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Soil organic matter (SOM) is an important natural resource. It is fundamental to soil and ecosystem functions across a wide range of scales, from site-specific soil fertility and water holding capacity to global biogeochemical cycling. It is also a highly complex material that is sensitive to direct and indirect human impacts. In SOM research, simulation models play an important role by providing a mathematical framework to integrate, examine, and test the understanding of SOM dynamics. Simulation models of SOM are also increasingly used in more 'applied' settings to evaluate human impacts on ecosystem function, and to manage SOM for greenhouse gas mitigation, improved soil health, and sustainable use as a natural resource. Within this context, there is a need to maintain a robust connection between scientific developments in SOM modeling approaches and SOM model applications. This need forms the basis of this review. In this review we first provide an overview of SOM modeling, focusing on SOM theory, data-model integration, and model development as evidenced by a quantitative review of SOM literature. Second, we present the landscape of SOM model applications, focusing on examples in climate change policy. We conclude by discussing five areas of recent developments in SOM modeling including: (1) microbial roles in SOM stabilization; (2) modeling SOM saturation kinetics; (3) temperature controls on decomposition; (4) SOM dynamics in deep soil layers; and (5) SOM representation in earth system models. Our aim is to comprehensively connect SOM model development to its applications, revealing knowledge gaps in need of focused interdisciplinary attention and exposing pitfalls that, if avoided, can lead to best use of SOM models to support policy initiatives and sustainable land management solutions.

PurposeFine roots are an important source of soil organic matter (SOM); however, it is unclear whether the cation exchange capacity (CEC) of leaf-derived and root-derived organic matter is comparable. We previously found that in surface soils of Cryptomeria japonica stands with low acid buffering capacity (ABC), fine-root biomass and total carbon (TC) content were higher than in high-ABC soils, but the CEC of the two soil types was almost the same. This result was unexpected, because TC is generally related to CEC. This study aimed to clarify why CEC was not enhanced in the low-ABC soils.MethodsSurface soils at eleven C. japonica stands with contrasting ABCs were separated into different density fractions: light (LF, containing unstabilized SOM), middle (MF, containing stabilized SOM), and heavy (HF, mainly minerals). The C composition of LF, MF, and tree organs was investigated by solid-state 13C NMR analysis.ResultsThe carboxyl C content of MF, which had a significant positive correlation with CEC, did not differ significantly between the low- and high-ABC soils, even though the alkyl/O-alkyl ratio suggested more SOM decomposition in the low-ABC soils. This ratio had a significant negative correlation with soil pH (H2O) in both LF and MF and a positive correlation with fine-root biomass in MF. Furthermore, aromatic C was preferentially decomposed in the low-ABC soils.ConclusionIn the low-ABC soils, the increase of root litter input did not increase the carboxyl group content in MF, and thus did not increase its CEC, owing to increased SOM degradation.

In this study, we analyzed the influence of soil mineral characteristics (e.g., clay concentration and mineralogical composition, iron and aluminum oxide concentration and crystallinity, specific surface area, and exchangeable cation concentration) on (i) organic carbon (OC) content (kg m−2) and (ii) the concentration (g kg−1), composition, and stability of the mineral‐associated organic matter (OM) of arable and forest topsoils. We selected seven soil types with different mineral characteristics for this study. For each soil type, samples were taken from topsoils of a deciduous forest and an adjacent arable site. The arable and forest sites have been used continuously for more than 100 years. Na‐pyrophosphate soluble OM fractions (OM(PY)), representing mineral‐associated OM, were extracted, analyzed for OC and 14C concentrations, and characterized by FTIR spectroscopy. For the forest and arable topsoils, a linear relationship was found between the OC content and exchangeable Ca. For the arable topsoils (pH 6.7–7.5), correlation analyses indicated that the OCPY concentration increased with an increase in oxalate soluble Fe and Al, exchangeable Ca, and Na‐pyrophosphate soluble Mg and Fe concentrations. The stability of OM(PY) determined by the 14C measurements of the near‐neutral arable topsoils was shown to increase with the specific surface area and the concentration of exchangeable Ca. For the acidic forest topsoils (pH <5), the stability of OM(PY) was found to increase as the pH, and the concentration of C=O groups and Na‐pyrophosphate soluble Mg increase. Key Points Land use–derived carbon accumulation is site specific Organic carbon stocks increase with increasing stocks of exchangeable Ca Na4P2O7‐extractable organic matter is more stable in arable than in forest soils

PurposeInformation related to spatial distribution and dominants of soil organic matter (SOM) is critical for evaluating soil quality and assessing the carbon sequestration capacity, which play essential role in soil management and climate change mitigation. Until now, no reported research has conducted an extensive survey to predict SOM content, analysed SOM spatial variability, and determined the main controls of SOM variation in areas around Dongting Lake in southern China. Therefore, this study aims to (1) explore the spatial variability of SOM content; (2) build a model to quantitatively predict SOM content with various sources of covariates and with the RF method; and (3) identify potential controls of SOM based on the relative importance of variables.Materials and methodsA total of 8040 soil samples were collected from Yueyang County in Eastern Dongting Lake Plain. Ordinary kriging was used to produce a map of SOM and then the random forest algorithms were used to predict SOM content with 17 covariates covered terrain attributes, land use, climate, soil management policies, soil properties, and geologic information. Finally, the main dominants of SOM variability were identified.Results and discussionThe SOM content in the survey region varied from 4.00 to 446.60 g kg−1 and had an average content of 33.17 g kg−1, which indicated fertile soil in the study area. SOM presented strong spatial variability in the area under study. The high SOM values were majorly located in the northeast and southwest parts of the survey regions. The R2 of our developed model was 0.74 and the RMSE was 0.16 g kg−1. The main controls of SOM variability in the study area were available phosphorus, precipitation, soil group, rotation system, available potassium, altitude, and slope.ConclusionOur developed model showed a good performance to estimate SOM content using auxiliary variables. Soil properties and agricultural management measures played the most important roles in predicting SOM in the study area. Results obtained from this study could provide new insights for estimating SOM and contribute to the sustainable development of agriculture and better regulation of soil quality in the study area.

Purpose Appropriate land management is important for improving the soil quality and productivity of the saline-sodic farmland. A recent study has revealed that flue gas desulfurization (FGD) gypsum and lignite humic acid application enhanced the salt leaching and crop production. The purpose of this study was to investigate the effects of applied FGD gypsum and lignite humic acid (powder) on the soil organic matter (SOM) content and physical properties. Materials and methods This study was based on a field experiment of five consecutive rapeseed-maize rotations in a saline-sodic farmland soil (Aquic Halaquepts) at coastal area of North Jiangsu Province, China. The soil is sandy clay loam texture with pH of 8.43 and clay content of 185 g kg −1 . Six treatments included three FGD gypsum rates (0, 1.6, and 3.2 Mg ha −1 ) and two lignite humic acid rates (0 and 1.5 Mg ha −1 ). The amendments were incorporated into 0–20 cm soil depth manually every year. Soil samples were collected from each treatment and analyzed for soil organic matter, water-stable aggregates (wet sieving method), bulk density (clod method), water retention capacity (pressure plate apparatus), total porosity (calculated from bulk density and particle density), and microporosity (calculated from water content at 0.01 MPa). Results and discussion After 5 years, the SOM and soil physical properties were significantly ( P  < 0.05) affected by the application of FGD gypsum and lignite humic acid, especially at the 0–20 cm soil depth. The highest amount of SOM with best soil physical condition was observed in the field which was treated with FGD gypsum at 3.2 Mg ha −1 with lignite humic acid, and the SOM, total porosity (TP), microporosity (MP), mean weight diameter (MWD), water-stable macroaggregate (WSMA), and available water content (AWC) were increased by 22.8, 6.34, 23.2, 48.1, 55.5, and 15.8 %, respectively, while the bulk density (BD) was decreased by 5.9 % compared to no amendments applied. The generalized linear regression analysis showed that the SOM explained 42.9, 55.0, 48.5, and 54.2 % of the variability for BD, MWD, WSMA, and MP, respectively. Conclusions This study illustrates the benefits of applying FGD gypsum and lignite humic acid for increasing the soil organic matter content and improving the soil physical properties and suggests a great potential for ameliorating saline-sodic farmland soil (Aquic Halaquepts) by using combined amendment of FGD gypsum with lignite humic acid.

PurposeThere is little knowledge on the organic matter fractions of salt-affected soil aggregates. This study aimed at investigating characteristics of salt-affected soil organic carbon components and the relationships between soil salt concentration and soil organic carbon component content.Materials and methodsFive typical salt-affected soils in Hetao region China were collected and analyzed for light (LF) and heavy fraction (HF) in different water-stable aggregates. And the soil organic carbon components were measured by Fourier transform infrared (FTIR) and pyrolysis-gas chromatography/mass spectrometer (Py–GC/MS).Results and discussionThe results showed that the salt-affected soils were dominant in 53–10-μm water-stable aggregates, 61–80% in the bulk soil, and very low in > 250-μm macro-aggregates, less than 7.06% in the bulk soil. The proportions of > 250-μm macro-aggregates and the mean weight diameter (MWD) were negatively correlated to Na+ concentration (p < 0.05). Furthermore, the macro-aggregates were generally higher in total organic carbon (TOC) and accordingly higher C/N ratio than those in micro-aggregates. Heavy fractions (HF) from both > 53 μm and < 53-μm soil aggregates accounted for 99.30–99.83% of the bulk soil and contained 89.6–98.5% lower TOC and accordingly 49.2–84.8% lower C/N ratio than those in light fractions (LF). The LFs were high in lignin (7.27–34.02% in total pyrolysis products, 19.89% on average) and alkane/alkene-derived compounds (9.51–37.21%, 23.18% on average), but low in N-containing compounds (0–3.64%, 1.71% on average), while HFs were high in both alkane/alkene (4.38–27.46%, 15.06% on average) and N-containing compounds (7.45–26.45%, 13.98% on average), but low in lignin-derived compounds (1.13–8.75%, 3.86% on average).ConclusionsThe tested salt-affected soils were predominant in 53–10-μm micro-aggregates, which was caused by the Na+ dispersion effect on soil aggregates. Most SOM was stored in HF that contained high N-containing compounds and low C/N ratios. Our results suggested that the components of SOM were mainly controlled by the soil Na+ concentration.

For the purposes of assessment of long-term changes, two sets of Chernozems soil samples were analysed and compared in parallel: ‘old’ file samples obtained during the Soil Survey 1960–1970 in the former Czechoslovakia and a ‘present’ (2013) set of samples from exactly the same sites as the archive samples. The recently collected samples revealed worse qualitative parameters (lower humic acid to fulvic acid (HA/FA) ratios and higher colour quotient Q4/6 values) than the file samples, for all the localities. On the other side, the quantitative soil organic matter (SOM) parameters (oxidizable carbon (Cox) and all its determined components) showed contrary results. The amount of total SOM at the same sites is higher now than it was about 50 years ago. It can be concluded that the current decline in SOM quality in Chernozems is partly compensated for by higher accumulation of SOM in the soils. All the analysed Chernozem samples were found to have much worse qualitative SOM parameters than the values mentioned for this soil type in the older literature. However, a comparison of the current data and the file data of Chernozem SOM quality can still be considered an open issue and require more complex research.

The extent to which ectomycorrhizal (ECM) fungi enable plants to access organic nitrogen (N) bound in soil organic matter (SOM) and transfer this growth-limiting nutrient to their plant host, has important implications for our understanding of plant–fungal interactions, and the cycling and storage of carbon (C) and N in terrestrial ecosystems. Empirical evidence currently supports a range of perspectives, suggesting that ECM vary in their ability to provide their host with N bound in SOM, and that this capacity can both positively and negatively influence soil C storage. To help resolve the multiplicity of observations, we gathered a group of researchers to explore the role of ECM fungi in soil C dynamics, and propose new directions that hold promise to resolve competing hypotheses and contrasting observations. In this Viewpoint, we summarize these deliberations and identify areas of inquiry that hold promise for increasing our understanding of these fundamental and widespread plant symbionts and their role in ecosystem-level biogeochemistry.

We used long-term laboratory incubations and chemical fractionation to characterize the mineralization dynamics of organic soils from tussock, shrub, and wet meadow tundra communities, to determine the relationship between soil organic matter (SOM) decomposition and chemistry, and to quantify the relative proportions of carbon (C) and nitrogen (N) in tundra SOM that are biologically available for decomposition. In all soils but shrub, we found little decline in respiration rates over 1 year, although soils respired approximately a tenth to a third of total soil C. The lack of decline in respiration rates despite large C losses indicates that the quantity of organic matter available was not controlling respiration and thus suggests that something else was limiting microbial activity. To determine the nature of the respired C, we analyzed soil chemistry before and after the incubation using a peat fractionation scheme. Despite the large losses of soil C, SOM chemistry was relatively unchanged after the incubation. The decomposition dynamics we observed suggest that tundra SOM, which is largely plant detritus, fits within existing concepts of the litter decay continuum. The lack of changes in organic matter chemistry indicates that this material had already decomposed to the point where the breakdown of labile constituents was tied to lignin decomposition. N mineralization was correlated with C mineralization in our study, but shrub soil mineralized more and tussock soil less N than would have been predicted by this correlation. Our results suggest that a large proportion of tundra SOM is potentially mineralizable, despite the fact that decomposition was dependent on ligning breakdown, and that the historical accumulation of organic matter in tundra soils is the result of field conditions unfavorable to decomposition and not the result of fundamental chemical limitations to decomposition. Our study also suggests that the anticipated increases in shrub dominance may substantially alter the dynamics of SOM decomposition in the tundra.

The rhizosphere priming effect (RPE) is a mechanism by which plants interact with soil functions. The large impact of the RPE on soil organic matter decomposition rates (from 50% reduction to 380% increase) warrants similar attention to that being paid to climatic controls on ecosystem functions. Furthermore, global increases in atmospheric CO2 concentration and surface temperature can significantly alter the RPE. Our analysis using a game theoretic model suggests that the RPE may have resulted from an evolutionarily stable mutualistic association between plants and rhizosphere microbes. Through model simulations based on microbial physiology, we demonstrate that a shift in microbial metabolic response to different substrate inputs from plants is a plausible mechanism leading to positive or negative RPEs. In a case study of the Duke Free-Air CO2 Enrichment experiment, performance of the PhotoCent model was significantly improved by including an RPE-induced 40% increase in soil organic matter decomposition rate for the elevated CO2 treatment – demonstrating the value of incorporating the RPE into future ecosystem models. Overall, the RPE is emerging as a crucial mechanism in terrestrial ecosystems, which awaits substantial research and model development.