Soil Organic Matter Stabilization Research Articles

Soil organic matter as affected by the conversion of natural tropical rainforest to monoculture rubber plantations under acric ferralsols

Land-use change (LUC) in the tropics, such as the exponential rate of conversion of natural habitats into intense monocultures focusing on cash-crop cultivation, is a major causal factor of global environmental change. To understand the effects of LUC on soil organic matter (SOM) stability and the dynamics of C and N within SOM fractions, we measured the C and N content and δ13C and δ15N in fractions of different aggregate- and density-size of acric ferralsols in tropical rainforests and rubber plantations. The proportion of macroaggregates, heavy and light fractions significantly decreased after LUC. The results showed that, in general, conversion of tropical rainforest to rubber plantation significantly decreased the C and N content in bulk soil and the aggregate- and density-size to 20 cm soil depth. The decrease in C and N content in bulk soil was mainly driven by decreasing C and N associated with macroaggregates and light fractions, which accounted for > 50%. We found significant correlations among mean weight diameter, aggregate-, and density-size fractions C, N, and C/N ratios. The conversion of tropical rainforest to rubber plantation significantly enriched soil δ13C while depleting δ15N. Enrichment of δ13C in rubber plantations could be explained by the mixing of old and fresh C. We conclude that, C and N dynamics within SOM fractions were greatly affected by LUC and the δ13C and δ15N signature confirms the changes in SOM stability after forest conversion. We suggest that planting intercrops within rubber monocultures may improve SOM accumulation, soil aggregation, and C and N sequestration.

Characterization of herbaceous encroachment on soil biogeochemical cycling within a coastal marsh

Vegetation transitions occur globally, altering ecosystem processing of organic matter and changing rates of soil biogeochemical cycling. In coastal marshes, more salt- and inundation-tolerant herbaceous species are encroaching on less tolerant species, concomitant with sea level rise. These species shifts could disrupt ecosystem services such as soil organic matter storage and the cycling of carbon (C), nitrogen (N), and phosphorus (P). To determine how these ecosystem processes were affected by encroachment, we characterized biogeochemical properties and functions along a transect of encroaching Distichlis spicata L. Greene (saltgrass) on Spartina bakeri Merr. (cordgrass), two herbaceous species. During both the wet and dry season, nine soil cores were obtained from three community types: saltgrass end member, transition zone, and cordgrass end member. Total soil C, N, and organic matter were greatest within the saltgrass and transition zones. The saltgrass and transition zone soils also supported higher rates of enzyme activity and potentially mineralizable N and P than cordgrass soils during the dry season, and greater potential CO2 production and microbial biomass C during the wet season. Generally, the transition zone functioned similarly to the saltgrass zone and the encroachment gradient coincided with a 33 cm elevation change. Seasonally, low extractable nutrient availability (nitrate and soluble reactive phosphorus) during the dry season was correlated with overall greater enzyme activity (N-acetyl-β-D-glucosidase, alkaline phosphatase, β-glucosidase, xylosidase, and cellobiosidase) and potentially mineralizable N and phosphorus (P) rates. This study demonstrates that shifts in dominant herbaceous species and accompanying abiotic gradients alters biogeochemical processing of organic matter within coastal marshes.

Diversity of Rhizobia and Importance of Their Interactions with Legume Trees for Feasibility and Sustainability of the Tropical Agrosystems

Symbiotic biological nitrogen fixation (BNF) is a complex process that involves rhizobia, a diverse group of α and β-proteobacteria bacteria, and legume species. Benefits provided by BNF associated with legume trees in tropical environments include improvements to efficiency of nitrogen (N) use, increase of soil carbon sequestration, stabilization of soil organic matter, decrease of soil penetration resistance, and improvement of soil fertility. All these benefits make BNF a crucial ecosystem service to the sustainability of tropical agriculture. Due to the importance of this ecological process and the high diversity of rhizobia, these bacteria have been extensively characterized worldwide. Currently, over 400 species of rhizobia are known, distributed into seven families. In the humid tropics, Leucaena leucocephala, Acacia mangium, Gliricidia sepium, and Clitoria fairchildiana are four of the most common species used by family farmers to create sustainable agricultural systems. These four legumes perform symbiosis with different groups of rhizobia. Exploring BNF could help to enable sustainable intensification of agriculture in the humid tropics, mainly because it can increase N use efficiency in an environment where N is a limiting factor to plant growth.

Abiotic processes dominate soil organic matter mineralization: Investigating the regulatory gate hypothesis by inoculating a previously fumigated soil with increasing fresh soil inocula

The Regulatory Gate Hypothesis proposes that the rate of soil organic C mineralization is largely controlled by abiotic processes rather than biological ones. To further test this hypothesis, a repeatedly fumigated soil was inoculated with increasing weights of the initial fresh soil (0, 2, 10 and 25%) and then incubated for 30 days. The CO2-C evolved, soil microbial biomass C and adenosine 5′-triphosphate were measured at days 1, 5, and 30. The fresh soil was incubated as a control. Our aim was to further test the Regulatory Gate hypothesis that the rate of mineralization of stabilized soil organic matter is not regulated by the size, activity or community structure of the microbial biomass.Cumulative CO2 evolved changed little, irrespective of weight of inoculum, in the inoculated soils during the incubation. Soil respiration rates of inoculated soils (21 µg CO2-C g−1 soil) were significantly higher than those of non-inoculated-fumigated soil (18 µg CO2-C g−1 soil) and fresh soil (18.5 µg CO2-C g−1 soil) at the beginning of the incubation (day 0–1). However, they were not significantly different (6.5 µg CO2-C g−1 soil in inoculated soils) from those in non-inoculated-fumigated and fresh soil (5.5 and 7.1 µg CO2-C g−1 soil respectively) at the end of the incubation. Soil adenosine 5′-triphosphate (ATP) proportionally increased in the inoculated soils, but remained much lower (0.49–0.78 nmol g−1 soil) than that in fresh soil (2.28 nmol g−1 soil) until the end of the incubation. The recoveries of the microbial communities and diversity in the inoculated soils were proportional to the inoculation rates. Proteobacteria, mainly Gammaproteobacteria, had the highest recovery in inoculated soils after incubation, and Nitrospira the lowest. We conclude that soil organic matter mineralization is not regulated by the size or composition of the soil microbial population, but mainly depends on the conversion of non-bioavailable SOC to bioavailable substrate C. The precise processes involved now require further research to improve our understanding of the mechanisms driving the mineralization of soil organic matter.

Selective stabilization of soil fatty acids related to their carbon chain length and presence of double bonds in the Peruvian Andes

Recent studies show increasing evidence for perseveration of soil organic matter (SOM) controlled by interactions with the soil matrix (i.e. mineral surfaces and aggregates) rather than chemical recalcitrance of the SOM. However, a consensus is still absent for potential controls of SOM chemical composition on SOM stabilization and persistence. Soil fatty acids (FAs), which form an important SOM component, can be used to investigate the effects of chemical properties on SOM stabilization because they are easily degraded by microorganisms but can be stabilized by the soil matrix against decomposition. Here we investigated whether inherent molecular properties of FAs control their stability in soils and their interactions with the soil matrix. Soil samples were collected from alpine grasslands of the Peruvian Andes (Andosols, Umbrisols and Phaeozems), as they are characterized by high carbon stocks and abundant aliphatics. We applied pyrolysis - gas chromatography/mass spectrometry analyses assisted by tetramethylammonium hydroxide (TMAH-pyrolysis-GC/MS) to determine the chemical composition of bulk SOM and FAs before and after a 76-day incubation experiment, comparing a situation with intact versus crushed soil aggregates. The results showed that the TMAH-pyrolysis-GC/MS yielded a large proportion of FAs (>60% relative abundance of identified compounds), with a major contribution of free FAs. FA stability was controlled by the presence of double bonds (unsaturated vs. saturated FAs) and carbon chain length. Unsaturated FAs significantly (P < 0.05) predicted soil organic carbon mineralization rates and were more depleted after the incubation compared to saturated FAs. The depletion of unsaturated FAs is likely explained by their easier degradation compared to saturated FAs. The easier degradation might be explained by the smaller extent of stabilization through association with mineral surfaces and/or chemical properties rather than stabilization through occlusion in aggregates. In terms of carbon chain length, FA stability decreased from short-chain to long-chain FAs. A possible explanation for this is that short-chain FAs received more protection by occlusion in aggregates compared to long-chain FAs or that short-chain FAs were produced during the incubation as a result of microbial transformation of FAs. Such microbial transformation has limited effects on the prediction of FA stability using double bonds and carbon chain length. However, we observed that soil types and horizons did influence the controls of double bonds and carbon chain length on FA stability. Our results corroborate the hypothesis that the inherent properties of soil FAs control their interactions with the soil matrix and indirectly govern their stabilization and persistence in the Peruvian Andean soils under study.

Effects of 22-year fertilisation on the soil organic C, N, and theirs fractions under a rice-wheat cropping system

ABSTRACT The quantification of soil organic carbon (SOC) and N fractions under the influence of long-term fertilisation is useful to understand soil organic matter (SOM) stabilization mechanism and to develop optimal management practices. This study was to investigate the responses of different SOC and N fractions to long-term fertilisation and their contributions to the total change in the bulk soil under a rice-wheat cropping system. Compared with soil with no fertilisation, the 22-year fertilisation improved the C and N content of the bulk soil and different fractions with the greatest increase in NPK fertilisers and rice straw treatment. The SOC and N changes of heavy fraction accounted for 92% and 88% of the total SOC increases, while the Fe/Al-bound organic carbon and humic organic carbon in heavy fraction accounted for 43% and 48%, respectively. The SOC and N contents of each fraction were positively correlated with the bulk soil and the others. It is concluded that the NPK fertilisers plus straw returning was efficient to enhance SOC and N accumulation; the chemical binding of Fe and Al in organic-mineral complexes and the further formation of recalcitrant humin might be the potential mechanism for SOM accumulation in the rice-wheat cropping system. Abbreviations: SOM: soil organic matter; SOC: soil organic carbon; STN: soil total nitrogen; POM: particulate organic matter; MOM: mineral associated organic matter; LF: light fraction; HF: heavy fraction; Ca-OC: Ca-bound organic carbon; Fe/Al-OC: Fe- or Al-bound organic carbon; Humin-OC: humic organic carbon

Preferential degradation of leaf- vs. root-derived organic carbon in earthworm-affected soil

Earthworms are integral parts of many ecosystems and may play a decisive role in determining whether soils function as carbon (C) sink or source. However, information on how earthworms affect the composition and stability of soil organic matter (SOM) is scarce. Particularly their effect on organic matter deriving from leaves and roots with distinct composition and, thus, susceptibility to decomposition and stabilization remains unclear. Here, we combine cutin- and suberin-derived lipids as specific markers for leaf- and root-derived SOM with their 13C composition and physical fractionations of soil. We show that earthworms overprint the protective role of organo-mineral associations and aggregates to favor the accumulation of root- relative to leaf-derived SOM. This gradual accumulation contributes to the often-observed dominance of root-derived organic matter in soil and emphasizes the need to consider molecular level effects of earthworms on SOM dynamics.

Barriers and Strategies to Boost Soil Carbon Sequestration in Agriculture

The Paris Agreement calls for limiting global warming below 2°C. The “4 per 1,000 Initiative: Soils for food security and climate” was launched in 2015 to increase soil organic carbon sequestration with three objectives: mitigation of climate change, adaptation to climate change and improved food security. One of the challenges of the Initiative relates to its feasibility in contrasted biophysical, social and economic environments, questioning the adoption rate of required new practices. We conducted participatory multi-stakeholder workshops in France and Senegal to collect knowledge and perception of farmers, NGOs, agro-industries, administrations, donors and researchers on barriers and coping strategies for 4 per 1,000 innovations. Results in both countries reveal the predominance of social and economic barriers such as lack of knowledge or training, increased difficulties of fieldwork, workload, risk handling, funding and social pressure. Biophysical constraints such as limited potential of soil organic matter storage or rainfall scarcity and variability appear more important in Senegal. Identified actions to foster the sequestration of soil carbon call for an improved policy context leading to innovations in land planning, stakeholder communication, demonstration facilities, capacity building or financial support. Fewer constraints and coping strategies mention technical issues, showing that fostering agricultural soil carbon sequestration is more a question of enabling environment than technical innovations or farmers' willingness for change. We conclude that actions to support the 4 per 1,000 Initiative need to include a variety of stakeholders such as extension services, private sector, civil society, local institutions, policy makers, consumers, and not only farmers.

Effects of different regeneration patterns on soil dissolved organic matter degradation in Castanopsis carlesii forests of subtropical China

Biodegradability of dissolved organic matter (DOM) affects stabilization and mineralization of soil organic matter, which is of great significance to soil nutrient cycling. In order to explore the effects of forest regeneration on soil DOM degradation, soil DOM solution was sampled in a natural Castanopsis carlesii forest (NF), a secondary forest of C. carlesii (SF), and an artificial-assisted regeneration forest of C. carlesii (AR) in a sub-tropical area and conducted 42-day laboratory incubation. The results showed that: 1) both the degradation rate of soil dissolved organic carbon (DOC) and the ratio of labile DOC were as follows: SF>AR>NF; dissolved organic nitrogen (DON) and microbial biomass carbon (MBC) are the factors significantly affecting the ratio of labile DOC; 2) stable DOC accounted for the majority of soil DOC in all the three forest types (72.3%-94.6%), which had long turnover time and contributed to the formation of stable soil organic matter (SOC); 3) the initial humification index in emission mode (HIXem) of soil DOM would affect the turnover time of labile DOC. The spectral structure of DOM changed dynamically during the degradation process, indicating that microorganism would turn to degrade aromatic and hydrophobic fractions for carbon source after the depletion of labile DOM. Overall, the transformation from NF of C. carlesii into SF and AR could increase the proportion of the easily degradable DOC, and enhance the biodegradability of soil DOM, which were not conducive to the accumulation of SOC.

XPS and two-dimensional FTIR correlation analysis on the binding characteristics of humic acid onto kaolinite surface

The stabilization and preservation of soil organic matter have been attributed to the strong reactive sites of mineral surfaces that cause physical isolation and chemical stabilization due to the organic-mineral interface. However, much of the micro-scale knowledge about interactions between organic ligands and minerals largely remains at the qualitative level, and neglects the heterogeneity of functional groups of organic matter. Here, we report the use of molecular-scale technologies of two-dimensional FTIR Correlation Spectroscopy (2D-FTIR-CoS) and X-ray Photoelectron Spectroscopy (XPS) to directly measure the binding processes of humic acid (JGHA) groups onto kaolinite surface. The spectroscopy results showed that the carboxylate groups, aliphatic OH and aromatic structure participate in the binding of JGHA on kaolinite surface. The carboxylic and phenolic hydroxyl interact with kaolinite surface through the interfacial COAl/Si bonds. Kaolinite prefers to adsorb C-groups at pH 4.0 and O-groups at pH 8.0. The interaction of COO– group at 1566 cm−1 of JGHA leads to the formation of inner-sphere complex first and then outer-sphere complex with increasing contact time. The interaction of COOH group at 1261 cm−1 with the AlOH2+ of kaolinite was could be ascribed to ligand exchange and/or electrostatic attraction, whose contribution was evaluated to be 13.90%, 7.65% and 0% at pH 4.0, 6.0 and 8.0, respectively. These results of molecular binding provide quantitative mechanistic insights into organic-mineral interactions and expound the effect of functional groups of HA on binding mechanisms, and thus bring important clues for better understanding the mobility and transformation of land‑carbon including mineral-bound carbon.

Assessment of the molecular composition of humic acid as an indicator of soil carbon levels by ultra-high-resolution mass spectrometric analysis

Long-term stabilization of soil organic matter (SOM) plays an important role in the carbon cycle. Hence, understanding biogeochemical mechanisms of soil C sequestration is crucial to control its release to the atmosphere. This study aims at investigating the biogeochemical mechanisms of soil C sequestration. An exploratory assessment was carried out on the information about the soil C levels provided by the molecular composition of humic acids (HAs) analyzed by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Significant PLS forecasting model for total soil C was obtained using as descriptors the 131 compounds in common in all the HAs detected by FTICR-MS, and its variable importance for projection (VIP) was plotted in the space defined by their atomic ratios using van Krevelen diagrams. The results indicated that significant relationship exists between the HAs molecular composition and the soil organic C levels. The VIP values for the different groups of compounds illustrate how HA contains information about the amounts of C stored in the soil: the HAs in the soils with high levels of organic C have significantly (P < 0.1) higher proportions of unsaturated lipid and lignin-derived compounds; on the other side, low soil organic C levels are associated to HAs with comparatively high proportions of saturated lipid compounds.

A quantitative assessment of amino sugars in soil profiles

Recent evidence suggests that a significant proportion of stable soil organic matter is derived from microbial necromass, which can be evaluated by measuring biomarker amino sugars. However, our understanding of the vertical distribution and control of amino sugars in soil profiles at the global scale is still limited. Here, we synthesized the concentrations of amino sugars relative to soil mass (mg/g soil) and to soil organic carbon (C) concentration (mg/g C) in various soil horizons (litter layer, O horizon soil and mineral soil at 0–20, 20–50, 50–100 and > 100 cm depths) from 131 forest, grassland and cropland sites. We found that the concentration of soil amino sugars declined with soil depth, ranging from 5.90 mg/g soil in O horizon soil and 1.56 mg/g soil at 0–20 cm depth to 0.24 mg/g soil at >100 cm depth. However, the proportion of amino sugars in soil organic C significantly increased from 18.1 mg/g C in O horizon soil to 61.9–83.3 mg/g C in mineral soil. The concentration of amino sugars was positively correlated with soil organic C and microbial biomass C concentrations. The proportion of amino sugars in soil organic C was significantly higher in microaggregates (<2 μm) and in soil with low carbon to nitrogen ratios. Microbial biomass carbon and soil pH had positive and negative effects, respectively, on the concentration of amino sugars in topsoil, and together explained 58.8% of the total variation in the concentration of amino sugars. These results suggest that soil biogeochemical properties are more important than climatic variables and latitude in determining the long-term persistence of microbial residues in soils.

Susceptible soil organic matter, SOM, fractions to agricultural management practices in salt-affected soils

In the recent decades, soil salinity became the main human-induced soil degradation causes in Egypt’s Nile Delta Valley (ENDV) by affecting the stabilization processes of soil organic matter (SOM). However, soil organic carbon (SOC) is highly conserved by aggregating the stabilized organic molecules under sound agricultural management. In particular, labile SOM fractions assumed to be dually influenced by salinity and agricultural management practices other than the stabilized fractions. This work aimed to study various labile and stable SOM fractions that are more susceptible to the current agricultural practices in salt-affected soils of the ENDV area. Three different agro-ecological sites were studied: Eastern (EH, EM soils) and Western (WM, WL soils) Delta regions dominated by Vertic Torrifluvents, and Coastal region (NCH, NCM soils) dominated by Typic Calcitorrerts of high CaCO3 contents. Two different salinity levels were detected in each site; low in WL soils, medium in WM, NCM, and EM soils, and high in EH and NCH soils. The least values in EM, WL, and NCM soils were due to the recurrent legume applications. The carbon content of glomalin-related soil protein (GRSP) (C-GRSP) was positively correlated with SOC and water-extractable organic carbon (WEOC) fraction confirming the contribution of GRSP to the stabilization of SOM. The lower soil β-glucosidase, phosphatase, and protease enzymes activities were in those soils with larger salinity levels in each site as NCH < NCM, WM < WL, and EH < EM reflecting the effect of soil salinity and CaCO3 contents on soil metabolic activities. Extracted organic carbon (EOC) in both humic and fulvic fractions was higher in EH, WM, WL, and EM soils than in NCH and NCM soils. The chemical composition of SOM obtained by the pyrolysis gas chromatography showed that lignocellulosic and condensed aromatic structures in SOM increased significantly with CaCO3 and salinity. In conclusion, the considered SOM fractions such as WEOC, EOC, GRSP, C-GRSP, together with the pyrolytic results can be considered as significant indicators in the dynamic stability of SOM. Intercropping with legumes may increase the stability of SOM fractions in salt-affected soils of degraded lands. In calcareous soils, severe alteration in SOC conservation was observed and negatively influenced the active constituents of SOM.

Do Grazing Systems and Species Composition Affect Root Biomass and Soil Organic Matter Dynamics in Temperate Grassland Swards?

Elevating soil organic matter (SOM) levels through changes in grassland management may contribute to lower greenhouse gas concentrations in the atmosphere and mitigate climate change. SOM dynamics of grassland soils may be affected by grazing systems and plant species composition. We analyzed the effects of simulated grazing systems (continuous (CG), rotational (RG), and lenient strip grazing (LG)) and species composition (monocultures of perennial ryegrass fertilized (LP+) and unfertilized (LP−)), tall fescue (fertilized, FA+), and a mixture of these two species with white clover (fertilized, LFT+)) on root biomass and SOM dynamics in field experiments on loamy and sandy soils in the Netherlands. Dried cattle manure was added to all fertilized treatments. We hypothesized that SOM accumulation would be highest under CG and LG, and FA+ and LFT+ as a consequence of greater belowground biomass production. SOM was monitored after conversion from arable land for a period of two years (loamy and sandy soil) and five years (sandy soil). We found that management practices to increase SOM storage were strongly influenced by sampling depth and length of the grassland period. SOM increased significantly in nearly all fertilized treatments in the 0–60 cm layer. No differences between species compositions were found. However, when only the 30–60 cm soil layer was considered, significantly higher SOM increases were found under FA+, which is consistent with its greater root biomass than the other species. SOM increases tended to be higher under LG than RG. The results of this study suggest that it seems possible to comply with the 4-thousandth initiative during a period of five years with fertilized perennial ryegrass or tall fescue in monoculture after conversion from arable land. It remains to be investigated to which extent this sequestration of carbon can be maintained after converting grassland back to arable land.

Carbon Sequestration in Soil Aggregates under Different Cropping Patterns of Bangladesh

Land use change and cropping patterns are important factors for controlling carbon sequestration in soils and they may also change the relative importance of different mechanisms of soil organic matter stabilization. The study was conducted to investigate the state of carbon sequestration in soil aggregates under different cropping patterns of Khulna, Jessore and Chapainawabganj districts in Bangladesh. Thirty-six soil samples were collected from (0 - 100 cm depth) above mentioned regions of three physiographic regions: Ganges Meander Floodplain, Ganges Tidal Floodplain and High Barind Tract. The texture of the samples varied within three soil texture groups, Silt Loam, Silty Clay Loam and Silty Clay. The highest NSI value (0.89) was found under Wheat-Fallow-T. Aman cropping pattern in Silty Clay soils (sample No 15) and lowest value (0.59) was found Vegetables/Mustard-Fallow-T. Aman cropping pattern in Silt Loam soils (sample No 17). The highest value (735.20 mg•kg−1) of active C was observed under Chickpea/mustard-T. Aman (Sample No 31) and the lowest value (619.23 mg•kg−1) was found in case of Wheat-Fallow-T. Aman cropping pattern (Sample No 30). The highest SOC stock (1.62 Kg C m−2) was found in Silty Clay Loam soil under Mungbean/Ash gourd-T. Aman cropping pattern (Sample no 4) and the lowest SOC stock (0.35 Kg C m−2) was found in Silt Loam soil under Cauliflower/Pumkin/Spinach-T. Aman Cropping pattern (Sample No 2). Soil organic carbon associated with different size aggregates was the highest (3.14%) under Mungbean/Ash gourd-T. Aman (Sample No 20) and was the lowest (0.36%) under Cauliflower/Pumkin/Spinach-T. Aman cropping pattern (Sample No 2). Organic carbon content in aggregate size ranges > 2000 μm (SOC1), 2000 - 250 μm (SOC2), 250-53 μm (SOC3), and 2000 μm, 2000 - 250 μm and 250 - 53 μm, aggregates. Significant positive correlations were found between SOC stock and SOC1, SOC stock and SOC2, SOC stock and SOC3, SOC stock and SOC4.

Humic Acids Isolated from Selected Soils from the Russian Arctic and Antarctic: Characterization by Two-Dimensional 1H-13C HETCOR and 13C CP/Mas NMR Spectroscopy

Here we describe the molecular composition and resistance to decomposition of humic acids isolated from selected soils of the Russian Arctic and Antarctic. The degree of soil organic matter stabilization was assessed using modern instrumental methods: nuclear magnetic resonance spectroscopy (cross peak magic-angle spinning (CP/MAS) 13C-NMR and 1H-13C heteronuclear-correlation (HETCOR)). Analysis of the humic acids showed that aromatic compounds prevail in the organic matter formed in cryoconites, located on the surfaces of the glaciers. The predominance of aliphatic fragments is revealed in the soils of the Yamal peninsula and Antarctica. This could be caused by sedimentation of fresh organic matter exhibiting low decomposition stage due to the severe climate and processes of hydrogenation in the humic acids, destruction of the C-C bonds, and formation of chains with high hydrogen content. These processes result in formation of aliphatic fragments in the humic acids.

Climate and land use as key factors of the stability of organic matter in soils

Stocks of organic carbon in the soils and its resistance to biodegradation depend on a combination of external and internal determinants which control not only the decomposition, but also the stabilization of organic matter (OM). Soil OM stability was evaluated: 1) by specific mineralization rate (SpMinR) for the forest ecosystems located in various climatic zones of Eurasia (temperate, Mediterranean, and tropical) and 2) by the biological stability index (IBS), determined for the main soil types of the European part of Russia under natural vegetation and agricultural crops. It is shown that the key factors determining the stability of OM of substances in soils are the climate and type of vegetation (land use). Thus, a close negative correlations between the SpMinR values and the annual precipitation and the average annual air temperature were found in the soils of the Eurasian transect. It was revealed that the share of stable and biologically conservative OM and IBS values in the soils under agricultural crops were significantly higher than in the soils of natural cenoses.

Influences of nitrogen addition and aboveground litter-input manipulations on soil respiration and biochemical properties in a subtropical forest

Atmospheric nitrogen (N) deposition has rapidly increased in subtropical ecosystems and may have altered the input of aboveground litter to soil, which substantially impacts soil carbon (C) and nutrient cycling. But how the soil processes and properties respond to N deposition under uncertain fresh litter input is poorly understood. In order to examine the responses of soil respiration and biochemical properties to N addition and aboveground litter manipulation, a field N addition and litterfall manipulation interaction experiment was performed in an evergreen broadleaf forest on the western edge of the Sichuan Basin in China. Three levels of N addition, including an N control (CN, ambient N input) and low N (LN, + 50 kg N ha−1 year−1) and high N (HN, + 150 kg N ha−1 year−1), and three levels of litterfall manipulation, including intact litter input (L0, no litter alteration), litter reduction (L−, reduced by 50%) and litter addition (L+, increased by 50%), were conducted monthly starting in January 2014 and August 2015, respectively. Soil respiration was measured monthly from January 2016 to December 2017. Soil samples were collected four times, in October 2016 and January, April and July 2017, to measure soil biochemical properties. The results showed that: (1) short-term N addition did not significantly alter the aboveground litter input in this forest; (2) soil respiration decreased with elevating N input and was associated with amount of litterfall input; (3) N addition increased the total organic C (TOC) concentration in topsoil in subplots without litterfall alteration but did not affect TOC in subplots with increased or decreased litter-input; (4) N addition decreased soil pH and did not affect soil microbial biomass regardless of whether litterfall was altered or not; (5) short-term litter manipulation did not affect any soil properties in the N control plots, but both litterfall reduction and addition tended to reduce surface soil TOC concentration in the N-added plots; and (6) both N addition and litterfall manipulation showed stronger effects on organic soil than on mineral soil. These findings indicated that elevated N input increased the surface soil C content by reducing soil respiration mainly via enhancing stabilization of soil organic matter rather than by reducing soil microbial biomass, and that altered litterfall may mitigate the N-induced increase in soil C. Because of temporal lag, long-term experimentation is needed to investigate the response of soil to altered litter input under different N addition conditions.

The multilayer model of soil mineral–organic interfaces—a review

AbstractAssociation of organic matter (OM) with minerals is an important pathway in the formation of stable OM in soil. While the importance of mineral–organic associations (MOA) in regulating soil carbon cycling has been rigorously demonstrated by empirical evidence, knowledge about the molecular‐scale arrangement of OM at mineral surfaces is still lacking. Such knowledge is urgently needed to disentangle the mechanisms of long‐term storage of soil OM. Based on indirect observations regarding the formation, composition, and structure of MOA, a conceptual multilayer model was proposed by Kleber et al. in 2007 to foster debate and help elucidating the structure and reactivity of MOA. According to this model, the associated OM at mineral surfaces is discrete and self‐organized into a multilayer structure. In this review, we aim to collect and evaluate existing studies that used this model to explain biogeochemical processes at mineral–organic interfaces, and based on this, assess the applicability of the model. The multilayer model has seen extensive adoption within soil science and related fields. In general, existing studies either support the concept of a patchy distribution of adsorbed OM on mineral surfaces or advocate that OM can be coprecipitated with nanosized poorly crystalline minerals or hydrolysable metals. However, the evidence for the patchy distribution of adsorbed OM cannot support the multilayer model on its own. There is little consensus about the role of N‐rich OM in forming the contact zone according to the multilayer model but surface conditioning by different classes of organic compounds appears to be an essential factor for the overall adsorption of OM. Nevertheless, large uncertainty still remains with respect to multilayer‐like organization of MOA. By taking advantage of recent developments in surface analytical sciences and computational chemistry, a rigid experimental testing of the multilayer model at the molecular level is still required and awaits to be integrated into improved concepts of MOA formation and OM stabilization.

Effects of mixing maize straw with soil and placement depths on decomposition rates and products at two cold sites in the mollisol region of China

Crop straw is often retained on the ground as mulch or incorporated into soil through tillage practices. These straw management practices may affect straw decomposition rates and subsequently decomposition products due to mixing straw with soil, straw placement depths and experimental sites. Using litterbags containing maize (Zea mays. L) straw only (STO) or straw-soil mixture (SSM), this study aimed to determine the influence of mixing straw with soil and straw placement depths on decomposition rates and decomposition products, and to understand the relation of chemical composition of straw residues with straw decomposition rates. The litterbags were placed at three soil depths (0, 15 and 30 cm) in the fields at two cold sites (Hailun and Harbin), Northeast China, which had similar precipitations but different temperatures. During the 17-month experiment, the decomposition rate constants were higher by 60 %–160 % at a comparable depth in SSM than in STO. The mixing straw with soil was a more important factor in controlling straw decomposition rates than placement depths and experimental sites at the studied region. The relative abundances of O–alkyl C groups in straw residues decreased during the experimental period, even in the winter, while those of aromatic groups increased profoundly. The relative abundances of O–alkyl C groups in straw residues were lower by 0.4 %–6.9 % and those of aromatic groups were higher by 0 %–4.6 % in SSM than in STO. These findings suggest that the conventional litterbag method underestimated straw decomposition rates. Incorporating straw into soil could enhance maize straw decomposition and the formation of more stable soil organic matter (SOM) in the cold high-latitude studied region.