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Physical soil organic matter (SOM) fractions provide increased insight into the biogeochemical functioning of soils. Several fractionation methodologies have been developed to separate particulate (POM) from mineral-associated organic matter (MAOM) fractions either by particle size, particle density, or some combination of these two properties. The proliferation of approaches to separation has led to ambiguity regarding what these methodologically defined fractions should be understood to conceptually represent, hindering robust data synthesis analyses and model development. Here we attempt to identify chemical similarities and differences amongst POM and MAOM fractions separated by different physical approaches, with the aim of guiding fractionation choices for future research. We obtained soils from 11 farms across the United States with variable parent materials, textures, and pHs, and fractionated them using 4 methods. We tested two single-step methods: a size fractionation (53 µm cutoff) and a density separation (1.85 g cm−3 cutoff), and two multi-step methods: a combined size and density separation after full dispersion, which isolates a third pool of organic matter associated with coarse, heavy particles (coarse heavy associated organic matter; CHAOM), and a combined method with delayed dispersion to separate the free POM from the occluded POM + CHAOM fraction. We analyzed all fractions for C and N concentrations, their isotopic composition, and for their chemical composition via mid-infrared spectroscopy. We found that MAOM fractions tended to be very homogenous in character regardless of fractionation schemes, while POM varied widely. In particular, to POM isolated by density floatation was distinct in both isotopic and spectroscopic signature compared to POM isolated by size fractionation alone. Indeed, our results indicated that POM isolated by size is a composite fraction of light POM and CHAOM, and does not map well onto a conceptual pool. We found the CHAOM fraction to be more similar to MAOM in terms of chemical and spectroscopic composition, indicating that one-step density separations may be the most effective means of isolating contrasting SOM pools in a time and cost-efficient matter. However, our analysis highlights that POM, CHAOM and MAOM exhibit distinct features, and the three fractions are worth separating via a combined size-density fractionation when possible.

To investigate the stabilization mechanism of microbial necromass carbon (C) in rice paddy soil, the accumulation of microbial necromass and its contribution to SOC in bulk soil and in various fractions were addressed under a 40-year fertilization trial. The fertilization regimes included no fertilizer (Control), nitrogen, phosphorus and potassium inorganic fertilizer (NPK), and inorganic fertilizer plus manure (NPKM). Coarse particulate organic matter (cPOM), fine inter-microaggregate POM (fPOM), intra-microaggregate POM (iPOM), non-occluded silt plus clay fraction (s + c_f), and silt plus clay occluded within microaggregates (s + c_m) fractions were isolated. The results indicated that fungal necromass were the main component of microbial necromass across the fractions; however, the bacterial role was enhanced in mineral fractions (s + c_f and s + c_m) compared to POM fractions (cPOM, fPOM, and iPOM). The iPOM and mineral fractions stored more than 90 % of microbial necromass C due to the physical protection (organo-mineral associations and occlusion within microaggregates). Compared to Control, the significantly increased content of microbial necromass C and its contribution to SOC in bulk soil were attributed to the enhanced content of fungal necromass in iPOM fraction under NPK treatment. The highest content of microbial necromass C was found under NPKM treatment; moreover, the increased content of microbial necromass C in iPOM and s + c_m fractions made up to 82.80 % of that in bulk soil compared to Control, while a saturation behavior was found in s + c_f fraction. The contribution of microbial necromass to SOC was not altered under NPKM treatment compared to Control. However, a reduced fungal/bacterial necromass C ratio was found in bulk soil and iPOM fraction under NPKM treatment compared to Control, mainly due to the supply of higher quality substrates and the increased soil pH. As a whole, this study revealed the diverse responses of microbial necromass C accumulation and its contribution to SOC under different fertilization regimes in paddy soil.

As atmospheric carbon (C) concentrations increase, so too has interest in understanding the mechanisms that preserve C within the soil organic matter (SOM). Mineral-associated organic matter (MAOM) is one pool of SOM recently shown to protect soil organic C from mineralization. However, most MAOM research has been in agriculture and forest mineral soils. Given the magnitude of soil C stored in wetlands, this study sought to determine the abundance of MAOM in wetlands. A standard method for quantifying MAOM in terrestrial soils was tested, modified, and optimized with three unique wetland soils. Using a physical fractionation method followed by a density fractionation, it was concluded that field moist soils and a dispersant were necessary for fractionating wetland soils (0–50 cm). The Bayhead Swamp soil had the greatest total C (484.07 ± 1.25 mg C g soil -1), but the smallest MAOM pool (0.63 %); most of the C was in the light fraction as particulate organic matter (POM, >98 %). The sandy-peat Cypress Dome soil also lacked MAOM (1.09 %) and was dominated by POM (66.26 %). The silt-rich Brackish Marsh had the least total C (94.37 ± 1.67 mg C g soil -1), but the largest fraction of protected C (24.60 % MAOM). Nitrogen (N) followed similar trends to C, except in the Brackish Marsh where nearly half the total N was MAOM. This research represents the first step in evaluating the role of MAOM in C persistence in organic-rich wetland soils; future work should consider C physical fraction, in addition to C quantity.

The long-term stabilization of organic matter in mineral soils is widely believed to be mainly driven by its interaction with the mineral matrix rather than by its recalcitrance. This interaction involves several processes, and the relative importance of each one may vary according to the ecosystem type. Here we investigate organo-mineral interactions in soils across different ecosystems (coniferous and broadleaved forests, grasslands, and technosols), widely differing in their soil organic matter (SOM) content, by an approach involving two consecutive steps: (i) a size fractionation by wet sieving after sonication, to isolate the particulate organic matter (POM, >20 μm) from the organo-mineral complex (OMC, <20 μm), and (ii) a characterization of the OMC by sequential extractions with different chemicals, each one disrupting a specific kind of bond between SOM and active mineral surfaces. The OMC accounted for >60% of total organic C and >75% of total N contents. Differences among ecosystems affected the total amount of SOM, but barely its distribution between POM and OMC. The sequential extractions showed that a substantial amount of the organic matter in the OMC (20–30%) was weakly bound to minerals. Other components of the OMC, in contrast, were much less dominant than expected, i.e., the organic matter stabilized by carbonate coating (5–6% of total organic C in the OMC) and by iron oxyhydroxydes (3–6%). By far, the main pool of the OMC was the final residue remaining after all extractions (34–48% of total organic C in the OMC). Because this residue remains unextractable with NaOH after removing all the active mineral components, such unextractability can be due only to its own chemical characteristics. Future research should aim at its in-depth characterization and at elucidating its ecological functions. • Size fractionation was coupled with an extensive set of chemical extractions. • The distribution of organic C and N among size fractions is little affected by land use. • Irrespective of land use, the organo-mineral complex (OMC) accounts for most of TOC and N. • A substantial part of the organic matter in the OMC is bound loosely. • For all land uses, the highest organic C contents are found in humin. [ABSTRACT FROM AUTHOR]

Soils on riverine floodplains in Central Europe are commonly enriched in soil organic matter (SOM). We analyzed the quantity and qualitative aspects of SOM in three soil profiles with mollic horizons along the Elbe River (Germany) after physical fractionation by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to gain deeper insights into the composition of SOM and the formation of these soils. In all horizons, the majority of SOM was recovered in the fraction of particulate OM and in that of sand and aggregates, while SOM in the silt and clay fraction always made up < 20% of soil organic carbon. The C:N ratios of most fractions did not decrease with depth, indicating less decomposed or pyrogenic OM or both. The presence of pyrogenic OM, as a characteristic feature of floodplain soils, was indicated in almost all physical fractions. As derived from DRIFT spectroscopy, SOM was less qualitatively differentiated among the fractions in the soil with the longest duration of flooding, while qualitative differences of SOM were more pronounced in the more aerated ones. The soils have developed from stratified fluviatile sediments and fulfil the criteria of a mollic horizon, irrespective of differences in the composition of SOM. From a soil-genetic point of view, we strongly suggest to classify these soils as Mollic Fluvisols instead of Fluvic Phaeozems, which would accord to the latest WRB classification. [ABSTRACT FROM AUTHOR]

• Chemical and thermal analyses of fractions from 75 soils were cross-correlated. • Observed strong correlations between broad thermal regions and chemical types. • EGA data generated stronger correlations with NMR than did DSC. • Demineralization treatments resulted in substantial differences in correlations. • Bioenergetics may be better approach than chemical composition for SOM reactivity. While many advanced analytical methods have been applied to soil organic matter (SOM), its highly complex and heterogeneous chemical composition still eludes complete characterization. Analytical thermal analysis has been proposed as a relatively rapid, inexpensive method for SOM characterization that requires no pre-treatment, but is challenging due to a lack of direct information about chemical composition. The goal of this study was to inform the interpretation of coupled differential scanning calorimetry and evolved gas analyses (DSC, CO2-EGA) using spectral correlations with solid phase 13C NMR data. We used a subset of soils collected as part of the Australian National Soil Carbon Research Program (SCaRP), which were physically fractionated and characterized using conventional analytical methods. Correlating the well-understood NMR spectra with the less-understood DSC and CO2-EGA thermograms provided some indications of which chemical compounds combust at which temperatures. Overall, the EGA data generated stronger correlations compared to correlations with DSC data, which was attributable to greater variability in DSC data due thermal reactions associated with minerals. Direct comparison of NMR and thermal data for the mineral associated organic matter in the fine (<50 µm) fraction was not possible due to the need to demineralize samples prior to NMR analyses. Thermal analyses showed substantial differences in samples pre- and post-HF pretreatment, and the NMR data for HF treated samples showed scattered and weak correlation patterns with DSC and EGA data for untreated samples. While precise chemical compositions cannot be gleaned directly from thermal analyses results, thermal approaches provide an avenue of investigation into SOM reactivity based on bioenergetics that may be a quantitative representation of SOM persistence. [ABSTRACT FROM AUTHOR]

• Bulk SOC increased under long-term organic fertilization. • No SOC accumulated in the MAOM fraction (<6.3 µm) over a 36-year period. • Bulk SOC increase under organic fertilization was solely attributed to the labile POM fractions. • oPOM-C contents are highly dynamic and depend on farming system. Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO 2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0–20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state 13C CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (<6.3 µm) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned. [ABSTRACT FROM AUTHOR]

• We investigated Fe speciation in fine sand (FSa) and fine silt plus clay (FSi + Cl). • The nature of Fe-SOM interactions vary with soil particle size and land use. • FSa fractions were characterized by more crystalline Fe phases, including goethite. • FSi + Cl fractions exhibit the presence of ferrihydrite. • SOM in the FSi + Cl fraction is more thermodynamically stable than in FSa. Iron (Fe) (oxyhydr)oxides represent a significant phase for the organic carbon (OC) stabilization. Due to their high surface areas, short-range-ordered Fe minerals, like ferrihydrite, show a higher ability to stabilize OC than crystalline secondary minerals, like lepidocrocite, goethite, and magnetite. However, how Fe phases and their crystallinity relate to soil organic matter (SOM) composition is still not completely known. We investigated Fe solid phase speciation in two soil particle size fractions (i.e., fine sand — FSa — and fine silt and clay — FSi + Cl —), isolated from coniferous (CF) and broadleaved forest soils (BF), grassland soils (GL), and technosols (TS). All samples were characterized by Fe K-edge extended X-ray absorption fine structure (EXAFS) and X-ray diffraction, and a subset by 57Fe Mössbauer spectroscopy. Further, ramped combustion thermal analyses (coupled differential scanning calorimetry (DSC) and CO 2 evolved gas analysis (CO 2 -EGA)) were used to evaluated SOM stability. With the only exception of TS, goethite was the main Fe phase in FSa fractions, whereas less crystalline phases (i.e., ferrihydrite, based on EXAFS) dominated the FSi + Cl fractions. The proportion of goethite and ferrihydrite in both fractions decreased with increasing OC content, while that of Fe(III)-SOM in FSa increased with increasing OC content. Mössbauer and EXAFS data clearly indicated a presence of hematite in GL soils. Our data suggest that more crystalline Fe forms, like goethite and hematite, may be important for OC abundance in the FSa fraction and in soils with high OC contents, like GL. Thermal analysis showed the dominance of mineral associated organic matter in low-OC soils, and of plant residues in high-OC soils. As a whole, we posit that the FSi + Cl fractions contain Fe phases of less crystallinity because of presumed association with SOM, and that SOM in the FSi + Cl fraction is also more thermodynamically stable than in FSa, although differences are observed across land uses. Our observations suggest that the nature of Fe-SOM interactions can vary substantially with soil particle size and land use, which has important implication for SOM persistence. [ABSTRACT FROM AUTHOR]

• We used stable isotope tracing to quantify litter decomposition and SOM formation. • After 13 months, C and N losses from the litter layer were 24% higher under irrigation. • Irrigation did not affect the amount of litter-derived C and N recovered in SOM. • We found more litter-derived C and N in macroaggregates of the irrigated treatment. • Irrigation had a larger effect on litter losses than on the formation of new SOM. In semiarid agricultural systems, irrigation may increase soil organic matter (SOM) due to higher biomass production. However, aboveground production tends to increase more than belowground biomass, and in no-till systems SOM responses to irrigation would strongly depend on the fate of crop residues left on the soil surface. Litter decomposition and SOM formation can also be affected by irrigation due to changes in water dynamics, but little is known about the impacts of irrigation on the pathways of SOM formation. We used dual 13C and 15N labeled maize litter to quantify the effect of irrigation on the contribution of aboveground crop residues to SOM formation in semiarid, no-till agricultural systems. We incubated the litter in a continuous maize cropping system with two treatments, dryland and irrigated, and measured litter decomposition and its contribution to different SOM pools. Irrigation had a larger effect on litter C and N losses than on the formation of new SOM. After 13 months the C and N losses from the litter layer were 24% higher under irrigation, but there were no treatment differences in the litter-derived C and N recovered in SOM. Most of the litter-derived organic matter (OM) was found in the mineral associated OM pool (MAOM), but irrigation did not affect the amount of litter-derived OM found as MAOM or as particulate organic matter. Irrigation increased the amount of litter-derived OM found in macroaggregates, and this observation was more related to a higher aggregation than to aggregate enrichment in the irrigated treatment. Our results suggest that a smaller proportion of aboveground crop residues will form SOM in irrigated systems compared to dryland conditions and may help to partially explain why irrigation has a stronger relative effect on crop productivity and C inputs than on SOC stocks in semiarid agricultural systems. [ABSTRACT FROM AUTHOR]

Abstract: A better understanding of the effects of climate and management on long-term soil organic carbon (SOC) sequestration necessitates separation of the stable from the labile SOC fraction. An array of physical and chemical methods is currently used for the separation of SOC fractions, but these methods have not been subjected to a comparative evaluation to understand their effectiveness. Therefore, the present study was conducted with the objective to evaluate the pool size and 13C isotopic signatures of stable SOC fractions derived from physical and chemical approaches applied separately and in combination on a Typic Fragiudalf of Ohio, USA. Five methods were evaluated: recovery of silt-associated SOC by physical method (method 1a), clay-associated SOC by physical method (method 1b), bulk soil-associated SOC by chemical method (method 2), chemically stable silt-associated SOC by combined physical and chemical methods (method 3a), and chemically stable clay-associated SOC by combined physical and chemical methods (method 3b). The corn-derived fraction of the stable SOC pool was calculated using 13C natural abundance technique because there was a clear vegetation shift from C3 (forest) to C4 species (corn, Zea mays L.) at the study site. Across land use and soil depth increments, the pool sizes of stable SOC isolated by chemical treatment, individually and in combination with physical method (methods 2, 3a, and 3b), were substantially lower (1.5% to 29.4% of total SOC) than that of standalone physical method (methods 1a and 1b) (17.3% to 64.8% of total SOC). However, among the chemical methods (method 2, 3a and 3b), the pool size of chemically resistant SOC was generally comparable. In the surface layer, the proportion of young, corn-derived SOC was lower after chemical treatment than after individual physical method. But, the chemically stabilized SOC in subsoil showed an enrichment of 13C signatures and increased proportion of C4–C than that of surface soil, and it is mostly similar to that derived from physical methods. The data obtained from this study indicate that integrating physical and chemical methods is effective in isolating the SOC fraction with greatest stability in the surface soil layer, but its effectiveness to separating the stable SOC fractions in the subsoil was comparable to individual physical and chemical methods. [Copyright &y& Elsevier]

An investigation was conducted on the amount and rate of soil organic matter (SOM) turnover, as influenced by agricultural operations in ultisols in Argentina. The decrease of particulate organic carbon (POC) and decomposition or relative stability of its free (POCf) and occluded (POCo) fraction were determined at sites under a forest, yerba mate (Ilex paraguariensis St. Hill), and in a yerba mate plantation using elephant grass as cover crop. Aggregate stability was measured by the dry sieving method for 5 min, using four sieves of different sizes and a Retsch stirrer. Wet sieving was performed to separate the mineral-associated organic matter (MOC) from the POC. The effect of 50 years of yerba mate cultivation resulted in a significant decrease of total POC. The highest POC were noted in the microaggregate (0.1–0.25 mm) fractions in the surface layers of the three ecosystems. The POCo concentration was higher than the POCf content in the soil within the plant rows and between the rows. This suggested that POCo was more resistant than POCf to decomposition, due to protection within the microaggregates. The few years under elephant grass has resulted in a significant increase in POC, probably due to the relatively slow rate of decomposition of fresh residues, characterized by high dry matter input and C/N ratios. Elephant grass-derived POC is believed to play an important role in the nutrient cycling of the degraded yerba mate soils. [Copyright &y& Elsevier]

[Display omitted] • The four most widely used lab methods for soil organic matter (OM) were compared. • Not only the fine earth but the silt-plus-clay was studied. • How the conversion coefficients between methods are related was revealed. • The conversion coefficients depend on OM quality chemical characteristics. • These OM characteristics are related to its stoichiometry and oxidation resistance. Nowadays, the continuous organic carbon (OC) assessment in soil and its various particle-size fractions is needed to correctly estimate the soil organic matter (OM) contents and dynamics. However, the existence of several widely used analytical methods for OC and OM determination hinders the comparison of OC and OM data taken by different laboratories, in different times, soil classes, horizons and particle-size fractions. Although these methods are usually related by means of empirical soil-dependent factors, how these coefficients vary among soils is seldom addressed. In the present work 67 samples from the A horizons (0 to 20–40 cm depth) of 58 forest and agricultural calcareous soils from the Valencia province (Eastern Spain) were taken, the silt-plus-clay separated, and both the fine earth and the silt-plus-clay analysed through the wet dichromate self-heated and externally heated oxidations, the dry combustion with CO 2 measurement and the loss-on-ignition. As a consequence, the readily oxidizable carbon (RXC), the total oxidizable carbon (TXC), OC and OM were obtained, respectively. Furthermore, the coefficients to convert among these properties, namely, the Walkley-Black factor (f WB), the oxidation recovery factor (f XR), the carbon valence in the organic matter (v C,OM), and the van Bemmelen (f VB) factor were assessed. The RXC in both fine earth and silt-plus-clay was 75% of the OC thus supporting a common f WB of 1.33. However, the OM in the fine earth presented less f XR than the silt-plus-clay. This apparent inconsistency between f WB and f XR was caused by the different v C,OM in the fine earth (3.45) and the silt-plus-clay (4.56) and hence, the different oxidation state of the OC in each fraction. This was revealed by how f WB depends on f XR and v C,OM through f WB = 4 f XR / v C,OM. Therefore, the different v C,OM exactly compensated for the different f XR in each fraction to give the same f WB for both. Besides, the different v C,OM in each fraction was consistent with the fact that carbon accounted for 61% and 71% of the OM mass in, respectively, the fine earth and the silt-plus-clay, thus supporting the use of a van Bemmelen factor below the standard of 1.72 and different, i.e., 1.64 and 1.41, for each particle-size fraction. This fact can be understood taking into account that v C,OM linearly depends on f VB through v C,OM = 4 – α – β f VB , where α and β are two empirical coefficients. Therefore, it has been shown how the conversion coefficients among the RXC, TXC, OC and OM depend on two independent chemical characteristics, one OM-stoichiometry-related, the carbon mass fraction, and another OM-reactivity-related, the oxidation recovery. As a consequence, it will be better understood that the determinations of OC and OM in soils, remarkably when the Walkley-Black method is used, vary not only because of changes in OC and OM magnitude, but also because of changes in OM stoichiometry and reactivity. The differences in the soil OC and OM data obtained by using different laboratory methods, and in different times, particle-size fractions, soil depths, classes, etc., will be thus better understood. [ABSTRACT FROM AUTHOR]

• After six months litter C was found in all soil fractions. • Similar amounts of litter C in the mineral fraction at six months and five years. • Sand content and precipitation best predict litter C formed in mineral fraction. • We found a positive feedback between new litter-derived SOM and soil C content. Understanding the mechanisms controlling the formation and persistence of soil organic matter (SOM) is important for managing soil health and sustainable food production. The formation of SOM and the degree to which it is protected from decomposition are important for determining the long-term persistence of SOM. We used soils collected in a 13C-labelled litter decomposition study established at agricultural sites in Canada to understand the formation and persistence of newly-formed SOM. The ten agricultural sites spanned a wide range of soil carbon contents, texture, and climatic conditions. We fractionated the soil to isolate water extractable organic matter (WEOM), free light POM (fPOM), sand-sized and occluded particulate organic matter (oPOM), and silt and clay sized particles, referred to as mineral-associated organic matter (MAOM). Quantitative isotope tracing was used to determine the litter-derived C in all fractions. We performed these analyses early (six months after incubation) and later (five years after incubation) in the decomposition process to evaluate factors that control the formation and persistence of POM and MAOM. After six months litter-derived C was found in all fractions, but after five years it had declined in all fractions except the MAOM. Formation of MAOM was related to high mean annual precipitation and low sand content, whereas occluded POM formation was related to high soil C content. Persistence of MAOM and POM during the incubation were associated with low soil temperature and high soil C content. There was no consistent indication that formation of MAOM occurred from the decomposition of POM, suggesting that MAOM and POM are formed by two separate pathways. This has important implications for SOC models, which assume that plant-derived C passes through a sequence of pools, becoming more stable along the way. [ABSTRACT FROM AUTHOR]