulate organic matter (POM) and light fractions (LF) within various soil aggregate‐size classes can be illuminated by isotopic 13C/12C (δ13C) and 15N/14N (δ15N) differences. The objective of this study was to evaluate the effects of residue level, residue burning, tillage, and irrigation on δ13C and δ15N values of the bulk‐soil, macro‐ (&gt;250 μm) and micro‐aggregate‐(53–250 μm), coarse‐ (&gt;250 μm), and fine‐ (53–250 μm) POM, and coarse‐ and fine‐LF in the top 10 cm following 13 yr of consistent management in a wheat (Triticum aestivum L.)–soybean [Glycine max (L.) Merr.] double‐crop system on a silt‐loam soil in eastern Arkansas. Various treatment combinations affected (p &lt; .05) δ13C values within the bulk‐soil and fine‐POM, as well as δ15N values within the bulk‐soil, macro‐aggregate, coarse‐LF, and fine‐LF fractions. Averaged across all other field treatments, macro‐aggregate δ15N was greater (p &lt; .01) in the no‐tillage (NT)‐low‐ (3.23%) compared with NT–high‐residue (3.05%) and CT‐high‐ and low‐residue combination, which did not differ and averaged 3.11%, indicating that more labile residue can be achieved in the NT–high‐residue treatment combination. Results showed significant variations in aggregate‐associated δ13C and δ15N, as affected by long‐term residue and water management practices that would otherwise not have been evident from simple, bulk‐soil analysis or a short‐term field study.

Fractionation of Soil Organic Matter into Labile and Stable Fractions

The present study aims to test and evaluate the efficiency of a new modified method of organic matter evaluation. It allows the assessment of the quality and quantity of the primary soil organic matter and the stable organic fractions separately. The new method was tested in six soil samples of different localities in the Czech Republic. This method is based on observing reaction kinetics during the oxidation of soil organic matter and measuring the cation-exchange capacity of stable organic fractions. The results were compared with classical methods, which rely on the isolation of humic substances, determination of the content of humic acids and fulvic acids and their ratio CHA:CFA, quotient E4/6, and fractionation of soil organic matter according to resistance to oxidation. It turned out that the results of the new modified method are more sensitive in comparison with the results obtained by classical procedures. The linear regression demonstrated the dependence between the amounts of soil organic matter determined by the classical method compared with the modified method. Moreover, the new modified method was found to be faster and not demanding on laboratory equipment. The new method has been improved to be easily repeatable, and some shortcomings of the previous method were eliminated. Based on our results and other recent studies, the modified method may be recommended for the practical evaluation of soil organic matter conditions.

Changes in soil organic matter fractions induced by cropland and pasture expansion in Brazil's new agricultural frontier

Recently, the so-called Matopiba region (northeastern Brazil) has experienced an accelerated agricultural expansion, with large areas of native vegetation (NV: Cerrado biome) converted to extensive pasture (PA) and cropland under no-tillage (CL). However, because soils from this region contain a large amount of sand (> 60%) and a high susceptibility to erosion, concerns have been raised about the impacts of land-use change (LUC) on soil organic matter (SOM) dynamics. Therefore, we assessed the magnitude that agricultural expansion affects SOM fractions in the most common LUC scenarios observed in the Matopiba region. Soil samples were collected to a depth of 1-m in three adjacent sites (i.e., NV, PA, and CL) located in a representative agricultural area of the region. Soil organic matter was physically fractioned into particulate organic matter (POM) and mineral-associated organic matter (MAOM); and carbon (C) and nitrogen (N) were determined in SOM fractions, in addition to in bulk soil samples. Additionally, the C management index (CMI) was used as a proxy to infer soil health changes. The conversion from NV to PA decreased C (by ~28 Mg ha−1 at 0–100 cm) and N (by ~0.5 Mg ha−1 at 0–50 cm) stocks at an annual loss rate of ~1.3 and 0.02 Mg ha−1, respectively. Carbon loss occurred in both POM and MAOM fractions, which were lower in ~36% and 34%, respectively, compared with NV (0–100 cm). This C depletion resulted in the smallest CMI value (75%) in the 0–30 cm soil layer in PA. The conversion from NV to CL had no effect on total C and N stocks (0–100 cm), but impacted C dynamics in SOM fractions. A lower value was observed for MAOM-C (~22%) in the 30–70 cm layer compared to NV, but the C accrual in the POM fraction (~24%) in the uppermost soil layers (0–30 cm) offset MAOM-C loss. This increase in the POM stocks enhanced the CMI, which was 37% higher than NV for the 0–30 cm layer. Our results revealed that PA expansion in the Matopiba region, besides declining C and N levels, reduced soil health as indicated by the low CMI values. On the other hand, land conversion from NV to CL did not impact C and N stocks and increased CMI. However, we advocated that agriculture expansion over NV areas is not a sensible option to promote the region's sustainable development. It should be prioritized on degraded PA areas and be based on best management practices as a strategy to recarbonize PA soils and restore soil health. This is paramount to mitigate climate change and increase sustainable food production in the Matopiba region over time.

Long-term effects of grazing intensities on soil aggregation and organic matter in a no-tilled integrated soybean-cattle system

Grazing intensity in integrated crop-livestock systems (ICLS) can affect soil aggregation and C stabilization and, consequently, the soil condition and agricultural sustainability. This study evaluate the influence of 13 years of different grazing intensities on (i) soil aggregation, (ii) C content in different aggregate size, and (iii) C and N content in different fractions of soil organic matter (SOM), in a no-tilled with integrated soybean-beef production system on a subtropical Oxisol. Treatments consisted of four steers grazing intensities defined by sward height of 10, 20, 30 and 40 cm on mixed black oat and Italian ryegrass pasture in the winter under continuous stocking, plus an ungrazed treatment, in a randomized complete block design with three replicates. Soil aggregation was not affected by grazing intensities. There was no effect of the grazing intensities on organic C content in all aggregate-size fractions and in all soil layers evaluated, being lower in microaggregates than in large and small macroaggregates. The macroaggregates were predominant, representing 97% of the soil mass, of which 74% were large macroaggregates (>2 mm). The high macroaggregates stability contributed to the lack of difference in the C and N content on free light fraction and the light occlusal fraction of SOM. Most of C and N were observed heavy fraction of SOM (53–68%), with greater stock in the no grazing compared to the grazed treatments. Even with the animal trampling in the grazed systems, the soil maintained high stability of aggregates and consequently the labile forms of SOM.

Long-term effects of tillage and nitrogen fertilization on soil C and N fractions in a corn–soybean rotation

Tillage and nitrogen (N) fertilization can influence soil organic matter (SOM) dynamics, but their interactive effects remain contradictory. A long-term (25 yr) corn (Zea mays L.)-soybean (Glycine max L. Merr.) rotation was used to investigate the effect of tillage [moldboard plow (MP) and no-till (NT)] and N rates (0, 80, and 160 kg N·ha−1) on soil organic carbon (SOC), total N (STN), respiration, and SOM fractions [particulate organic matter (POMC, POMN), mineral-associated organic matter (MAOMC, MAOMN), and microbial biomass (MBC, MBN)]. Results indicate that NT had 27% higher SOC and 24% higher STN than MP in the 0–20 cm depth. Furthermore, SOC and STN stocks (0–20 cm) were 22% and 20% higher, respectively, under NT than MP. There was significant stratification under NT, with a rather uniform distribution under MP. The SOM fractions and soil respiration were 28%–275% and 20%–83% higher at the 0–5 and 5–10 cm depths, respectively, under NT than MP. Interestingly, N fertilizer rate or its interaction with tillage had no impact, except for respiration (tillage × N rate and N rate × depth). Hence, while N addition was required for adequate grain production and increased cumulative plant C and N inputs, our findings indicate that the vertical distribution of SOC, STN, and SOM fractions was affected by tillage, thereby influencing resource accessibility and subsequent dynamics of SOM fractions. Taken together, our results support the adoption of NT and judicious use of N fertilizers for enhancing topsoil SOM storage and fertility under humid temperate conditions.

Spatiotemporal Dynamics of Nitrous Oxide Emission Hotspots in Heterogeneous Riparian Sediments

AbstractNitrous oxide (N2O) is a potent ozone‐depleting greenhouse gas produced by incomplete denitrification. Recent works on riverine N2O emissions focus mainly on contributions from in‐channel, benthic, and fluvial hyporheic environments under assumptions of steady‐state conditions and homogeneous sediment hydraulic conductivity (K). However, riparian floodplains are also a potentially important N2O source characterized by complex sediment heterogeneity and dynamic surface and groundwater interactions. We use numerical flow and reactive transport models to investigate the influence of complex sedimentary architecture and high‐flow events (e.g., storms) on N2O production. We interpret the correlation between flow and solute fields with the flow topological Okubo‐Weiss metric (OW) and the scalar dissipation rate weighted by soil organic matter (OM) fraction and soil saturation. We model a heterogeneous riparian floodplain based on field observations from the Theis Environmental Monitoring and Modeling Site, Ohio, USA. N2O production is greatest within intermediate‐K sediments (e.g., sands) where denitrification rates are highest, and emissions increase by more than an order of magnitude during storms. Sensitivity analysis reveals that the denitrification rate is most influential for N2O flux, accounting for nearly 46% of the variance in production rates. Denitrification rates adapt to spatial changes in the flow topology (measured by OW) related to sediment heterogeneity and are strongly influenced by subsurface mixing dynamics. Mixing is greatest in shear strain‐dominated regions, while vorticity promotes OM dissolution and prolongs residence times. Accurate lithologic representation is imperative for characterizing subsurface N2O production dynamics, especially given growing concern regarding climate change driven hydrologic changes within watersheds worldwide.

Distribution of soil organic matter fractions are altered with soil priming

Soil organic matter (SOM) plays a central role in mediating soil productivity through its impacts on nutrient cycling and retention, aggregate stability and water retention. Thus, management techniques or technologies including novel soil amendments could benefit farmers through the accumulation of carbon (C) and other nutrients in SOM. However, these same inputs can also lead to accelerated mineralization of native SOM through the process known as priming. This unresolved paradox may be due to the limited understanding of how different SOM fractions respond to priming and in which direction. In this study, we examine the response of functionally distinct SOM fractions to priming when soils are amended with lactobionate, a low molecular weight sugar acid byproduct of cheese manufacturing. Liquid-based 13C lactobionate was added to an agricultural silty loam soil to study its persistence, priming effects, and response of different SOM fractions to lactobionate over 84 days. Cumulative soil carbon dioxide (CO2) was greater in lactobionate-amended soils versus control and by the end of the experiment, 53% of added lactobionate was mineralized. In total, positive priming of 40% of extant SOM was observed from 14 to 84 days. Lactobionate-induced changes to SOM fractions were determined at days 14, 28, 56 and 84 of the incubation to examine if and how priming altered the distribution of C between fast and slow-cycling SOC fractions. In response to lactobionate, the total C content of the water extractable organic matter (WEOM) fraction initially increased by 100% from the dissolved lactobionate we added, but then declined and at a faster rate than other SOM fractions. In addition, the total C of the light-fraction particulate organic matter (LF-POM) fraction also declined. At the same time, we observed total C increases in the slower-cycling sand-sized POM (H-POM) and mineral-associated organic (MAOM) C fractions, in response to lactobionate additions. We also saw a marginal increase in total soil C in the lactobionate-amended soils. Our findings therefore suggest that the application of lactobionate to soils may induce positive priming of the faster cycling LF-POM and WEOM fractions, but also concurrent gains in the H-POM and MAOM C fractions associated with long-term persistence and relative resiliency to disturbance with no net loss of total soil carbon. Thus, the application of low-molecular weight C-based materials such as lactobionate presents an avenue to building more persistent SOM through its impacts on the internal cycling and transformation of SOM fractions.

Fractionation, source, and ecological risk assessment of heavy metals in cropland soils across a 100-year reclamation chronosequence in an estuary, South China

Coastal reclamation for cropland has led to the accumulation of heavy metals in soils, bringing about pervasive and severe risks for environment and human health. However, less is known about the influence of long-term reclamation on heavy metals risk, mobility and bioavailability in cropland soil. In this study, we determined six heavy metals (Cd, Cr, Ni, Cu, Zn and Pb) and their fractionations in soils from five croplands across a 100-year reclamation chronosequence in the Pearl River estuary. Results showed that across five reclaimed soils, Cd posed seriously ecological risk and bioavailability according to assessments based on both total contents (single-metal pollution index: Cd > Cu > Zn > Ni > Cr > Pb) and fractionations (risk assessment code: Cd > Zn > Cu > Ni > Pb > Cr). Cr, Ni, Cu, Zn and Pb posed slightly to moderately ecological risks, and were mainly bound to residual (73.70%) and reducible (15.86%) fractions with lower mobility and bioavailability. With the highest risks level, mobility, toxicity and bioavailability (5.67% exchangeable and 11.75% carbonate fractions bound), Cd was identified as the main pollution factor in study area. Principal component analysis and Pearson's correlation analysis revealed that anthropogenic reclamation activities (including phosphate fertilizers, pesticides and sewage irrigation) were the major sources of these heavy metals. Long-term reclamation activities induced the increases of soil organic matter, clay contents, total concentrations and non-residual fractions of heavy metals by 46.14%, 538.98%, 42.87% and 219.78%, respectively, demonstrating significant promotions in level and mobility of heavy metals due to longer-term agricultural activities, higher soil clay and organic matter content.

Distribution of soil organic carbon and nitrogen under reduced nitrogen application in the Guanzhong plain of China#

AbstractBackgroundThe problems of soil quality degradation and environmental pollution caused by excessive use of nitrogen fertilizer seriously affect sustainable development of agriculture.AimsWe investigated the effects of reduced nitrogen application with simultaneous full straw return on carbon input, aggregate size distribution, aggregate‐associated organic carbon and total nitrogen (TN), soil organic carbon (SOC) stock, SOC fractions, and crop yield.MethodsA long‐term field experiment was performed in the dryland region of Guanzhong. The study involved three nitrogen fertilizer (NF) amounts: 337.5 (conventional NF, CNF), 286.4 (15%NF reduction, 15%RNF), and 236.3 (30%NF reduction, 30%RNF) kg N ha–1 y–1 with full straw return.ResultsCompared with CNF, 30%RNF increased the content of soil organic carbon by 5.7% and increased maize yield by 13.1%, under straw return conditions. The 30%RNF + straw application treatment promoted the formation of large macro‐aggregates more effectively than CNF application. The 30%RNF + straw treatment significantly increased SOC by 10.6% and 4.7%, respectively, in large and small macroaggregates relative to conventional NF. Moreover, the 30%RNF + straw treatment resulted in a significant increase in the SOC content in the density fractions of mineral soil organic matter (mSOM) for both small and large macro‐aggregates. This treatment also resulted in a significant increase in the content of TN in the mSOM and density fractions of particulate matter that had coarse and fine intra‐aggregates of the large macro‐aggregates.ConclusionFull straw return with reduced nitrogen (N) application could be a promising option to improve the structure of soil aggregates, soil organic carbon, and nitrogen sequestration.

Black Carbon and Its Effect on Carbon Sequestration in Soil

The properties of black carbon (BC) are described very differently in the literature, even when determined by the same methodological procedure. To clarify this discrepancy, BC was investigated in the clay Cambisols of southern Bohemia, Czech Republic, in groups of soils with lower and higher deposition of its atmospheric fallout. The BC determination was performed according to a modified method of Kuhlbusch and Crutzen (1995). The amount of the free light fraction, the occluded light fraction of soil organic matter and its ratio, the amount of heavy soil fraction DF, and its soil organic matter DFOM were determined. Other soil characteristics were identified. It was found that there are two very different types of BC in soils. Historical BC from biomass fires, and new, anthropogenic, from the furnace and transport fumes. Historical BC has a significant effect on the organic matter of the heavy soil fraction, on the ratio of the free and occluded soil organic matter fraction, and the number of water-resistant soil aggregates. Anthropogenic BC does not have this effect. Because this form of BC is not significantly stabilized by the colloidal mineral fraction, it is necessary to take general data on BC’s high stability and resistance to mineralization in the soil with circumspection.

Soil Organic Matter Fractions and Enzymes Activities under No-tillage System: Effects of Organomineral and Mineral Fertilizer with Humic Substances

ABSTRACT The use of organomineral fertilizer can promote increments of organic matter and affect the enzymatic activity of the soil. The objective of this research was to evaluate the effect of organomineral and mineral fertilizer with humic substances on soil organic matter fractions and enzymes activities. The study was set up in a Xanthic Ferrasol, in the Cerrado region, Brazil. Treatments consisted of two fertilizers and four rates of P fertilizer. The following fertilizers were studied: mineral fertilizer enriched with humic substances (03% of N, 25% of P2O5, 08% of K2O, with 1% humic substance); and granulated organomineral produced from poultry litter and mineral fertilizer (MAP – mono-ammonium phosphate, with 26.6% of P2O5 and 18.2% of organic carbon). Five rates of P were also evaluated (0, 20, 40, 60, and 80 kg ha−1 of P2O5). The crop production, soil organic matter fraction and enzymes were assessed in the 2014/2015 and 2015/2016 growing seasons. Contents of the total organic carbon, particulate organic carbon, humic acid, fulvic acid, and maize and soybean grain yield were positively influenced by the fertilizer application. Fertilization treatments did not affect the enzymes activities, but they showed positive correlations with particulate organic carbon.

Chemistry and Oxidation State of Soil Organic Matter Fractions Under Long-Term Mineral and Manure Amendments in Ferralic Cambisol

Chemical properties of organic matter are fundamental for soil fertility. However it is unclear how physical size and fertilization regime shifts the chemical composition and oxidation state of soil organic matter (SOM). Here we studied this issue among three physical size classes under four 32-years amendments: control (unfertilized), urea (N), urea+Ca(H2PO4)2+KCl (NPK), and NPK + pig manure (NPKM). Coarse particulate (cPOM, >250 μm), fine particulate (fPOM, 53–250 μm) and mineral-associated OM (MAOM, <53 μm) fractions were separated and analyzed by pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS). Over the 32-years, the SOM of control, N, NPK and NPKM increased by 10.6%, 14.2%, 23% and 52% compared with the initial level, respectively. The conversion efficiency from fertilization-induced input-C to soil organic carbon (SOC) was 6.8%. Physical size but not fertilization primarily regulated the molecular composition of SOM, with relative selective retention of aromatics and lignin in cPOM and fPOM, whereas N-containing compounds, especially amino-N, were enriched in MAOM because of their high chemical affinities to 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 on physical size and fertilization. In conclusion, physical size, not amendment regime, primarily regulated the chemical composition and oxidation state of SOM, suggesting that physical fractionation (proxy of microbial decomposition degree) plays greater role in SOM chemistry than the quality of amendments.

Particle size primarily shifts chemical composition of organic matter under long‐term fertilization in paddy soil

AbstractThe chemistry of soil organic matter (SOM) is critical for soil functions. However, the chemical composition and oxidation state of physical SOM fractions under long‐term mineral and organic amendments in paddy soils are still unclear. Here, we investigated this issue using three particle size classes of SOM sampled from five long‐term (37‐years) fertilisation regimes in a subtropical paddy soil which were analysed using pyrolysis–gas chromatography–mass spectrometry (Py–GC/MS). Coarse particulate organic matter (cPOM, &gt;250 μm), fine particulate organic matter (fPOM, 53–250 μm) and mineral‐associated organic matter (MOM, &lt;53 μm) were isolated. The five fertilisation regimes were (1) unfertilised (control), (2) nitrogen (N, urea), (3) nitrogen + phosphorus + potassium (NPK, urea + Ca(H2PO4)2 + KCl), (4) NPK + straw (NPKS) and (5) NPK + Astragalus sinicus L. + pig manure (NPKM). After 37‐years, the SOM content of the control, N, NPK, NPKS and NPKM treatments had increased by 16.3%, 12.3%, 31.6%, 41.7% and 59.8% compared with the initial level, respectively. The pools of soil organic carbon (SOC) and total N (TN), as well as the C/N ratio in the soil matrix, were localised by particle size, with the majority in the mineral‐associated organic matter (MOM). Particle size, not fertilisation regime, primarily associated with shifts in the chemical composition of SOM, with selective preservation of lignin and polysaccharides in the particulate organic matter (POM) and enrichments of lipids and N‐containing compounds in the MOM. The origin (plant‐ and microbial‐derived) and oxidation state of SOM were also regulated by particle size (especially the coarse particles) but not fertilisation treatment. Despite long‐term persistence, the MOM had a lower C oxidation state (implying higher vulnerability to microbial utilisation) than the coarse POM (cPOM) and fine POM (fPOM). In conclusion, fertilisation regime regulated the bulk SOM (SOC and TN) pool, whereas physical particle size generated a divergence trend in the chemical composition and oxidation state of SOM in the subropical paddy soil, indicating the greater role of physical arrangement in SOM chemistry than the quality of amendments.Highlights Particle size, not fertilisation regime, regulated chemical composition and oxidation state of SOM. POM is enriched in lignin and polysaccharides while MOM is enriched in lipids and N‐containing compounds. Physical arrangement plays a greater role in SOM chemistry than the quality of amendments.