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Fractionation of soil organic carbon (SOC) is crucial for mechanistic understanding and modeling of soil organic matter decomposition and stabilization processes. It is often aimed at separating the bulk SOC into fractions with varying turnover rates, but a comprehensive comparison of methods to achieve this is lacking. In this study, a total of 20 different SOC fractionation methods were tested by participating laboratories for their suitability to isolate fractions with varying turnover rates, using agricultural soils from three experimental sites with vegetation change from C3 to C4 22–36 years ago. Enrichment of C4-derived carbon was traced and used as a proxy for turnover rates in the fractions. Methods that apply a combination of physical (density, size) and chemical (oxidation, extraction) fractionation were identified as most effective in separating SOC into fractions with distinct turnover rates. Coarse light SOC separated by density fractionation was the most C4-carbon enriched fraction, while oxidation-resistant SOC left after extraction with NaOCl was the least C4-carbon enriched fraction. Surprisingly, even after 36 years of C4 crop cultivation in a temperate climate, no method was able to isolate a fraction with more than 76% turnover, which challenges the link to the most active plant-derived carbon pools in models. Particles with density >2.8 g cm −3 showed similar C4-carbon enrichment as oxidation-resistant SOC, highlighting the importance of sesquioxides for SOC stabilization. The importance of clay and silt-sized particles (<50 μm) for SOC stabilization was also confirmed. Particle size fractionation significantly outperformed aggregate size fractionation, due to the fact that larger aggregates contain smaller aggregates and organic matter particles of various sizes with different turnover rates. An evaluation scheme comprising different criteria was used to identify the most suitable methods for isolating fractions with distinct turnover rates, and potential benefits and trade-offs associated with a specific choice. Our findings can be of great help to select the appropriate method(s) for fractionation of agricultural soils. [ABSTRACT FROM AUTHOR]

Long-term effect of mungbean inclusion in lowland rice-wheat and upland maize-wheat systems on soil carbon (C) pools, particulate organic C (POC), and C-stabilization was envisaged in organic, inorganic and without nutrient management practices. In both lowland and upland systems, mungbean inclusion increased very-labile C (Cfrac1) and labile C (Cfrac2) in surface soil (0-0.2 m). Mungbean inclusion in cereal-cereal cropping systems improved POC, being higher in lowland (107.4%). Lowland rice-based system had higher passive C-pool (11.1 Mg C ha−1) over upland maize-based system (6.6 Mg C ha−1) indicating that rice ecology facilitates the stabilization of passive C-pool, which has longer persistence in soil. Organic nutrient management (farmyard manure + full crop residue + biofertilizers) increased Cfrac1 and carbon management index (CMI) over inorganic treatment. In surface soil, higher CMI values were evident in mungbean included cropping systems in both lowland and upland conditions. Mungbean inclusion increased grain yield of cereal crops, and yield improvement followed the order of maize (23.7-31.3%) > rice (16.9-27.0%) > wheat (lowland 7.0-10.7%; upland 5.4-16.6%). Thus, the inclusion of summer mungbean in cereal-cereal cropping systems could be a long-term strategy to enrich soil organic C and to ensure sustainability of cereal-cereal cropping systems. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) turnover increasingly is conceptualized as a tension between accessibility to microorganisms and protection from decomposition via physical and chemical association with minerals in emerging soil biogeochemical theory. Yet, these components are missing from the original mathematical models of belowground carbon dynamics and remain underrepresented in more recent compartmental models that separate SOM into discrete pools with differing turnover times. Thus, a gap currently exists between the emergent understanding of SOM dynamics and our ability to improve terrestrial biogeochemical projections that rely on the existing models. In this opinion paper, we portray the SOM paradigm as a triangle composed of three nodes: conceptual theory, analytical measurement, and numerical models. In successful approaches, we contend that the nodes are connected—models capture the essential features of dominant theories while measurement tools generate data adequate to parameterize and evaluate the models—and balanced—models can inspire new theories via emergent behaviors, pushing empiricists to devise new measurements. Many exciting advances recently pushed the boundaries on one or more nodes. However, newly integrated triangles have yet to coalesce. We conclude that our ability to incorporate mechanisms of microbial decomposition and physicochemical protection into predictions of SOM change is limited by current disconnections and imbalances among theory, measurement, and modeling. Opportunities to reintegrate the three components of the SOM paradigm exist by carefully considering their linkages and feedbacks at specific scales of observation. [ABSTRACT FROM AUTHOR]

Abstract: Soil degradation processes due to intensified agriculture may be counteracted by an extended knowledge on soil humus dynamics at molecular level. Here, we show that the molecular composition of the soil Humeome changes substantially after 1 and 3 years of conventional tillage under maize cropping. Application of Humeomics sequential chemical fractionation not only separated from soil more than twice the organic carbon extracted by traditional methods, but also enabled to identify 10 times more molecular structures using GC–MS and high‐resolution ESI‐Orbitrap‐MS. The fine molecular description of the soil Humeome revealed a decrease of soil organic matter (SOM) hydrophobic protection with prolonged tillage and a potential loss of small nitrogen‐rich compounds. We showed for the first time an abundance of heterocyclic nitrogen compounds that are persistent in SOM due to a covalent binding to soil iron. The formation of organo‐Fe complexes appeared not only as a major mechanism of molecular stabilization of SOC but also as an adsorption substrate for other nitrogen‐rich soil components. The detailed molecular insight of SOM composition reached by Humeomics may become the key to develop new technologies for the control of soil carbon. [ABSTRACT FROM AUTHOR]

Agricultural soils degradation is a global challenge that affects food security, climate change, environmental risks, and the loss of biodiversity and ecosystem services. In this on-farm study conducted in the Mediterranean mountain area of southern Spain, we used a natural area (NAT) as reference and compared the effect of three tree cropping systems – conventional olive (COL), organic olive (OLO) and organic almond (OAL)– on soil aggregation, aggregates-associated soil organic carbon and carbon preservation capacity. This was conducted to evaluate the hypothesis that the organic management system affects soil stability, soil organic carbon sequestration and soil organic carbon distribution among aggregate fractions. Soil samples were collected at five different depths (0–10, 10–20, 20–30, 30–40 and 40–50 cm depth) to analyse the management impacts on deeper soil layers. Aggregation indices showed an increase in the topsoil layers (0–10 and 0–20 cm depth) under organic managements with higher large macroaggregates (> 2000 µm) percentages in OLO (+10.9 % and 21.8 %) and OAL (+15.25 % and 28.5 %) respectively compared to COL. A significant decarbonization process was detected and the total carbon preservation capacity (0–50 cm depth) decreased from NAT (159.24 g kg—1) to cultivated plots, that stored a − 53.8 % OLO (73.45 g kg—1), − 66.3 % OAL (53.69 g kg—1) and − 70.4 % COL (47.22 g kg—1). In this regard, the management change from conventional to organic increased total carbon preservation capacity and caused a change in the distribution of soil organic carbon pool observed in a higher carbon content in large macroaggregates (+47.3 %). These findings indicate that organic farming affects not only to the soil organic carbon sequestration but also to the extent of soil organic carbon protection. • Organic management increased soil quality by improving macroaggregation and stability. • Organic fertilisation improved SOC storage and sequestration under organic farming. • Organic olive-grove improved SOC protection by higher SOC in the macroaggregates. • Rainfed Mediterranean woody crops had potential for mitigating climate change. [ABSTRACT FROM AUTHOR]

In the present study, three soils (Ferralsol, Luvisol topsoil, and Luvisol subsoil) were amended with biochars (charcoal, hydrothermal carbonization coal (HTC) of bark, and low-temperature conversion coal of sewage sludge), wheat straw and a control (no amendment) and incubated over a period of 365 days. Each amendment was applied at a rate of 11.29 g C/kg soil. After incubation, the soils were analyzed to retrieve three density fractions (free fraction (FF), intra-aggregate fraction (IAF), and heavy fraction) which were analyzed for total carbon (TC) contents and scanned by fourier transform infrared spectroscopy (FTIR). The biochars and straw significantly increased the TC contents of soils as compared to control. Among soil organic matter (SOM) density fractions, higher TC contents were documented in the FF and IAF from biochar treatments as compared to the straw. The FTIR spectra of the FF from the charcoal and HTC treatments showed the presence of aluminosilicate minerals on surfaces of SOM. There were slight changes in the FF of straw and HTC treatments as compared to spectra of original amendments. The study suggests that the stability of charcoal and HTC in soils is due to the recalcitrant nature of biochar followed by occlusion into soil micro-aggregates.

Hydrofluoric acid (HF) is a powerful tool in the investigation of soil organic matter (SOM) due to its ability to dissolve minerals but not break the chemical bonds of organic matter. These properties make the use of HF a common pretreatment step for removing paramagnetic interferences and concentrating carbon prior to solid-state 13 C NMR spectroscopy with the working assumption that any SOM lost during HF treatment will not bias the resulting NMR spectra. Hydrofluoric acid is also used to isolate a mineral-stabilized OM fraction with the working assumption that most mineral-stabilized OM is primarily low molecular weight compounds bound to mineral surfaces and when the minerals are dissolved in HF, the OM bound to these surfaces will be lost to solution. The working assumptions behind these two uses of HF dissolution appear to be contradictory. To address this apparent conundrum, we treated a number of simple organic compounds, soil and sediment samples with HF in 2 and 10% concentrations and tracked C and N loss as well as chemical shifts observed in solid-state 13 C NMR spectra. For the soil and sediment samples there were inconsistent C and N losses but no difference in loss between the 2% and 10% HF concentrations. There were no obvious soil properties that could explain the differences in C or N loss. Overall, there were significant shifts in NMR-observable organic chemistry after treatment with both 2 and 10% HF with anoxic fine grained sediments under a seagrass meadow exhibiting strong preferential loss of O-alkyl C while terrestrial soils generally lost OM with more of a mixed chemical character. For many samples, the degree of selective loss was enough to significantly bias the interpretation of OM composition. Given the lack of ability to explain the large differences in C loss between samples with observed soil properties, this study suggests that caution should be used when interpreting HF-soluble C to indicate a mineral-stabilized fraction without considering the soil physicochemical environment and putative mechanisms for organo-mineral associations in that particular soil. [ABSTRACT FROM AUTHOR]

Background and aims: Legumes are used in forage agriculture to replace mineral N fertilizer by biological N fixation. We hypothesised that N addition through biological N fixation may have different effects on soil organic matter (SOM)112 quantity and composition compared to mineral N fertilization. In this study we introduced lucerne into grasslands and aimed to examine the resulting effects on C stocks and the molecular C signatures of roots and soil. Methods: Lucerne, cocksfoot and tall fescue were grown for five years as monocultures or mixtures. Grass monocultures were N-fertilised (SNF), while lucerne monoculture and mixtures received N through biological N fixation (BNF). For root and soil samples from 30 cm depth, we analysed quantity and composition of (1) non-cellulosic neutral carbohydrates after acid hydrolysis and (2) lignin after CuO oxidation. Results: Our data showed species-specific chemical signatures for roots extracted from soil. Lucerne presence increased the N content of roots and lowered their lignin/N ratio. Contrasting root input over four growing seasons did not impact SOM stocks, as they were similar in all treatments. We observed different chemical signatures for soil under mixtures as compared to lucerne monoculture. In particular, the state of SOM degradation was enhanced under mixtures. Conclusion: A greater input of higher quality litter to soils receiving BNF did not increase SOM storage. When lucerne was grown in mixture with grass, we observed lignin and carbohydrate signatures that indicated a more advanced state of degradation as compared to monocultures. Therefore, we suggest that the introduction of lucerne into grassland influences OM degradation more than its stabilization. After four growing seasons, SOM molecular signatures were not significantly influenced by type of N fertilization and showed little difference among the treatments despite species specific root composition. [ABSTRACT FROM AUTHOR]

A Humeomics sequential chemical fractionation coupled to advanced analytical identification was applied directly to soil for the first time. Humeomics extracted ~ 235% more soil organic carbon (SOC) than by the total alkaline extraction traditionally employed to solubilise soil humic molecules (soil Humeome). Seven fractions of either hydro- or organo-soluble components and a final unextractable humic residue were separated from soil. These materials enabled an unprecedented structural identification of solubilised heterogeneous humic molecules by combining NMR, GC–MS, and ESI-Orbitrap-MS. Identified molecules and their relative abundance were used to build up structure-based van Krevelen plots to show the specific contribution of each fraction to SOC. The stepwise isolation of mostly hydrophobic and unsaturated molecules of progressive structural complexity suggests that humic suprastructures in soil are arranged in multi-molecular layers. These comprised molecules either hydrophobically adsorbed on soil aluminosilicate surfaces in less stable fractions, or covalently bound in amorphous organo-iron complexes in more recalcitrant fractions. Moreover, most lipid molecules of the soil Humeome appeared to derive from plant polyesters rather than bacterial metabolism. An advanced understanding of soil humic molecular composition by Humeomics may enable control of the bio-organic dynamics and reactivity in soil. [ABSTRACT FROM AUTHOR]

The potential of storing carbon (C) in soil and offsetting the atmospheric CO 2 depends on the management practices. This study investigated the long-term (15 years) effects of using nitrogen: phosphorous: potassium (NPK) fertilizers with and without organic manures namely farmyard manure (FYM), green manure (GM), and straw incorporation (SI) on soil C stabilization, and C management index (CMI) in intensive rice-wheat system in Typic Ustochrepts soil of a semiarid region in Punjab, India. The long-term effects of different fertilizer treatments as control, NPK, NPK + GM, NPK + SI, and NPK + FYM on soil organic carbon (SOC), labile fractions of SOC (LFSOC), C stabilization, C sequestration, and C management index (CMI) were determined. Prolonged application of NPK fertilizers alone or in combination with organic manures significantly decreased soil bulk density (from 7.3 to 16.6%) and increased total soil porosity of (from 11.7 to 26.2%) as compared to the control in surface soil (0–15 cm). Fertilizations had positive and variable effects on different LFSOC, namely water soluble C (WSC), labile carbon (LC), particulate organic C (POC), hot water soluble carbon (HWSC), and total organic carbon (TOC). Long-term intensive cultivation and fertilization significantly increased the SOC in surface soil (0–15 cm) up to 5.16 g kg −1 as compared with its antecedent value of 2.42 g kg −1 in 1999. The sensitivity index indicated that among LFSOC, LC (51–85%), WSC (67–131%), POC (64–159%), and HWSC (38–131%) were more sensitive compared with SOC (29–59%). NPK + SI (5.06 Mg C ha −1 ) or NPK + FYM (5.31 Mg C ha −1 ) along with NPK sequestered significantly higher C than the application of NPK (3.57 Mg C ha −1 ) or NPK + GM (4.10 Mg C ha −1 ). The fraction of C in stable fractions (C frac3 + C fract4 ) increased from 7 to 29% with application of NPK fertilizers and organic manures compared with that of the control. The CMI indicated that the use of NPK + SI or NPK + FYM was statistically superior to the use of NPK + GM for improving the SOC status. The study concluded that a relatively high sensitivity index of LFSOC may be used as an early indicator for determining the potential of different management practices for improving SOC on short-term basis. The results indicated that the stability of SOC varies with the nature of the added organic manures, suggesting that the selection of organic manure is important for long-term C sequestration. [ABSTRACT FROM AUTHOR]

Residue retention and reduced tillage are both conservation agricultural practices that may enhance soil organic carbon ( SOC) stabilization in soil. We evaluated the long-term effects of no-till ( NT) and stover retention from maize on SOC dynamics in a Rayne silt loam Typic Hapludults in Ohio. The six treatments consisted of retaining 0, 25, 50, 75, 100 and 200% of maize residues on each 3 × 3 m plot from the crop of previous year. Soil samples were obtained after 9 yrs of establishing the experiment. The whole soil (0-10 and 10-20 cm of soil depths) samples under different treatments were analysed for total C, total N, recalcitrant C (Na OCl treated sample) and 13C isotopic abundance (0-10 cm soil depth). Complete removal of stover for a period of 9 yrs significantly ( P < 0.01) decreased soil C content (15.5 g/kg), whereas 200% of stover retention had the maximum soil C concentration (23.1 g/kg). Relative distribution of C for all the treatments in different fractions comprised of 55-58% as labile and 42-45% as recalcitrant. Retention of residue did not significantly affect total C and N concentration in 10-20 cm depth. 13C isotopic signature data indicated that C4-C (maize-derived C) was the dominant fraction of C in the top 0-10 cm of soil layer under NT with maize-derived C accounting for as high as 80% of the total SOC concentration. Contribution of C4-C or maize-derived C was 71-84% in recalcitrant fraction in different residue retained plots. Residue management is imperative to increase SOC concentrations and long-term agro-ecosystem necessitates residue retention for stabilizing C in light-textured soils. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) is usually said to be well correlated with soil texture and soil aggregation. These relations generally suggest a physical and physicochemical protection of SOC within soil aggregates and on soil fine particles, respectively. Because there are few experimental evidences of these relations on tropical soils, we tested the relations of soil variables (SOC and soil aggregate contents, and soil texture) with the amount of SOC physically protected in aggregates on a set of 15 Malagasy soils. The soil texture, the SOC and water stable macroaggregate (MA) contents and the amount of SOC physically protected inside aggregates, calculated as the difference of C mineralized by crushed and intact aggregates, were characterized. The relation between these variables was established. SOC content was significantly correlated with soil texture (clay+fine silt fraction) and with soil MA amount while protected SOC content was not correlated with soil MA amount. This lack of correlation might be attributed to the highest importance of physicochemical protection of SOC which is demonstrated by the positive relation between SOC and clay+fine silt fraction.

The objectives of the study were to (i) investigate whether short term mineralization varies to a greater extent by charring organic matter (OM) or by exposure to different mineral surfaces, (ii) examine the effect of clay mineralogy on mineralization of either charred or uncharred OM and (iii) quantify these differences in mineralization in the context of C evolution during charring. Mineralization of different combinations of OM types (corn stover, cellulose, glucose, lignin and arginine) or their corresponding pyrogenic organic matter (PyOM) with minerals (quartz, corundum, goethite, kaolinite and kaolinite mixed with goethite) were assessed in incubation experiments at 30 °C for 196 days. All PyOM materials showed lower mineralization extent (3.8–7.5% of initial C) than uncharred OM (7.6–89% of initial C). Charring decreased mineralization to a greater extent (by 4.7–95%) than differences in the minerals (by 3.7–70%). The relative standard deviation of C mineralization of individual charred OM materials caused by differences in mineralogy was smaller (8.7–16%) than that of uncharred OM (8.7–65%). For the ratio of C remaining after both mineralization and charring to mineralization of uncharred biomass, the relative standard deviation was higher for different OM types (51–84%) than mineral types (1.1–47%), suggesting that mineralogy was less important than OM properties. Therefore, the importance of mineralogy was lower for charred than uncharred OM and the type of PyOM more strongly controlled mineralization than the type of mineral during the initial decomposition process. [ABSTRACT FROM AUTHOR]

The importance of lignin as a recalcitrant constituent of soil organic matter (SOM) remains contested. Associations with iron (Fe) oxides have been proposed to specifically protect lignin from decomposition, but impacts of Fe-lignin interactions on mineralization rates remain unclear. Oxygen (O 2 ) fluctuations characteristic of humid tropical soils drive reductive Fe dissolution and precipitation, facilitating multiple types of Fe-lignin interactions that could variably decompose or protect lignin. We tested impacts of Fe addition on 13 C methoxyl-labeled lignin mineralization in soils that were exposed to static or fluctuating O 2 . Iron addition suppressed lignin mineralization to 21% of controls, regardless of O 2 availability. However, Fe addition had no effect on soil CO 2 production, implying that Fe oxides specifically protected lignin methoxyls but not bulk SOM. Iron oxide-lignin interactions represent a specific mechanism for lignin stabilization, linking SOM biochemical composition to turnover via geochemistry. [ABSTRACT FROM AUTHOR]

Co-composting biowastes such as manures and biosolids can be used to stabilize carbon (C) without impacting the quality of these biowastes. This study investigated the effect of co-composting biowastes with alkaline materials on C stabilization and monitored the fertilization and revegetation values of these co-composts. The stabilization of C in biowastes (poultry manure and biosolids) was examined by their composting in the presence of various alkaline amendments (lime, fluidized bed boiler ash, flue gas desulphurization gypsum, and red mud) for 6 months in a controlled environment. The effects of co-composting on the biowastes' properties were assessed for different physical C fractions, microbial biomass C, priming effect, potentially mineralizable nitrogen, bioavailable phosphorus, and revegetation of an urban landfill soil. Co-composting biowastes with alkaline materials increased C stabilization, attributed to interaction with alkaline materials, thereby protecting it from microbial decomposition. The co-composted biowastes also increased the fertility of the landfill soil, thereby enhancing its revegetation potential. Stabilization of biowastes using alkaline materials through co-composting maintains their fertilization value in terms of improving plant growth. The co-composted biowastes also contribute to long-term soil C sequestration and reduction of bioavailability of heavy metals. [ABSTRACT FROM AUTHOR]

Alum water treatment sludge is composed of amorphous hydroxyl-Al, which has variable charge surfaces with a large Brunauer-Emmett-Teller (BET) surface area (103 m g) capable of specific adsorption of organic matter molecules, phosphate, and heavy metals. The effects of adding dried, ground, alum water treatment sludge (10 % w/ w) to the feedstock for composting municipal green waste alone, green waste plus poultry manure, or green waste plus biosolids were determined. Addition of water treatment sludge reduced water soluble C, microbial biomass C, CO evolution, extractable P, and extractable heavy metals during composting. The decrease in CO evolution (i.e., C sequestration) was greatest for poultry manure and least for biosolid composts. The effects of addition of water treatment sludge to mature green waste-based poultry manure and biosolid composts were also determined in a 24-week incubation experiment. The composts were either incubated alone or after addition to a soil. Extractable P and heavy metal concentrations were decreased by additions of water treatment sludge in all treatments, and CO evolution was also reduced from the poultry manure compost over the first 16-18 weeks. However, for biosolid compost, addition of water treatment sludge increased microbial biomass C and CO evolution rate over the entire 24-week incubation period. This was attributed to the greatly reduced extractable heavy metal concentrations (As, Cr, Cu, Pb, and Zn) present following addition of water treatment sludge, and thus increased microbial activity. It was concluded that addition of water treatment sludge reduces concentrations of extractable P and heavy metals in composts and that its effect on organic matter stabilization is much greater during the composting process than for mature compost because levels of easily decomposable organic matter are initially much higher in the feedstock than those in matured composts. [ABSTRACT FROM AUTHOR]

In a warmer world, soil CO2 emissions are likely to increase. There is still much discussion about which soil organic C (SOC) pools are more sensitive to increasing temperatures. While the temperature sensitivity of C stabilized by biochemical recalcitrance or by sorption to mineral surfaces has been characterized, few studies have been carried out on the temperature sensitivity-expressed as Q10-of C physically protected inside soil macroaggregates (0.2-2 mm). It has been suggested that increasing the availability of labile SOC by exposing C through macroaggregate crushing would decrease Q10, i.e., the temperature dependence of soil CO2 emissions. To test this hypothesis, the temperature dependence of CO2 emissions from C physically protected in macroaggregates was measured through 21-d laboratory incubations of crushed and uncrushed soils, at 18°C and 28°C. 199 topsoil samples, acidic or calcareous, with SOC ranging from 2 to 121 g kg−1 soil were investigated. The CO2 emissions were slightly more sensible to temperature than to C deprotection: about 0.3 mg C g−1 soil (= 13 mg C g−1 SOC) and 0.2 mg C g−1 (= 12 mg C g−1 SOC) were additionally mineralized, in average, by increasing the temperature or by disrupting the soil structure, respectively. The mean Q10 index ratio of CO2 emitted at 28°C and 18°C was similar for crushed and uncrushed soil samples and equaled 1.6. This was partly explained because Q10 of macro-aggregate-protected C was 1. The results did not support the initial hypothesis of lower temperature dependence of soil CO2 emissions after macroaggregate disruption, although a slight decrease of Q10 was noticeable after crushing for soils with high amounts of macroaggregate-protected C. Field research is now needed to confirm that soil tillage might have no effect on the temperature sensitivity of SOC stocks. [ABSTRACT FROM AUTHOR]

Drying and rewetting of soil can have large effects on carbon (C) and nitrogen (N) dynamics. Drying-rewetting effects have mostly been studied in the absence of plants, although it is well known that plant-microbe interactions can substantially alter soil C and N dynamics. We investigated for the first time how drying and rewetting affected rhizodeposition, its utilization by microbes, and its stabilization into soil (C associated with soil mineral phase). We also investigated how drying and rewetting influenced N mineralization and loss. We grew wheat (Triticum aestivum) in a controlled environment under constant moisture and under dry-rewetting cycles, and used a continuous 13C-labeling method to partition plant and soil organic matter (SOM) contribution to different soil pools. We applied a 15N label to the soil to determine N loss. We found that dry-rewetting decreased total input of plant C in microbial biomass (MB) and in the soil mineral phase, mainly due to a reduction of plant biomass. Plant derived C in MB and in the soil mineral phase were positively correlated (R² = 0.54; P = 0.0012). N loss was reduced with dry rewetting cycles, and mineralization increased after each rewetting event. Overall drying and rewetting reduced rhizodeposition and stabilization of new C, primary through biomass reduction. However, frequency of rewetting and intensity of drought may determine the fate of C in MB and consequently into the soil mineral phase. Frequency and intensity may also be crucial in stimulating N mineralization and reducing N loss in agricultural soils. [ABSTRACT FROM AUTHOR]

Goethite, hematite, ferrihydrite, and other iron oxides bind through various sorption reactions with humic substances (HS) in soils creating nano-, micro-, and macro-aggregates with a specific nature and stability. Long residence times of soil organic matter (SOM) have been attributed to iron-humic substance (Fe-HS) complexes due to physical protection and chemical stabilization at the organic–mineral interface. Humic acids (HA) and fulvic acids (FA) contain many acidic functional groups that interact with Fe oxides through different mechanisms. Due to the numerous interactions between mineral Fe and natural SOM, much research has led into a better identification and definition of HS. In this review, we first focus on the surface colloidal properties of Fe oxides and their reactivity toward HS. These minerals can be efficiently identified by usual techniques, such as XRD, FTIR spectroscopy, XAS, Mössbauer, diffuse reflectance spectroscopies (DRS), HRTEM, ATM, NanoSIMS. Second, we present the recent state of art regarding the adsorption/precipitation of HS onto iron mineral surfaces and their effects on binding metalloid and trace elements. Finally, we consider future research directions based on recent scientific literature, with particular focus on the ability of Fe nano-particles to increase Fe bioavailability, improve carbon sequestration, reduce greenhouse gas emissions, and decrease the impact of persistent organic and inorganic pollutants. The methodology in this field has rapidly developed over the last decade. However, new procedures to estimate the nature of Fe-HA bonds will be important contributions in clarifying the role of natural iron oxides in soil for carbon stabilization. [Display omitted] • Surface properties of Fe oxides, and their reactivity toward HS are examined. • A metal binding mechanism between Fe and HS is resumed. • HS impacts on chemical and oxidative/reductive reactivity of iron oxide is reviewed. • Stabilization mechanism of HA with Fe oxides is related with longer turnover of SOM. • Fe-HS is associated with transport and bioavailability of persistent organic pollutants. [ABSTRACT FROM AUTHOR]

Detecting soil organic carbon (SOC) gain or loss is challenging due to large uncertainties deriving from temporal and spatial variability of organic carbon concentrations, even at the field scale. In order to reduce these uncertainties, we used the organic carbon associated with clay and fine silt particles (fine fraction) rather than SOC in bulk soil for assessing decadal changes. This approach reduces the impact of the inherent variability of labile carbon on SOC estimates. We analysed archived soil samples taken in 1970 and recent ones taken in 2012 from an on-going long-term field trial in the Hesbaye region in Belgium. The experiment started in 1959 and contains three contrasting management practices (3 × 6 replicates): residue export (RE), farmyard manure (FYM) and residue restitution (RR). After 42 years, there are no significant differences in bulk soil organic carbon concentrations between treatments (RE = 9.2 g C kg −1 soil; FYM = 10.4 g C kg −1 soil; RR = 10.1 g C kg −1 soil). In contrast, there are significant differences ( p < 0.05) in stable carbon concentration (C associated to the fine fraction) between treatments over the same time period (RE = 13.2 g C kg −1 clay and fine silt; FYM = 16.6 g C kg −1 clay and fine silt; RR = 15.4 g C kg −1 clay and fine silt). Moreover, we can be 99% confident that stable carbon in the fine fraction increased between 1970 and 2012 in FYM (+19%, p < 0.01) and RR plots (+14%, p < 0.01). There was a small, but significant, change of stable carbon in RE plots over the same period. In 1970, no differences in stable carbon concentration were detected between residue treatments. Labile carbon did not change significantly from 1970 to 2012 but its variability increased for all plots except for the RE treatment. We used the Rothamsted carbon model (RothC-26.3) to describe SOC changes under the different residue treatments. For bulk soil, observed trends in FYM and RR SOC concentrations are in line with the ones predicted. Modeled SOC changes from 1962 to 2012 are −14% (RE) and +10% (FYM). We also used RothC-26.3 to understand the evolution of the sensitive and slow fractions over time. On the one hand, we found that RothC was not capable to simulate the range of observed SOC concentrations inter-annual variability. On the other hand, the increase of the RothC pool with slow decomposition (HUM) was similar to the trend in the carbon associated with the fine fraction observed in the FYM and RR plots. This finding highlights that residue management can increase the storage of C in more stable fractions in agricultural soils, even when no changes are detected in bulk soil C. [ABSTRACT FROM AUTHOR]

Understanding how atmospheric nitrogen (N) deposition affects carbon (C) stabilization in forest soils has become an important focus as humans continue to alter global C and N cycles. Recent reviews have found a positive effect of increased N inputs on C stabilization in soils of temperate forest ecosystems. However, there is still uncertainty about the role and magnitude of the effect of chronic N inputs on forest soil C sequestration and how different tree species can modulate this effect. We evaluated the response of soil C lability to experimental N additions across plots with different dominant tree species (Acer saccharum, Fagus grandifolia, Betula alleghaniensis, Tsuga canadensis and Quercus rubra). We used a 14-year N addition experiment with a single-species, paired-plot design, and several measurements to estimate C lability, including soil laboratory incubations and density fractionation. Our two principal measures of C lability showed statistically significant interactive effects of N treatment and tree species composition: soils from maple (Acer) stands showed the greatest effect of added N on the light fraction mass in the mineral horizon (a 69% increase), and soils from beech (Fagus) stands showed the greatest N effect on potentially mineralizable C (a 23% decrease). Decreases in soil decomposition and respiration rates in organic and mineral horizons in response to N addition across all five species suggest a significant suppression of C mineralization, particularly in the first few weeks of the incubation, with the strongest responses in beech and oak (Quercus) stands. Our results confirm that increased N additions significantly reduce soil organic matter decomposition rates and the lability of soil C for some tree species, and indicate that mechanisms other than organo-mineral associations could play an important role in the stabilization of C in these soils. Further, our research illustrates the need to consider varying responses among different tree species when predicting future consequences of N inputs on soil C storage. [ABSTRACT FROM AUTHOR]

Models of ecosystem carbon (C) balance generally assume a strong relationship between NPP, litter inputs, and soil C accumulation, but there is little direct evidence for such a coupled relationship. Using a unique 50-year detrital manipulation experiment in a mixed deciduous forest and in restored prairie grasslands in Wisconsin, combined with sequential density fractionation, isotopic analysis, and short-term incubation, we examined the effects of detrital inputs and removals on soil C stabilization, destabilization, and quality. Both forested sites showed greater decline in bulk soil C content in litter removal plots (55 and 66 %) compared to increases in litter addition plots (27 and 38 % increase in surface soils compared to controls). No accumulation in the mineral fraction C was observed after 50 years of litter addition of the two forested plots, thus increases in the light density fraction pool drove patterns in total C content. Litter removal across both ecosystem types resulted in a decline in both free light fraction and mineral C content, with an overall 51 % decline in mineral-associated carbon in the intermediate (1.85-2.4 g cm) density pool; isotopic data suggest that it was preferentially younger C that was lost. In contrast to results from other, but younger litter manipulation sites, there was with no evidence of priming even in soils collected after 28 years of treatment. In prairie soils, aboveground litter exclusion had an effect on C levels similar to that of root exclusion, thus we did not see evidence that root-derived C is more critical to soil C sequestration. There was no clear evidence that soil C quality changed in litter addition plots in the forested sites; δC and ΔC values, and incubation estimates of labile C were similar between control and litter addition soils. C quality appeared to change in litter removal plots; soils with litter excluded had ΔC values indicative of longer mean residence times, δC values indicative of loss of fresh plant-derived C, and decreases in all light fraction C pools, although incubation estimates of labile C did not change. In prairie soils, δC values suggest a loss of recent C4-derived soil C in litter removal plots along with significant increases in mean residence time, especially in plots with removal of roots. Our results suggest surface mineral soils may be vulnerable to significant C loss in association with disturbance, land use change, or perhaps even climate change over century-decadal timescales, and also highlight the need for longer-term experimental manipulations to study soil organic matter dynamics. [ABSTRACT FROM AUTHOR]

Aggregate formation is a key process of soil development, which promotes carbon (C) stabilization by hindering decomposition of particulate organic matter (POM) and its interactions with mineral particles. C stabilization processes lead to 13C fractionation and consequently to various δ13C values of soil organic matter (SOM) fractions. Differences in δ13C within the aggregates and fractions may have two reasons: 1) preferential stabilization of organic compounds with light or heavy δ13C and/or 2) stabilization of organic materials after passing one or more microbial utilization cycles, leading to heavier δ13C in remaining C. We hypothesized that: 1) 13C enrichment between the SOM fractions corresponds to successive steps of SOM formation; 2) 13C fractionation (but not the δ13C signature) depends mainly on the transformation steps and not on the C precursors. Consequently, minimal differences between Δ13C of SOM fractions between various ecosystems correspond to maximal probability of the SOM formation pathways. We tested these hypotheses on three soils formed from cover loam during 45 years of growth of coniferous or deciduous forests or arable crops. Organic C pools in large macroaggregates, small macroaggregates, and microaggregates were fractionated sequentially for four density fractions to obtain free POM with ρ < 1.6 g cm−3, occluded POM with two densities (ρ < 1.6 and 1.6-2.0 g cm−3), and mineral fraction (ρ > 2.0 g cm−3). The density fractions were 13C enriched in the order: free POM < light occluded POM < heavy occluded POM < mineral fraction. This, as well as their C/N ratios confirmed that free POM was close to initial plant material, whereas the mineral fraction was the most microbially processed. To evaluate the successive steps of SOM formation, the Δ13C values between δ13C of SOM fractions and δ13C of bulk SOM were calculated. The Δ13C indicated that, parallel with biochemical transformations, the physical disintegration strongly contributed to the formation of free and occluded light POM. In contrast, biochemical transformations were more important than physical disintegration for formation of heavy occluded POM from light occluded POM. This was confirmed by review of 70 Δ13C values from other studies analyzed Δ13C depending on the density of SOM fractions. Accordingly, the successive steps of SOM formation were: fLF<1.6 = oLF<1.6 → oDF1.6-2.0 = MF>2.0. The obtained steps of C stabilization were independent on the initial precursors (litter of coniferous forest, deciduous forest or grasses). To test the second hypothesis, we proposed an extended scheme of C flows between the 3 aggregate size classes and 4 SOM fractions. Δ13C enrichment of the SOM fractions showed that the main direction of C flows within the aggregates and SOM fractions was from the macroaggregate-free POM to the mineral microaggregate fraction. This confirmed the earlier concept of SOM turnover in aggregates, but for the first time quantified the C flows within the aggregates and SOM density fractions based on δ13C values. So, the new 13C natural abundance approach is suitable for analysis of C pathways by SOM formation under steady state without 13C or 14C labeling. [ABSTRACT FROM AUTHOR]

Soils play an important role in carbon cycling. An important approach to terrestrial carbon sequestration is to make use of currently underutilized and waste/degraded lands. The addition of fly ash can ameliorate the adverse conditions of wastelands through a variety of mechanisms, besides helping in stabilization of the soil carbon. Carbon mineralization and humification studies were carried out with soil, fly ash, and their mixtures. Organic monomers included for the humification reactions were resorcinol, p-hydroxybenzoic acid, L-glycine, and L-serine. Results showed that the humification pattern was higher for Associate Cement Companies (0.154 λ485) and Khaparkheda (0.119 λ485) fly ashes, than soil (0.110 λ485). In the carbon mineralization experiment, the soil carbon stabilization enzyme peroxidase activity was higher at soil amended with fly ash (0.052 μM/g/h) than soil alone (0.013 μM/g/h). The dissolved organic carbon was almost two times lower in fly ash amended soil, which revealed the adsorption of carbon in fly ash. The adsorption of soluble organic compounds on the solid surfaces is one of the mechanisms of fixing of soil organic carbon. The cumulative CO2liberation due to plant litter addition was not affected by fly ash. To conclude, fly ash is abundantly available and is considered as a waste in many thermal power plants, which could be sustainably utilized in the agriculture and forestry sectors, both under arable and waste degraded land for enhancing terrestrial carbon sequestration, besides increasing the plant growth and yield. [ABSTRACT FROM PUBLISHER]

A combination of intensive soil manipulation, low water and input efficiencies has resulted in declining soil and crop productivity, and a worsening soil organic carbon (SOC) stock and crop productivity in the eastern Himalayan agro-ecosystem. There is an urgent need to improve the carbon (c) stability with water-cum-energy efficient management for long-term sustainability. Experimental field was practiced under limited tillage with power tiller-based tilling for ten years (2001–2010). This experiment was designed for two years (2011 and 2012) with three tillage practices in the main-plots: i) conventional tillage (CT), ii) minimum tillage (MT), and iii) zero tillage (ZT), while in the sub-plots crop residues mulch (CRM): a) CRM at 4 Mg ha−1 and b) no mulch (NM) was practiced under rainfed conditions. Results show that grain yield of maize improved by 16–31% in CT, whereas rapeseed yield improved by 26–28% with MT as compared to ZT during both the years. CRM improved the yield by 6–11% in maize and 20–25% in rapeseed over NM. Moreover, per plant biomass accumulation improved with CT+CRM by 42–44% in maize and MT+CRM by 10–24% in rapeseed during the both years. Likewise, CT maize and MT rapeseed along with CRM used water was recorded 378–467 and 200–201 mm. Moreover, similar trend was observed on water use efficiency (WUE) by 8.5–10.2 and 4.9–5.4 kg ha−1 mm−1, respectively. In terms of energy, CT and MT along with CRM were more energy exhaustive practices in both the crops; while in ZT recorded higher energy use efficiency (9.3 and 7.7) and productivity (0.7 and 0.4 kg MJ−1) in CT and MT, respectively. Adoption of ZT also increased bulk density (ρb) by 3% over CT. The ZT along with CRM improved C stability by 6 folds than CT and 58% over NM. Considering all these facts, CT/MT along with CRM may be recommended to obtain higher yield, WUE, energy productivity and C stability in (Zea mays L.)- rapeseed (Brassica campestris L.) system in the eastern Himalayan agro-ecosystem. • Higher yield obtained under CT with CRM in maize and MT with CRM in rapeseed. • Water use efficiency enhanced under CT in maize and MT in rapeseed coupled with CRM. • ZT and CRM augmented energy parameters, SOC and C stabilization. [ABSTRACT FROM AUTHOR]

• The C accumulation in soil particles was assessed by density-and size-fractionation. • High C input led to saturation of mineral-associated C pool under long-term no-till. • The C saturation deficit decreased to 20 cm depth, but mostly in the top 5 cm soil. • Beyond saturation of mineral-associated, C accumulation continues in particulate fraction. • The potential C stabilization in long-term no-till remains mainly at deep layers. The capacity of soils to stabilize carbon (C) may decrease over time, limiting the potential of no-till soil to act as a C sink in the long-term. Our objectives were to evaluate the effects of long-term no-till cropping systems on (i) C storage in soil, (ii) C stabilization in the fine silt plus clay-size (<20 μm) fraction and its relationship with the decrease of C saturation deficit (CSD) in this fraction, and (iii) on C accumulation in labile fractions of soil organic matter (SOM) in 0–2.5, 2.5–5, 5–10 and 10–20 cm layers of a subtropical Acrisol. The study was based on a long-term (36 years) no-till experiment where five cropping systems, with variable annual C inputs, were assessed: [i] bare-soil, [ii] black oat/maize, [iii] black oat + vetch/maize + cowpea, [iv] lablab + maize and [v] pigeon pea + maize. Cropping systems including maize and tropical legumes (lablab, pigeon pea and cowpea) with high C input led to the highest C storage in the top layers (up to 10 cm depth) of this no-till soil. Also, a decrease of CSD in fine silt plus clay-size fraction was observed in all soil layers to 20 cm depth, but the most expressive impact on CSD occurred in the topsoil (0–2.5 cm), where the capacity to further stabilize more carbon decreased by 90–97% when compared to bare soil. Considering the full C saturation level of the silt plus clay-size fraction and the current C contents in the soil, the remaining capacity of C sequestration up to 20 cm was estimated as ranging from 22.5 to 32.8 Mg C ha−1, and much of it (58–75%) was in the 10–20 cm layer. Our results highlight the importance of diversified cropping systems with high input (quantity and quality) crop residues to C sequestration in soil. Moreover, although the mineral-associated SOM of the top layer reached a C stabilization limit, C accumulation continues in non-saturated labile fractions, and in non-saturated fine silt plus clay-size fraction in deeper layers of subtropical no-till soils. [ABSTRACT FROM AUTHOR]

The interactions of organic carbon (OC) with ferric iron (Fe) oxides play an important role in OC stabilization, but reductive dissolution of Fe(III) oxides under anoxic conditions potentially compromises the stabilization of Fe-bound OC (Fe-OC). Up to date, there are very limited studies on the fate of Fe-OC in sulfidic conditions commonly encountered in OC-rich marine sediments. Herein, multi-pronged studies were implemented to quantitatively characterize the capacity of metastable iron sulfide (FeS) to sequester OC extracted from marine sediments and to unravel the fate of ferrihydrite-bound OC complexes (Fh-OCs) when exposed to dissolved sulfide. Results show that, for a given mechanism of OC association (adsorption or coprecipitation), FeS has DOC retention capacity generally comparable to Fh, implying that the metastable phase could serve as an important OC sink in anoxic marine sediments. During reduction of Fh-OCs by dissolved sulfide, Fh reduction was progressively retarded with increasing OC pre-adsorption, while Fh reduction was promoted after DOC pre-coprecipitation. Despite high extents of Fe reduction by dissolved sulfide, OC release from Fh-OC coprecipitates was low due to efficient re-sequestration of released DOC by coprecipitation with FeS, and an increase in OC pre-loading could facilitate Fe2+ sulfidization and thus re-sequestration of released DOC by FeS. When Fh-OC adsorption complexes were exposed to dissolved sulfide, substantial OC release was restricted to the complexes with lower OC pre-adsorption (C/Fe ≤ 1.5). Overall, reactive Fe reduction by dissolved sulfide may not lead to substantial release of associated OC as long as reduced Fe could be sulfidized to efficiently re-sequester concomitantly released DOC. Results of carbon-1 s near-edge X-ray absorption fine structure spectroscopy indicate that, upon reduction of Fh-OCs by dissolved sulfide and subsequent FeS formation, carboxyl-C was selectively released into the solution but aromatic-C selectively remained on the solid matrixes after Fe reduction, suggesting that OC sink switching from Fe oxides to Fe sulfides could result in molecular fractionations of porewater DOC to some extent, with potential implications for OC biogeochemical cycling. Ferrihydrite reduction by dissolved sulfide and concomitant release of bound DOC, FeS formation, and re-sequestration of DOC by coprecipitation with FeS. [Display omitted] [ABSTRACT FROM AUTHOR]

Serious concerns about carbon (C) sequestration capacity and the stabilization of sequestered C in forested soils have emerged in the context of global climate change. The organic C in soil and in soil fractions at four sampling times in Acacia auriculiformis plantations afforested in 1991 were investigated with a combination of density fractionation and acid hydrolysis techniques. The results showed that the accumulation of C in the forested soils had accelerated because the afforestation of wasteland with A. auriculiformis. The C accumulation rates of the surface and subsurface soils averaged 0.38 and 0.17 t C/ha/yr, respectively, during the 19 yr following the afforestation. The percentage of organic C in heavy fraction relative to total soil organic C at the surface soil was 71% in 2003. This value was significantly ( P < 0.05) higher than that in 2010 (68%). The chemical recalcitrant C index of light fraction was significantly ( P < 0.05) higher than that of heavy fraction in 2003 regardless of soil depth, but both decreased with time. ca. 58-68% of the newly sequestered C was protected by physical mechanism, and 41-50% was transferred into the acid nonhydrolysable fraction during the 12-19 yr after the trees were planted. The chemical stability of the physically protected C remained lower than that of the unprotected C following the afforestation in the valley-type savannah. However, both the stability values showed a decline with time. [ABSTRACT FROM AUTHOR]

Abstract: Soils that exhibit soil organic carbon (SOC) saturation provide an opportunity to examine mechanisms of C storage in soils with increasingly limited C-stabilization potential. A manure rate experiment in Lethbridge, Alberta, in which SOC responded asymptotically to long-term manure C additions, allowed us to assess changes in SOC biochemical composition in response to soil C saturation. By quantifying the cupric oxide oxidation products of lignin, cutin, and suberin in fractionated SOC pools that are characterized by chemical (i.e., mineral-associated), physical (i.e., microaggregate-associated), or no protection (i.e., free particulate organic matter), we evaluated the interaction between C saturation and the biochemical characteristics of SOC. We tested the specific responses of soil fraction lignin, cutin, and suberin to C saturation level by using the bulk soil to approximate C-input composition across manure input treatments. Carbon-normalized lignin (lignin-VSC/OC) in the chemically protected fractions did not differ, while in the non-protected and physically protected soil fractions, it decreased with C saturation level. Neither the stabilization of cutin and suberin, nor the lignin:cutin + suberin ratio, differed in any of the measured soil fractions in response to C saturation level. These results indicate that with C saturation and decreased C stabilization potential, lignin, cutin, or suberin were not preferentially stabilized or depleted in mineral protected soil C pools. The lack of evidence for biochemical preference in mineral associations with C saturation supports the existence of an outer kinetic zone of organomineral associations, in which partitioning of organic compounds, rather than sorption, controls mineral SOC accumulation at high SOC loadings. Furthermore, despite theories of inherent lignin recalcitrance, depleted lignin concentrations with C saturation in the non-protected and aggregate protected fractions indicate that lignin was, in this study, preferentially decomposed when not protected by association with mineral phases in the soil. In conclusion, C-input quantity, and not quality, combined with physical and chemical protection mechanisms that govern long-term C storage, appeared to control C saturation and stabilization at this site. [Copyright &y& Elsevier]

The number of studies focused on the transformation and sequestration of soil organic carbon (C) has dramatically increased in recent years due to growing interest in understanding the global C cycle. While it is readily accepted that terrestrial C dynamics are heavily influenced by the catabolic and anabolic activities of microorganisms, the incorporation of microbial biomass components into stable soil C pools (via microbial living cells and necromass) has received less attention. Nevertheless, microbial-derived C inputs to soils are now increasingly recognized as playing a far greater role in stabilization of soil organic matter than previously believed. Our understanding, however, is limited by the difficulties associated with studying microbial turnover in soils. Here, we describe the use of an Absorbing Markov Chain (AMC) to model the dynamics of soil C transformations among three microbial states: living microbial biomass, microbial necromass, and C removed from living and dead microbial sources. We find that AMC provides a powerful quantitative approach that allows prediction of how C will be distributed among these three states, and how long it will take for the entire amount of initial C to pass through the biomass and necromass pools and be moved into atmosphere. Further, assuming constant C inputs to the model, we can predict how C is eventually distributed, along with how much C sequestrated in soil is microbial-derived. Our work represents a first step in attempting to quantify the flow of C through microbial pathways, and has the potential to increase our understanding of the microbial role in soil C dynamics. [ABSTRACT FROM AUTHOR]

Abstract: Soils contain an abundance of nano-size particles. Because of their tendency to aggregate and associate with organic colloids, however, soil nanoparticles are difficult to obtain and characterize. Here we report on a simple and rapid method of extracting mesoporous nanomaterials from the clay fraction of an Andisol with narrow size distribution. The clay and nanoclay were characterized by elemental analysis, pyrolysis GC/MS, electron and atomic force microscopy, infrared spectroscopy, and electrophoresis. The nanoclay dominantly consists of hollow allophane spherules forming globular aggregates of about 100nm in diameter. The nanoclay contains more organic matter (carbon and nitrogen) with a larger proportion of polysaccharides and nitrogen containing compounds, and has a lower isoelectric point, than the clay. Treatment with hydrogen peroxide causes a large decrease in the organic matter contents of both nanoclay and clay. The aggregates of allophane nanoparticles retain a significant amount (~12%) of carbon against intensive peroxide treatment. Thus, besides playing an important role in carbon stabilization, these naturally occurring nanomaterials are potentially useful for developing a low-cost carbon sequestration technology. [Copyright &y& Elsevier]

Boreal forests, containing >20% of the total organic carbon (OC) present at the surface of the Earth, are expected to be highly vulnerable to global warming. The objective of this study was to compare soil OC stocks and chemistry in jack pine stands located along a latitudinal climatic transect in central Canada. Total OC stocks (0-1 m) increased with decreasing mean annual temperature (MAT). We used a combination of physical fractionation of soil OC pools, C isotopic determination and cross-polarization, magic-angle spinning C nuclear magnetic resonance (NMR) spectroscopy to further characterize OC composition at all sites. Soil OC was dominated by labile pools. As illustrated by the C/N ratios, δC data and results from the C NMR analysis, the light fraction showed little alteration within the soil profiles. Instead, this fraction reflected the importance of fresh litter inputs and showed an increase in root contribution with depth. As opposed to the light fraction, the clay- and silt-stabilized OC exhibited an increase in δC and a decrease in C/N with depth, indicating an increase in its degree of decomposition. These changes with depth were more marked at the southern than the northern sites. Results hence suggest that if the MAT were to increase in the northern boreal forest the overall jack pine soil OC stocks would decrease but the remaining OC would become more decomposed, and likely more stabilized than what is currently present within the soils. [ABSTRACT FROM AUTHOR]

Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary. [ABSTRACT FROM AUTHOR]

Abstract: Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite–citrate–bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium–oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides. [Copyright &y& Elsevier]

Abstract: The invasive earthworm (Pontoscolex corethrurus) is commonly found in rubber plantations of Xishuangbanna, southwestern China. To understand the long-term impact of this invasive earthworm on soil labile organic carbon (LOC), we examined changes in LOC that was protected and unprotected by soil aggregates during a worm-exclusion experiment in rubber plantations of Xishuangbanna. We found that the presence of this invasive earthworm for 20 months increased LOC (up to 35%) protected by aggregates in surface soil layer (0–5 cm). In contrast, the presence of this earthworm increased LOC unprotected by the aggregates and the turnover rate of LOC protected by the aggregates in subsurface soil layer (5–15 cm). Soil total LOC did not differ between the control and worm-exclusion treatments. These findings suggest that the invasion of P. corethrurus can redistribute LOC along soil vertical profiles with accumulation of protected LOC on surface soil layer and unprotected LOC on subsurface soil layer. Earthworms'' redistribution of LOC between the protected and unprotected forms and along soil profile may affect long-term soil carbon cycling. [Copyright &y& Elsevier]