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Alpine wetlands are critical ecosystems for global carbon (C) cycling and climate change mitigation. Ecological restoration projects for alpine grazing wetlands are urgently needed, especially due to their critical role as carbon (C) sinks. However, the fate of the C pool in alpine wetlands after restoration from grazing remains unclear. In this study, soil samples from both grazed and restored wetlands in Zoige (near Hongyuan County, Sichuan Province, China) were collected to analyze soil organic carbon (SOC) fractions, arbuscular mycorrhizal fungi (AMF), soil properties, and plant biomass. Moreover, the Tea Bag Index (TBI) was applied to assess the initial decomposition rate (k) and stabilization factor (S), providing a novel perspective on SOC dynamics. The results of this research revealed that the mineral-associated organic carbon (MAOC) was 1.40 times higher in restored sites compared to grazed sites, although no significant difference in particulate organic carbon (POC) was detected between the two site types. Furthermore, the increased MAOC after restoration exhibited a significant positive correlation with various parameters including S, C and N content, aboveground biomass, WSOC, AMF diversity, and NH4+. This indicates that restoration significantly increases plant primary production, litter turnover, soil characteristics, and AMF diversity, thereby enhancing the C stabilization capacity of alpine wetland soils. [ABSTRACT FROM AUTHOR]

Rhizosphere engineering is the targeted manipulation of plants, soil, microorganisms, and agricultural management to shift pools and processes in the rhizosphere for specific aims. Physical, chemical, and biological approaches as well as farming practices allow engineering of the rhizosphere to increase carbon (C) sequestration. Rhizosphere engineering approaches focus on the accumulation and stabilization of C in the soil either directly or indirectly through: (i) raising root-derived C inputs; (ii) increasing the production of microbial biomass and necromass; and (iii) enhancing C stabilization in the soil. Rhizosphere engineering is crucial to manage rhizodeposition, microbial activities, and plant–soil–microbial interactions, and thus soil C sequestration under global change and human impacts. The rhizosphere is the central hotspot of water and nutrient uptake by plants, rhizodeposition, microbial activities, and plant-soil-microbial interactions. The plasticity of plants offers possibilities to engineer the rhizosphere to mitigate climate change. We define rhizosphere engineering as targeted manipulation of plants, soil, microorganisms, and management to shift rhizosphere processes for specific aims [e.g., carbon (C) sequestration]. The rhizosphere components can be engineered by agronomic, physical, chemical, biological, and genomic approaches. These approaches increase plant productivity with a special focus on C inputs belowground, increase microbial necromass production, protect organic compounds and necromass by aggregation, and decrease C losses. Finally, we outline multifunctional options for rhizosphere engineering: how to boost C sequestration, increase soil health, and mitigate global change effects. [ABSTRACT FROM AUTHOR]

Prescribed fire has been increasingly promoted to reduce wildfire risk and restore fire‐adapted ecosystems. Yet, the complexities of forest ecosystem dynamics in response to disturbances, climate change, and drought stress, combined with myriad social and policy barriers, have inhibited widespread implementation. Using the forest succession model LANDIS‐II, we investigated the likely impacts of increasing prescribed fire frequency and extent on wildfire severity and forest carbon storage at local and landscape scales. Specifically, we ask how much prescribed fire is required to maintain carbon storage and reduce the severity and extent of wildfires under divergent climate change scenarios? We simulated four prescribed fire scenarios (no prescribed fire, business‐as‐usual, moderate increase, and large increase) in the Siskiyou Mountains of northwest California and southwest Oregon. At the local site scale, prescribed fires lowered the severity of projected wildfires and maintained approximately the same level of ecosystem carbon storage when reapplied at a ~15‐year return interval for 50‐year simulations. Increased frequency and extent of prescribed fire decreased the likelihood of aboveground carbon combustion during wildfire events. However, at the landscape scale, prescribed fire did not decrease the projected severity and extent of wildfire, even when large increases (up to 10× the current levels) of prescribed fire were simulated. Prescribed fire was most effective at reducing wildfire severity under a climate change scenario with increased temperature and precipitation and on sites with north‐facing aspects and slopes greater than 30°. Our findings suggest that placement matters more than frequency and extent to estimate the effects of prescribed fire, and that prescribed fire alone would not be sufficient to reduce the risk of wildfire and promote carbon sequestration at regional scales in the Siskiyou Mountains. To improve feasibility, we propose targeting areas of high concern or value to decrease the risk of high‐severity fire and contribute to meeting climate mitigation and adaptation goals. Our results support strategic and targeted landscape prioritization of fire treatments to reduce wildfire severity and increase the pace and scale of forest restoration in areas of social and ecological importance, highlighting the challenges of using prescribed fire to lower wildfire risk. [ABSTRACT FROM AUTHOR]

Mulched drip irrigation (MDI) is generally accepted as a method to decrease soil salinization and improve crop yields in arid and semi-arid regions. However, there remain gaps in how MDI drives soil organic carbon (SOC) dynamic microbial assembly processes with time, and the mediating role of microorganisms remains unclear. In this study, we investigated the aforementioned issues across soil profiles in cotton fields with different years of MDI. The results showed that MDI did not cause the differences in SOC, particular organic carbon (POC), and mineral-associated organic carbon (MOC) in soil layers. The POC and MOC contents had a parabola relationship with time, and showed an opposite trend in soil. After 15 years of MDI, the ratio of MOC/SOC increased to a peak value of 50 % and 52 % in topsoil and subsoil, respectively; the ratio of POC/SOC decreased to valley values of 50 % and 48 %, respectively (P < 0.05). Long-term MDI reduced the differences in oxidase between soil layers but accelerated SOC loss by increasing polyphenol oxidase activity (P < 0.05). Compared with that of other years, with 10 years of MDI, bacterial Shannon diversity decreased to a valley value, and fungal Shannon diversity reached to a top value in subsoil (P < 0.05). In general, stochastic processes were mainly controlled by dispersal limitation, and undominated processes dominated microbial assembly; however, there was a close relationship between bacterial communities and organic carbon fractions. The high percentage of positive linkages among microorganisms indicated that long-term MDI was beneficial for carbon fixation. Additionally, a decrease of fungal oligotroph/copiotroph ratio, the relative abundance of Ascomycota and Basidiomycota was beneficial for the accumulation of SOC and POC in topsoil (P < 0.05). In conclusion, long-term MDI is useful for the fixation of organic carbon via improving soil POC content and strengthening linkages within community assemblies.

Background: Microaggregates are suspected to protect soil organic carbon (SOC) from microbial decay, but its residence time is not well understood. Aims: We aimed at unraveling the relevance of microaggregates for C storage and testing the hypothesis that C in the interior of aggregates is older, compared to the exterior. Methods: We sampled soil under C3 vegetation and at a site where cropping shifted to C4 vegetation 36 years ago. We isolated free and macroaggregate‐occluded size fractions (250–53 µm) by wet sieving and ultrasound, manually isolated aggregates therefrom, and analyzed whether vegetation‐related differences in δ13C could be traced at the interior and exterior of microaggregate cross‐sections using elemental and laser ablation‐isotope ratio mass spectrometry. Results: Size fraction weights comprised <5% of microaggregates. Based on a source partitioning approach including C3‐ and C4‐derived C, we found mean residence times of SOC in occluded and free microaggregates of 62 and 105 years, respectively. Thus, C storage was longer than that in size fractions (35 years) and bulk soil (58 years). The small‐scale variability of δ13C within aggregate cross‐sections was considerable, both in C3 and C4 soil, yet without significant (p = 0.46) differences between interior and exterior locations. Conclusions: We conclude that microaggregates do not persist in an intact form in such a long‐term that systematic differences in δ13C patterns between exterior and interior parts can develop. [ABSTRACT FROM AUTHOR]

Abstract Prescribed fire has been increasingly promoted to reduce wildfire risk and restore fire‐adapted ecosystems. Yet, the complexities of forest ecosystem dynamics in response to disturbances, climate change, and drought stress, combined with myriad social and policy barriers, have inhibited widespread implementation. Using the forest succession model LANDIS‐II, we investigated the likely impacts of increasing prescribed fire frequency and extent on wildfire severity and forest carbon storage at local and landscape scales. Specifically, we ask how much prescribed fire is required to maintain carbon storage and reduce the severity and extent of wildfires under divergent climate change scenarios? We simulated four prescribed fire scenarios (no prescribed fire, business‐as‐usual, moderate increase, and large increase) in the Siskiyou Mountains of northwest California and southwest Oregon. At the local site scale, prescribed fires lowered the severity of projected wildfires and maintained approximately the same level of ecosystem carbon storage when reapplied at a ~15‐year return interval for 50‐year simulations. Increased frequency and extent of prescribed fire decreased the likelihood of aboveground carbon combustion during wildfire events. However, at the landscape scale, prescribed fire did not decrease the projected severity and extent of wildfire, even when large increases (up to 10× the current levels) of prescribed fire were simulated. Prescribed fire was most effective at reducing wildfire severity under a climate change scenario with increased temperature and precipitation and on sites with north‐facing aspects and slopes greater than 30°. Our findings suggest that placement matters more than frequency and extent to estimate the effects of prescribed fire, and that prescribed fire alone would not be sufficient to reduce the risk of wildfire and promote carbon sequestration at regional scales in the Siskiyou Mountains. To improve feasibility, we propose targeting areas of high concern or value to decrease the risk of high‐severity fire and contribute to meeting climate mitigation and adaptation goals. Our results support strategic and targeted landscape prioritization of fire treatments to reduce wildfire severity and increase the pace and scale of forest restoration in areas of social and ecological importance, highlighting the challenges of using prescribed fire to lower wildfire risk.

Alpine wetlands, critical ecosystems in high-altitude mountain areas, play essential roles in water conservation, biodiversity protection, and carbon (C) sequestration. These ecosystems are particularly sensitive to climate change, with temperature and precipitation variations significantly impacting their structure and functional processes, such as litter decomposition, a key mechanism for C stabilization. This study focused on the Zoige wetland, a representative alpine wetland located on the eastern edge of the Qinghai-Tibet plateau. An incubation experiment was conducted with soil samples under simulated wet and dry moisture conditions to evaluate the environmental impacts on litter decomposition within this ecosystem. In this study, we employed the Tea Bag Index method, utilizing standardized litter, alongside native litter bags to compare the decomposition processes. This comparison between the uniform composition of standardized litter and the chemically diverse native plant litter aims to provide a comprehensive understanding of litter decomposition dynamics in alpine wetland ecosystems. Our finding showed that both standardized and native litter decomposition significantly decreased under dry conditions, in contrast to wetter condition. The components of both standardized and native litter exhibited a decrease over time, with the easily decomposable fractions breaking down swiftly, in contrast to the slower decomposition of the more resistant components. Furthermore, soil exo-enzyme activities varied significantly with environmental conditions. Wet conditions were observed to enhance soil microbial activity, whereas dry conditions resulted in shifts in microbial biomass C and nitrogen, indicative of drought resilience. The correlation analysis revealed that the composition of native litter is the primary factor influencing its decomposition. In contrast, the decomposition of standardized litter was influenced by both its composition and soil microbial activity. Thus, the distinction between the influences on native and standardized litter decomposition highlights the necessity of considering both litter quality and microbial interaction in ecological studies. This approach offers critical insight into the advantages and limitations of each decomposition methodology within alpine wetland ecosystems.

Herein, we report a carbon-stabilized porous silicon (pSi)-based electrochemical biosensing platform for the label- and amplification-free detection of bacterial 16S rRNA gene fragments that facilitates pan-bacterial detection. The sensing approach combines thermally carbonized pSi (THCpSi) structures as novel porous electrochemical transducers, and a highly sensitive sensing mechanism based on partial blockage of the pores caused by hybridization of 16S rRNA gene fragment to the DNA capture probe immobilized within the pores. Pore blockage upon RNA hybridization was quantified via differential pulse voltammetry as a decrease in the oxidation current of the redox pair ([Fe(CN)6]3/4−) added to the measuring solution. The use of carbon-stabilized pSi to build the biosensor has additional benefits: it favors high density of the immobilized bioreceptors and a large electroactive surface area, both further enhancing the overall sensitivity of the biosensor. The easily adjustable pSi morphology is key to design diagnostic tools fit-for-purpose. By tailoring the pore diameter, pore blockage upon analyte hybridization can be maximized, thus enhancing sensitivity. By tailoring film thickness, the surface area can be adjusted to optimize the amount of immobilized bioreceptors and the electroactive surface area. An excellent sensing performance was achieved by building the biosensor on THCpSi structures featuring a 27 nm pore diameter and a 1.6 μm film thickness, whose external surface was coated with a thin layer of silicon nitride (Si3N4), the latter contributing to maximize the pore blockage. The biosensor achieved a limit of detection of 2.3 pM when tested in 5% fetal bovine serum.

Litter decomposition is the main source of soil organic carbon (SOC) pool, regarding as an important part of terrestrial ecosystem C dynamics. The turnover of SOC is mainly regulated by extracellular enzymes secreted by microorganisms. However, the response mechanism of soil C-degrading enzymes and SOC in litter decomposition remains unclear. To clarify how SOC fraction dynamics respond to C-degrading enzymes in litter decomposition, we used field experiments to collect leaf litter and SOC fractions from the underlying layer in Robinia pseudoacacia plantations on the Loess Plateau. Our results showed that SOC, easily oxidizable organic C, dissolved organic C, and microbial biomass C increased significantly during the decomposition process. Litter decomposition significantly decreased soil hydrolase activity, but slightly increased oxidase activity. Correlation analysis results showed that SOC fractions were significantly positively correlated with the litter mass, lignin, soil moisture, and oxidase activity, but significantly negatively correlated with cellulose content and soil pH. Partial least squares path models revealed that soil C-degrading enzymes can directly or indirectly affect the changes of soil C fractions. The most direct factors affecting the SOC fractions of topsoil during litter decomposition were litter lignin and cellulose degradation, soil pH, and C-degrading enzymes. Furthermore, regression analysis showed that the decrease of SOC stability in litter decomposition was closely related to the decrease of soil hydrolase to oxidase ratio. These results highlighted that litter degradation-induced changes in C-degrading enzyme activity significantly affected SOC fractions. Furthermore, the distribution of soil hydrolases and oxidases affected the stability of SOC during litter decomposition. These findings provided a theoretical framework for a more comprehensive understanding of C turnover and stabilization mechanisms between plant and soil. [ABSTRACT FROM AUTHOR]

Chemical structure of organic matter (OM) pools in the arable layers of agrochernozems (non-eroded, eroded, and depositional has been studied by solid-state 13C-NMR spectroscopy. As has been shown, two competing processes simultaneously take place in the erosional zone: decomposition of the OM of the underlying horizon exposed due to erosion, and the stabilization of fresh OM having entered with the crop residues (OM dynamic replacement). Analytical data suggest that the OM dynamic replacement in the erosional zone efficiently compensate for the OM decomposition, as is evidenced by the highest C/N ratio of all studied OM pools in the eroded agrochernozem along with the absence of statistically significant differences in the integral characteristics of their chemical structure. However, a continuous removal of the upper soil layer from the eroded agrochernozem with each erosion event does not fully compensate for the quantitative OM losses there. The most labile OM part can be mineralized during transportation of the eroded material to the depositional zone. Accordingly, the OM entering the depositional zone is to a greater degree transformed as compared with that of the eroded agrochernozem. Nevertheless, characteristic of the depositional agrochernozem is an increased accumulation of organic carbon in the bulk soil and all examined OM pools. Correspondingly, the continuous OM inputs from the slope position subject to erosion with its subsequent burial after each consecutive erosion event, as well as the repacking/aggregation of the newly deposited OM, efficiently contribute to the deposition of organic carbon in the depositional zone. [ABSTRACT FROM AUTHOR]

Biochar is useful for soil organic carbon (SOC) sequestration. However, the effects of biochar aging and addition rates on SOC stabilization are unclear. A field experiment with four biochar application rates (0% (control), 1% (LB), 2% (MB), and 4% (HB) of dry fluvo-aquic soil) was conducted. Soil samples were sampled after 8, 12, and 24 months of its application to clarify the question. In general, SOC gradually increased with the biochar application rate. SOC with HB was higher than that in other treatments, while the ratio of microbial biomass carbon (MBC)/SOC and readily oxidizable carbon (ROC)/SOC with HB was lower than that in other treatments (p < 0.05), indicating a positive effect of HB for C stabilization over time. The effects of biochar on the soil carbon pool management index (CPMI) changed from negative to positive after 8 and 24 months of biochar application. The activities of β-D-glucosidase (βG), cellobiohydrolase (CBH), and β-N-acetylglucosaminidase (NAG) under HB were higher than with other treatments after 12 and 24 months of biochar application (p < 0.05) and negatively correlated with the ratio of MBC/SOC and ROC/SOC over time. The CPMI was positively related with βG and CBH activities after 8 and 24 months of biochar application, respectively (p < 0.05). HB increased the relative abundance of oligotrophs, including Acidobacteria, Actinobacteria, and Chloroflexi, but decreased the relative abundance of copiotrophs, including γ-Proteobacteria and Bacteroidetes over time (p < 0.05). The ratio of dissolved organic carbon (DOC)/SOC was positively correlated with the bacterial oligotroph/copiotroph ratio and significantly affected the oligotrophic and copiotrophic bacterial communities, especially after 8 and 12 months of biochar application (p < 0.05). These findings reinforce that increasing the biochar application rate and time enhances SOC stabilization by decreasing the proportions of labile organic carbon and making oligotrophic/copiotrophic communities and enzyme activities more conducive to C sequestration. [ABSTRACT FROM AUTHOR]

Highlights Increasing HTC temperature (190, 210 and 230 °C) resulted in physico-chemical and structural differences in the HCs. HC addition to a Podzol potentially resulted in a positive priming effect in the free OM fraction after 1 year. The more stable SOM fractions of the HC-amended Podzol contained more OC than the (HC free) control after 1 year.

Natural 13C abundances in different organic matter (OM) pools in the water-stable macro- and free microaggregates of Haplic Chernozem under contrasting land uses (steppe and long-term bare fallow) are described. The 13C fractionation at individual stages in the formation of OM pools is relatively constant regardless of the level of soil structural organization. The proposed conceptual scheme, allowing for quantification of the carbon (C) fluxes in the system of aggregate/OM pool, illustrates this assertion. As is shown, the main C fluxes in the OM pools go from the free OM (LFfr) to the residue fraction (Res) through microaggregates within water-stable aggregates (mWSAs), the components of which are the occluded OM (LFocc) and clay fraction (Clay). With a high degree of probability, C migrates from macroaggregates (WSAma) to free microaggregates (WSAmi). However, a higher probability of the C flux from the mWSA into the Res as compared with the direct C fluxes from the LFocc and Clay supports the hypothesis that the Res is to a greater degree represented by pieces/fragments (50–1 μm in size) of disintegrated mWSAs. Regardless of the size, the WSAs contain labile OM (as the components of mWSAs) along with LFfr (only in macroaggregates) and stable OM (Res). The labile OM (LFocc and Clay) within WSAmi displays a lower degree of microbial transformation (a "lighter" isotopic signature) compared to that in WSAma, which is a result of its better "physical" protection against microbial attacks. However, the most stable OM pool, concentrated in the Res within WSAmi, is enriched in 13C as compared with that in WSAma. Considering that the Res determines the total isotopic composition of C in WSAs, the OM of free microaggregates is generally characterized by a higher degree of microbial processing compared to that of macroaggregates. Free microaggregates are pieces of disintegrated macroaggregates. [ABSTRACT FROM AUTHOR]

The impact of cropping systems on soil aggregation and associated carbon (C) and nitrogen (N) stabilization in relation to soil's antecedent C level has not been well addressed, which is essential for prioritization of agricultural soils for C and N sequestration. In this background, the present study investigated the influence of maize-wheat (MW) and soybean-wheat (SW) cropping systems and continuous fallow (CF) on soil aggregation and associated C and N storage in four soils of same type and texture (sandy loam textured Typic Ustorthents) but different antecedent C levels: a low-C soil (Soil 1, 5.6 g C kg–1), two medium-C soils (Soil 2, 9.0 g C kg–1 and Soil 3, 9.6 g C kg–1) and a high-C soil (Soil 4, 12.9 g C kg–1). While in low-C soil, MW outperformed the SW in increasing aggregate mean weight diameter (MWD) by 13% and C and N preservation capacities of macroaggregate fractions by 5–52%; in medium-C soils, the opposite occurred where SW showed 4–9% increased MWD and 8–76% increased C and N preservation capacity over MW. Contrarily in high-C soil, the two cropping systems behaved similarly; these decreased the aggregate MWD by 6–10% and the macroaggregate-preserved C and N by 8–39% compared to the CF. Changes in macroaggregate C and N storage were significantly related to bulk soil C and N levels (R2 = 0.65–0.85, p < 0.05). Conclusively, the selection of cropping systems to improve aggregate C and N storage must preconsider the antecedent soil C level; because the magnitude and direction of cropping impacts on C and N storage depend on soil's antecedent C level. For a greater physical stabilization of sequestered C and N, in the Entisols of northwest India, MW and SW may be promoted to low- and medium-C soils, respectively. [ABSTRACT FROM AUTHOR]

Agricultural activities generate CO2, CH4, and N2O, affecting the global climate and the sustainability of agricultural production systems. This topic is essential in those areas where agriculture has caused soil decarbonization. The soil can regenerate by implementing sustainable soil management (SSM), and this regeneration is finite. Therefore, it is necessary to determine the maximum carbon (C) storage capacity to establish the most SSM for soil recarbonization. This research analyzes the C storage capacity in soils with rainfed olive groves and traditional tillage in the largest olive-oil-producing area in the world (Jaén, Andalusia, Spain). The results show that these soils had low soil organic C (SOC) content, ranging from 5.16 g kg−1 (topsoil) to 1.60 g kg−1 (subsoil) and low SOC stock (SOC-S) (43.12 Mg ha−1; 0–120 cm depth). In addition, the SOC fractionation showed that the highest SOC concentrations were in the particulate organic C form. The SOC-S linked to the fine mineral fraction (<20 µm) in topsoil was 21.93 Mg C ha−1, and the SOC-S saturated ranged between 50.69 and 33.11 Mg C ha−1. Therefore, on the soil surface (0–32.7 cm depth), these soils have a C storage maximum capacity of 28.76 Mg C ha−1, with a net C sink capacity of 105.55 Mg ha−1 of CO2-eq. All this suggests that these soils could have a high recarbonization capacity, and applying SSM (in the coming years) could be an essential C sink. [ABSTRACT FROM AUTHOR]

Soil organic matter is the dominant pool of carbon (C) in terrestrial ecosystems. Recent advances in understanding of the mechanisms of C stabilization in the soil emphasize microbes as the main drivers. Special attention is placed on the accumulation of bacterial and fungal necromasses. This calls for development of fast and reliable methods to estimate microbial necromass in a various type of soils, including peat soils. Here we provide precise method to measure fungal and bacterial necromasses with high-pressure liquid chromatography-fluorescence detector (HPLC-FLD) and its comparison with gas chromatography method. Purity of the chromatographic peaks was confirmed with mass spectrometry. The HPLC-FLD method provides reliable results for mineral, organic and highly organic peat soils. • we provide a method to study bacterial and fungal necromass using HPLC-FLD. • we confirmed purity of the chromatographic peaks with mass spectrometry. • method enables to measure microbial necromass from mineral and highly organic soils. [ABSTRACT FROM AUTHOR]

Mulched drip irrigation (MDI) is generally accepted as a method to decrease soil salinization and improve crop yields in arid and semi-arid regions. However, there remain gaps in how MDI drives soil organic carbon (SOC) dynamic microbial assembly processes with time, and the mediating role of microorganisms remains unclear. In this study, we investigated the aforementioned issues across soil profiles in cotton fields with different years of MDI. The results showed that MDI did not cause the differences in SOC, particular organic carbon (POC), and mineral-associated organic carbon (MOC) in soil layers. The POC and MOC contents had a parabola relationship with time, and showed an opposite trend in soil. After 15 years of MDI, the ratio of MOC/SOC increased to a peak value of 50 % and 52 % in topsoil and subsoil, respectively; the ratio of POC/SOC decreased to valley values of 50 % and 48 %, respectively (P < 0.05). Long-term MDI reduced the differences in oxidase between soil layers but accelerated SOC loss by increasing polyphenol oxidase activity (P < 0.05). Compared with that of other years, with 10 years of MDI, bacterial Shannon diversity decreased to a valley value, and fungal Shannon diversity reached to a top value in subsoil (P < 0.05). In general, stochastic processes were mainly controlled by dispersal limitation, and undominated processes dominated microbial assembly; however, there was a close relationship between bacterial communities and organic carbon fractions. The high percentage of positive linkages among microorganisms indicated that long-term MDI was beneficial for carbon fixation. Additionally, a decrease of fungal oligotroph/copiotroph ratio, the relative abundance of Ascomycota and Basidiomycota was beneficial for the accumulation of SOC and POC in topsoil (P < 0.05). In conclusion, long-term MDI is useful for the fixation of organic carbon via improving soil POC content and strengthening linkages within community assemblies. • The content of MOC increased with prolonging of MDI. • Long-term MDI weakens the effect of soil depth on microbial community structure. • Bacterial and fungal community assemblies were dominated by stochastic processes. • Long -term MDI decreased soil TDS, and increased MOC by improving PPO activity. • Fungal oligotroph/copiotroph ratio regulates SOC fractions in topsoil. [ABSTRACT FROM AUTHOR]

Biochar is useful for soil organic carbon (SOC) sequestration. However, the effects of biochar aging and addition rates on SOC stabilization are unclear. A field experiment with four biochar application rates (0% (control), 1% (LB), 2% (MB), and 4% (HB) of dry fluvo-aquic soil) was conducted. Soil samples were sampled after 8, 12, and 24 months of its application to clarify the question. In general, SOC gradually increased with the biochar application rate. SOC with HB was higher than that in other treatments, while the ratio of microbial biomass carbon (MBC)/SOC and readily oxidizable carbon (ROC)/SOC with HB was lower than that in other treatments (p < 0.05), indicating a positive effect of HB for C stabilization over time. The effects of biochar on the soil carbon pool management index (CPMI) changed from negative to positive after 8 and 24 months of biochar application. The activities of β-D-glucosidase (βG), cellobiohydrolase (CBH), and β-N-acetylglucosaminidase (NAG) under HB were higher than with other treatments after 12 and 24 months of biochar application (p < 0.05) and negatively correlated with the ratio of MBC/SOC and ROC/SOC over time. The CPMI was positively related with βG and CBH activities after 8 and 24 months of biochar application, respectively (p < 0.05). HB increased the relative abundance of oligotrophs, including Acidobacteria, Actinobacteria, and Chloroflexi, but decreased the relative abundance of copiotrophs, including γ-Proteobacteria and Bacteroidetes over time (p < 0.05). The ratio of dissolved organic carbon (DOC)/SOC was positively correlated with the bacterial oligotroph/copiotroph ratio and significantly affected the oligotrophic and copiotrophic bacterial communities, especially after 8 and 12 months of biochar application (p < 0.05). These findings reinforce that increasing the biochar application rate and time enhances SOC stabilization by decreasing the proportions of labile organic carbon and making oligotrophic/copiotrophic communities and enzyme activities more conducive to C sequestration.

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 (O2) 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 13C methoxyl-labeled lignin mineralization in soils that were exposed to static or fluctuating O2. Iron addition suppressed lignin mineralization to 21% of controls, regardless of O2 availability. However, Fe addition had no effect on soil CO2 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.

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 (O2) 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 13C methoxyl-labeled lignin mineralization in soils that were exposed to static or fluctuating O2. Iron addition suppressed lignin mineralization to 21% of controls, regardless of O2 availability. However, Fe addition had no effect on soil CO2 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.

To successfully reduce atmospheric CO2 by sequestering additional soil carbon, it is essential to understand the potential of a given soil to store carbon in a stable form. Carbon that has formed organo-mineral complexes with silt and clay particles is believed to be less susceptible to decay than non-complexed, or particulate, organic carbon. Using direct measurements of mineral associated organic matter (MAOC) on a subset of samples, and an approach developed previously for primarily allophanic soils, we took a modeling approach to estimate MAOC for 537 samples of much coarser and younger soils from 99 non-cultivated and agricultural sites in the Okanagan Valley, British Columbia, Canada. Using specific surface area (SSA) or soil texture as indicators of the mineral surface area available for sorption of organic matter, we used both Random Forest (RF) and Stepwise Multiple Regression with Akaike Information Criterion (SMR) to determine a best fit model for predicting MAOC. Random Forest modeling using SSA in addition to total SOC, exchangeable calcium, exchangeable potassium, and soil pH performed better than SMR for determining MAOC in these soils (R²: 0.790 for RF; R²: 0.713 for SMR). To determine if a MAOC deficit existed for these soils, we then applied a quantile regression approach wherein the predicted 90th quantile of MAOC represents the MAOC formation capacity. We determined that MAOC deficits were present in all soils and increased with depth. Moreover, clay rich soils had greater MAOC deficits (1.62 g kg-1 for 0-15 cm, 4.01 g kg-1 for 15-30 cm, and 5.80 g kg-1 for 30-60 cm), than sandier soils (1.01 g kg-1 for 0-15 cm, 2.72 g kg-1 for 15-30 cm, and 3.69 g kg-1 for 30-60 cm). Furthermore, the upper 30 cm of these soils have the potential to increase MAOC stocks by 29% (48.0 million kg of MAOC over 8,501 ha) before they reach formation capacity. This study highlights the variability in MAOC formation capacity of soils with different physicochemical properties and provides a framework for estimating MAOC concentrations and deficits for soils with a wide range of physicochemical properties. [ABSTRACT FROM AUTHOR]

To successfully reduce atmospheric CO2 by sequestering additional soil carbon, it is essential to understand the potential of a given soil to store carbon in a stable form. Carbon that has formed organo-mineral complexes with silt and clay particles is believed to be less susceptible to decay than non-complexed, or particulate, organic carbon. Using direct measurements of mineral associated organic matter (MAOC) on a subset of samples, and an approach developed previously for primarily allophanic soils, we took a modeling approach to estimate MAOC for 537 samples of much coarser and younger soils from 99 non-cultivated and agricultural sites in the Okanagan Valley, British Columbia, Canada. Using specific surface area (SSA) or soil texture as indicators of the mineral surface area available for sorption of organic matter, we used both Random Forest (RF) and Stepwise Multiple Regression with Akaike Information Criterion (SMR) to determine a best fit model for predicting MAOC. Random Forest modeling using SSA in addition to total SOC, exchangeable calcium, exchangeable potassium, and soil pH performed better than SMR for determining MAOC in these soils (R2: 0.790 for RF; R2: 0.713 for SMR). To determine if a MAOC deficit existed for these soils, we then applied a quantile regression approach wherein the predicted 90th quantile of MAOC represents the MAOC formation capacity. We determined that MAOC deficits were present in all soils and increased with depth. Moreover, clay rich soils had greater MAOC deficits (1.62 g kg−1 for 0–15 cm, 4.01 g kg−1 for 15–30 cm, and 5.80 g kg−1 for 30–60 cm), than sandier soils (1.01 g kg−1 for 0–15 cm, 2.72 g kg−1 for 15–30 cm, and 3.69 g kg−1 for 30–60 cm). Furthermore, the upper 30 cm of these soils have the potential to increase MAOC stocks by 29% (48.0 million kg of MAOC over 8,501 ha) before they reach formation capacity. This study highlights the variability in MAOC formation capacity of soils with different physicochemical properties and provides a framework for estimating MAOC concentrations and deficits for soils with a wide range of physicochemical properties.

Ice-rich permafrost has been subject to abrupt thaw and thermokarst formation in the past and is vulnerable to current global warming. The ice-rich permafrost domain includes Yedoma sediments that have never thawed since deposition during the late Pleistocene and Alas sediments that were formed by previous thermokarst processes during the Lateglacial and Holocene warming. Permafrost thaw unlocks organic carbon (OC) and minerals from these deposits and exposes OC to mineralization. A portion of the OC can be associated with iron (Fe), a redox-sensitive element acting as a trap for OC. Post-depositional thaw processes may have induced changes in redox conditions in these deposits and thereby affected Fe distribution and interactions between OC and Fe, with knock-on effects on the role that Fe plays in mediating present day OC mineralization. To test this hypothesis, we measured Fe concentrations and proportion of Fe oxides and Fe complexed with OC in unthawed Yedoma and previously thawed Alas deposits. Total Fe concentrations were determined on 1,292 sediment samples from the Yedoma domain using portable X-ray fluorescence; these concentrations were corrected for trueness using a calibration based on a subset of 144 samples measured by inductively coupled plasma optical emission spectrometry after alkaline fusion (R2 = 0.95). The total Fe concentration is stable with depth in Yedoma deposits, but we observe a depletion or accumulation of total Fe in Alas deposits, which experienced previous thaw and/or flooding events. Selective Fe extractions targeting reactive forms of Fe on unthawed and previously thawed deposits highlight that about 25% of the total Fe is present as reactive species, either as crystalline or amorphous oxides, or complexed with OC, with no significant difference in proportions of reactive Fe between Yedoma and Alas deposits. These results suggest that redox driven processes during past thermokarst formation impact the present-day distribution of total Fe, and thereby the total amount of reactive Fe in Alas versus Yedoma deposits. This study highlights that ongoing thermokarst lake formation and drainage dynamics in the Arctic influences reactive Fe distribution and thereby interactions between Fe and OC, OC mineralization rates, and greenhouse gas emissions.

The rice-wheat cropping system covers a considerable area of the Indo-Gangetic Plain zone (IGPZ) due to agro-pedological compatibility. The nutrient requirements of the rice-wheat system in an organic mode are met through locally available organic matter (OM) is a subject of investigation from the viewpoint of declining the underlying mechanisms. A long-term organic farming experiment was carried out at the research farm of ICAR-Indian Agricultural Research Institute, New Delhi, India. Locally available sources of OM's such as Sesbania green manuring, Leucaena green leaf manuring, farmyard manure (FYM), blue-green algae, and Azotobacter were evaluated for changes in soil physico-chemical properties and crop yield response after 15 years continuous applied of OM's. Our results indicate that treatment involving applied Sesbania green manure + farmyard manure + blue-green algae to rice + Leucaena green leaf manuring + farmyard manure + Azotobacter to wheat [SFB(R) + LFA(W)] showed a sharp decline in soil pH by 5.5%, soil electrical conductivity (EC) by 24.0%, and soil bulk density by 14.0% over the control within the 0–15 cm soil depth. While, the treatment SFB(R) + LFA(W) improved the soil large macroaggregate (>2.0 mm) by 85.0% in 0–7.5 cm, and 92.8% in 7.5–15 cm in soil depth. Similarly, the treatment SFB(R) + LFA(W) showed 3.2-, 2.8-fold higher mean weight diameter (MWD) in corresponding soil depths of 0–7.5 cm, 7.5–15 cm compared to the control. Treatment SFB(R) + LFA(W) increased in soil organic carbon (SOC) holding by 5-folds in soil large macroaggregate, 4-folds by soil small macroaggregate (2.0–0.25 mm), 5-folds by micro-aggregate (0.25–0.053 mm), and 9-folds by silt + clay fraction (<0.053 mm) than control in 0–15 cm soil depth. These observations strongly support greater carbon recalcitrance with a higher half-life in soil silt + clay fraction than in other soil fractions. Further, treatment SFB(R) + LFA(W) maintained higher SOC by 79.4% and carbon stock by 76.6% over control in 0–15 cm soil depth. These responses on soil aggregates and SOC changes translated into significant crop responses. The treatment SFB(R) + LFA(W) thus, showed correspondingly higher grain yield (5.41 Mg ha–1 and 4.69 Mg ha–1) of rice and wheat. Our study though showed on par agronomic response between low-quality OM's and mixed-quality OM's in 15-years. But considering the loading of SOC in the silt + clay fraction of soil, the study foresees a higher recalcitrant of SOC compared to any other soil fractions. This could well strengthen the process of soil aggregation having cascading response on other soil health-defined parameters a requisite for sustaining the rice-wheat sequence in the IGPZ. • 15-year application of organic materials (OM's) improves soil properties. • Applied mixed-quality OM's had decreased the soil bulk density, EC, and pH. • Applied mixed-quality OM's significantly increased the MWD and soil aggregation. • The soil small macroaggregate found a higher organic carbon holding in the plough layer. • The mixed-quality OM's recorded a 54.78 Mg ha–1 carbon stock in the plough layer. [ABSTRACT FROM AUTHOR]

[Display omitted] • Compares how native and standardized litter decompose in wet and dry alpine wetland conditions. • Examines how wet and dry conditions affect soil microbial activity in alpine wetlands. • Identifies different factors influencing the decomposition of native vs. standardized litter. Alpine wetlands, critical ecosystems in high-altitude mountain areas, play essential roles in water conservation, biodiversity protection, and carbon (C) sequestration. These ecosystems are particularly sensitive to climate change, with temperature and precipitation variations significantly impacting their structure and functional processes, such as litter decomposition, a key mechanism for C stabilization. This study focused on the Zoige wetland, a representative alpine wetland located on the eastern edge of the Qinghai-Tibet plateau. An incubation experiment was conducted with soil samples under simulated wet and dry moisture conditions to evaluate the environmental impacts on litter decomposition within this ecosystem. In this study, we employed the Tea Bag Index method, utilizing standardized litter, alongside native litter bags to compare the decomposition processes. This comparison between the uniform composition of standardized litter and the chemically diverse native plant litter aims to provide a comprehensive understanding of litter decomposition dynamics in alpine wetland ecosystems. Our finding showed that both standardized and native litter decomposition significantly decreased under dry conditions, in contrast to wetter condition. The components of both standardized and native litter exhibited a decrease over time, with the easily decomposable fractions breaking down swiftly, in contrast to the slower decomposition of the more resistant components. Furthermore, soil exo -enzyme activities varied significantly with environmental conditions. Wet conditions were observed to enhance soil microbial activity, whereas dry conditions resulted in shifts in microbial biomass C and nitrogen, indicative of drought resilience. The correlation analysis revealed that the composition of native litter is the primary factor influencing its decomposition. In contrast, the decomposition of standardized litter was influenced by both its composition and soil microbial activity. Thus, the distinction between the influences on native and standardized litter decomposition highlights the necessity of considering both litter quality and microbial interaction in ecological studies. This approach offers critical insight into the advantages and limitations of each decomposition methodology within alpine wetland ecosystems. [ABSTRACT FROM AUTHOR]

Evidence-based decisions governing sustainable agricultural land management practices require a mechanistic understanding of soil organic matter (SOM) transformations and stabilization of carbon in soil. Large amounts of carbon from organic fertilizers, root exudates, and crop residues are input into agricultural soils. Microbes then catalyze soil biogeochemical processes including carbon extracellular transformation, mineralization, and assimilation of resources that are later returned to the soil as metabolites and necromass. A systems biology approach for a holistic study of the transformation of carbon inputs into stable SOM requires the use of soil “omics” platforms (metagenomics, metatranscriptomics, metaproteomics, and metabolomics). Linking the data derived from these various platforms will enhance our knowledge of structure and function of the microbial communities involved in soil carbon cycling and stabilization. In this review, we discuss the application, potential, and suitability of different “omics” approaches (independently and in combination) for elucidating processes involved in the transformation of stable carbon in soil. We highlight biases associated with these approaches including limitations of the methods, experimental design, and soil sampling, as well as those associated with data analysis and interpretation.

In the Arctic zone, where up to 1024 × 1013 kg of organic matter is stored in permafrostaffected soils, soil organic matter consists of about 50% humic substances. Based on the analysis of the molecular composition of humic acids, we assessed the processes of accumulation of the key structural fragments, their transformations and the stabilization rates of carbon pools in soils in general. The landscape of the Lena River delta is the largest storage of stabilized organic matter in the Arctic. There is active accumulation and deposition of a significant amount of soil organic carbon from terrestrial ecosystems in a permafrost state. Under ongoing climate change, carbon emission fluxes into the atmosphere are estimated to be higher than the sequestration and storing of carbon compounds. Thus, investigation of soil organic matter stabilization mechanisms and rates is quite an urgent topic regarding polar soils. For study of molecular elemental composition, humic acids were separated from the soils of the Lena River delta. Key structural fragments of humic matter were identified and quantified by CP/MAS 13C NMR spectroscopy: carboxyl (-COOR); carbonyl (-C=O); CH3-; CH2-; CH-aliphatic; -C-OR alcohols, esters and carbohydrates; and the phenolic (Ar-OH), quinone (Ar = O) and aromatic (Ar-) groups as benchmark Cryosols of the Lena delta river terrestrial ecosystem. Under the conditions of thermodynamic evolutionary selection, during the change between the dry and wet seasons, up to 41% of aromatic and carboxyl fragments accumulated in humic acids. Data obtained showed that three main groups of carbon played the most important role in soil organic matter stabilization, namely C, H-alkyls ((CH2)n/CH/C and CH3), aromatic compounds (C-C/C-H, C-O) and an OCH group (OCH/OCq). The variations of these carbon species' content in separated humics, with special reference to soil-permafrost organic profiles' recalcitrance in the current environment, is discussed. [ABSTRACT FROM AUTHOR]

Alpine wetlands are critical ecosystems for global carbon (C) cycling and climate change mitigation. Ecological restoration projects for alpine grazing wetlands are urgently needed, especially due to their critical role as carbon (C) sinks. However, the fate of the C pool in alpine wetlands after restoration from grazing remains unclear. In this study, soil samples from both grazed and restored wetlands in Zoige (near Hongyuan County, Sichuan Province, China) were collected to analyze soil organic carbon (SOC) fractions, arbuscular mycorrhizal fungi (AMF), soil properties, and plant biomass. Moreover, the Tea Bag Index (TBI) was applied to assess the initial decomposition rate ( k ) and stabilization factor ( S ), providing a novel perspective on SOC dynamics. The results of this research revealed that the mineral-associated organic carbon (MAOC) was 1.40 times higher in restored sites compared to grazed sites, although no significant difference in particulate organic carbon (POC) was detected between the two site types. Furthermore, the increased MAOC after restoration exhibited a significant positive correlation with various parameters including S , C and N content, aboveground biomass, WSOC, AMF diversity, and NH 4 + . This indicates that restoration significantly increases plant primary production, litter turnover, soil characteristics, and AMF diversity, thereby enhancing the C stabilization capacity of alpine wetland soils., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Tang, Li, Bao, Tang, Li, Ding, Luo, Zeng, Liu, Shu, Liu and Du.)

The interaction between tannins and proteins has been studied intensively for more than half a century as a result of its significance for various applications. In chemical ecology, tannins are involved in response to environmental stress, including biotic (pathogens and herbivores) and abiotic (e.g., drought) stress, and in carbon (C) and nutrient cycling. This perspective summarizes the newest insights into the role of tannins in soil processes, including the interaction with fungi leading to C stabilization. Recent knowledge presented here may help to optimize land management to increase or preserve soil C to mitigate climate change.

An improved identification of the environmental variables that can be used to predict the content of soil organic carbon (SOC) stored belowground is required to reduce uncertainties in estimating the response of the largest terrestrial carbon reservoir to environmental change. Recent studies indicate that some metal cations can have an active role in the stabilization of SOC, primarily by coordinating the interaction between soil minerals and organic matter through cation bridging and by creating complexes with organic molecules when their hydration shells are displaced. The effective cation exchange capacity (CEC eff.) is a measure that integrates information about available soil surfaces to which metal cations are retained. Therefore, we critically tested the relationship between CEC eff. and SOC content using regression analyses for more than 1000 forest sites across Switzerland, spanning a unique gradient of mean annual precipitation (640–2500 mm), elevation (277–2207 m a.s.l), pH (2.8–8.1) and covering different geologies and vegetation types. Within these sites, SOC content is significantly related to CEC eff., in both topsoils and subsoils. Our results demonstrate that, on a pH-class average, in Swiss forest topsoils ( 5.5, between 59 and 83% of subsoil CEC eff. originates from exchangeable calcium, whereas in acidic soils exchangeable aluminum contributes between 21 and 44% of the CEC eff. Exchangeable iron contributes to less than 1% of the variability in CEC eff. Overall this study indicates that in Swiss forests subsoils, CEC eff. strongly reflects the surface of soil minerals to which SOC can be bound by metal cations. The strength of the relationship between CEC eff. and SOC content depends on the pH of the soil, with the highest amount of variation of SOC content explained by CEC eff. in subsoils with pH > 5.5.

Aims: Mineral-associated organic matter, mainly derived from microbial by-products, persists longer in soil compared to particulate organic matter (POM). POM is highly recalcitrant and originates largely from decomposing root and shoot litter. Theory suggests that root traits and growth dynamics should affect carbon (C) accumulation into these different pools, but the specific traits driving this accumulation are not clearly identified. Methods: Twelve herbaceous species were grown for 37 weeks in monocultures. Root elongation rate (RER) was measured throughout the experiment. At the end of the experiment, we determined morphological and chemical root traits, as well as substrate induced respiration (SIR) as a proxy for microbial activity. Carbon was measured in four different soil fractions, following particle-size and density fractionation. Results: Root biomass, RER, root diameter, hemicellulose content and SIR (characteristic of N2-fixing Fabaceae species), were all positively correlated with increased C in the coarse silt fraction. Root diameter and hemicellulose content were negatively correlated with C in the POM fraction, that was greater under non N2-fixing Poaceae species, characterized by lignin-rich roots with a high carbon:nitrogen ratio that grew slowly. The accumulation of C in different soil pools was mediated by microbial activity. Conclusions: Our results show that root traits determine C input into different soil pools, mediated primarily by microbial activity, thus determining the fate of soil organic C. We also highlight that C in different soil pools, and not only total soil organic C, should be reported in future studies to better understand its origin, fate and dynamics. [ABSTRACT FROM AUTHOR]

Specific surface area can be a strong predictor of organic carbon (SOC) contents in soils. Specific surface area can be estimated reliably and cost‐effectively from water adsorption by air‐dry soil samples, but SOC itself can also adsorb water. For estimating the mineral component of specific surface area, it is, therefore, necessary to exclude water‐adsorption by SOC. Here, we refer to "apparent specific surface area" for measurements that include water adsorption by both mineral soil and SOC. We used a mathematical approach to estimate water adsorption by SOC so that this component can be subtracted from measurements of apparent specific surface area. We used a dataset of apparent specific surface area and soil carbon at seven depths from 50 soil cores collected from a research farm in the Manawatu region in New Zealand. Both apparent specific surface area and SOC content decreased with soil depth with very high correlation (r2 = 0.98). We estimated the SOC contribution to apparent specific surface area from the slope of the relationship between changes in apparent specific surface area and SOC content. For our soils, the SOC contribution to apparent specific surface area was estimated as 0.43 ± 0.02 m2 mgC−1. This parameter allows apparent specific surface area measurements to be corrected for the water adsorption by SOC to calculate the functionally relevant mineral specific surface area. Highlights: Soil surface area can be estimated from the H2O content of air‐dry soil but SOC also adsorbs H2O.We developed a mathematical approach to estimate water adsorption by SOC.We estimated the contribution of SOC to apparent specific surface area as 0.43 ± 0.02 m2 mgC−1.Mineral specific surface area can be inferred by subtracting SOC‐based H2O adsorption. [ABSTRACT FROM AUTHOR]

Against the background of current understanding of dissolved organic carbon (DOC) adsorption onto clay surfaces, it remains unclear if bulk DOC or its fractions contribute to microaggregate formation in the top layers of coarse‐textured soils. We therefore investigated the effects of long‐term inputs of biochemically contrasting organic residues on the chemical characteristics and vertical distribution of DOC in a coarse‐textured Ultisol. During 2007–2008, DOC samples were extracted from soil profiles of a long‐term residue quality field experiment initiated in 1995. In this field experiment, groundnut stover, dipterocarp and tamarind leaf litter, as well as rice straw of contrasting biochemical quality, were applied yearly at 10 Mg ha−1. Groundnut, dipterocarp and tamarind produced large amounts (7.1–11.8 g C m−2) of high‐molecular‐weight (HMW; > 10 kDa) DOC, which was found in high concentrations (30–50 mg C kg−1) in the topsoil (0–15 cm). Rice straw, however, produced large amounts (3.5 g C m−2) of low‐molecular‐weight (LMW; < 1 kDa) DOC during the initial stage of decomposition. Although the HMW DOC was retained in the topsoil (0–15 cm), the LMW DOC was rapidly translocated to lower soil depths (60–80 cm). This translocation was facilitated by the low adsorption potential of the rice straw‐derived LMW DOC on colloidal surfaces of the topsoil. There was a significant positive correlation of C in the HMW DOC with that in fine particles, indicating their contribution to microaggregate formation and thus C accumulation. It was concluded that biochemical quality of residues as a determinant of concentration and chemistry of DOC and its vertical dynamics along the soil profile must be considered for SOC accumulation in coarse‐textured soils. Furthermore, we found reasonable indications that HMW DOC contributes to microaggregate formation in topsoils. Highlights: Residue quality determined vertical dynamics of DOC in coarse– textured Ultisols.Lignin‐ and polyphenol‐rich residues produced HMW DOC in topsoil.LMW DOC derived from cellulose‐rich residues was translocated to the subsoil.There were indications that HMW DOC supported microaggregate formation in topsoils. [ABSTRACT FROM AUTHOR]

Sugarcane straw is identified as a potential energy feedstock to increase bioenergy production, but advances are required in understanding the straw removal impacts on soil organic carbon (SOC) stability and storage. The main objective of this study was to evaluate the quantity, quality and physical protection of SOC under areas with different straw removal rates over a six-year period. A field experiment was conducted in randomized blocks with four replications, including the following straw removal treatments: total removal (TR), high removal (HR), low removal (LR), and no removal (NR), corresponding to the quantities of 0, 5, 10 and 15 Mg ha−1 of dry straw maintained on soil surface, respectively. Effects of straw removal on SOC stocks and its temporal dynamics, as well as the rates of carbon (C) incorporated into the soil were evaluated to a 0.3-m depth. The effects on particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions were evaluated through physical fractionation of SOM. Finally, it was evaluated the distribution of aggregates, the C content of aggregates and the C preservation capacity (CPC) in fractions of aggregates. Our findings revealed that TR and HR decreased SOC stocks at 0.0–0.1 m and 0.0–0.3 m depths. The data indicated that about 24% of the C added via straw were retained in soil, and NR showed a SOC accumulation rate of 1.42 Mg ha-¹ year-¹ relative to baseline. Low straw removal increased POM (46.2%) and MAOM (12.4%) in surface layer as compared to TR. The straw maintenance in the field increased the proportion of soil macroaggregates, resulting in higher amounts of C preserved in this fraction. Our findings suggest that maintaining 10 Mg ha-¹ of straw was enough to favor physical protection and sustain storage of SOC over time. Therefore, the surplus of straw could be removed to produce bioenergy, although this management may hinder soil C sequestration and its benefits for sugarcane production and climate change mitigation. • High and total removal rates of sugarcane straw reduced soil carbon stocks. • Low straw removal increased POM (46.2%) and MAOM (12.6%) compared to total removal. • Straw retention increased soil aggregation and preserved C in macroaggregates. • Low removal rate was enough to sustain physical protection and storage of soil C. [ABSTRACT FROM AUTHOR]

In the Arctic zone, where up to 1024 × 1013 kg of organic matter is stored in permafrost-affected soils, soil organic matter consists of about 50% humic substances. Based on the analysis of the molecular composition of humic acids, we assessed the processes of accumulation of the key structural fragments, their transformations and the stabilization rates of carbon pools in soils in general. The landscape of the Lena River delta is the largest storage of stabilized organic matter in the Arctic. There is active accumulation and deposition of a significant amount of soil organic carbon from terrestrial ecosystems in a permafrost state. Under ongoing climate change, carbon emission fluxes into the atmosphere are estimated to be higher than the sequestration and storing of carbon compounds. Thus, investigation of soil organic matter stabilization mechanisms and rates is quite an urgent topic regarding polar soils. For study of molecular elemental composition, humic acids were separated from the soils of the Lena River delta. Key structural fragments of humic matter were identified and quantified by CP/MAS 13C NMR spectroscopy: carboxyl (–COOR); carbonyl (–C=O); CH3–; CH2–; CH-aliphatic; –C-OR alcohols, esters and carbohydrates; and the phenolic (Ar-OH), quinone (Ar = O) and aromatic (Ar–) groups as benchmark Cryosols of the Lena delta river terrestrial ecosystem. Under the conditions of thermodynamic evolutionary selection, during the change between the dry and wet seasons, up to 41% of aromatic and carboxyl fragments accumulated in humic acids. Data obtained showed that three main groups of carbon played the most important role in soil organic matter stabilization, namely C, H-alkyls ((CH2)n/CH/C and CH3), aromatic compounds (C-C/C-H, C-O) and an OCH group (OCH/OCq). The variations of these carbon species’ content in separated humics, with special reference to soil–permafrost organic profiles’ recalcitrance in the current environment, is discussed.

• Macroaggregation was greater under NT compared to CT. • NT promotes occluded-C accumulation mainly in macroaggregates. • The increase in occluded-C accumulation leads to organomineral association. • Legume cover cropping favors the mineral-associated C enrichment in microaggregates. Both no-tillage and legume cover crops have been shown to increase soil organic carbon (SOC) in subtropical soils. However, the mechanisms underpinning management system effects on SOC accumulation are still not well understood. We used a combination of aggregate size and density fractionation to elucidate these mechanisms at a 30-year old experiment on an Acrisol in southern Brazil. The effects of two tillage systems [conventional system (CT) and no-tillage (NT)] combined with three cropping systems [oat/maize (O/M), vetch/maize (V/M) and oat + vetch/maize + cowpea (OV/MC)] were evaluated in the top 20 cm soil layer. Overall, macroaggregation (>0.25 mm) was significantly influenced by tillage with NT showing values 14% greater than CT in the 0–5 cm soil depth. On average, the occluded light fraction-C content in macroaggregates was more than twice as high under NT compared to CT (4.4 vs. 1.8 g kg−1). This effect was more pronounced when legume cover crops were grown. However, the most significant effect of cover crops was observed in the organomineral fraction of microaggregates, especially under NT (12.1 under O/M and 19.8 g kg−1 under OV/MC). Our results suggest that, although NT increased the occluded light fraction-C compared to CT, this effect was smaller than the gains that legume cover crops offered in organomineral association. [ABSTRACT FROM AUTHOR]

Abstract Tillage intensive cropping practices have deteriorated soil physical quality and decreased soil organic carbon (SOC) levels in rice–growing areas of South Asia. Consequently, crop productivity has declined over the years demonstrating the need for sustainable alternatives. Given that, a field experiment was conducted for six years to assess the impact of four tillage based crop establishment treatments [puddled transplant rice followed by conventional tillage in wheat/maize (CTTPR–CT), non–puddled transplant rice followed by zero–tillage in wheat/maize (NPTPR–ZT), zero–till transplant rice followed by zero–tillage in wheat/maize (ZTTPR–ZT), zero–tillage direct seeded rice followed by zero–tillage in wheat/maize (ZTDSR–ZT)], two residue management treatments [residue removal, residue retention (~33%)], and two cropping systems [rice–wheat, rice–maize] on soil aggregation, carbon pools, nutrient availability, and crop productivity. After six years of rotation, in top 0.2 m soil depth, zero–till crop establishment treatments (ZTTPR–ZT and ZTDSR–ZT) had higher (p < 0.05) total organic carbon (TOC) over conventional tillage treatment (CTTPR–CT). Zero–till crop establishment treatments increased very–labile C faction (C frac 1) by 21% followed by labile fraction (C frac 2) (16%), non–labile fraction (C frac 4) (13%) and less–labile fraction (C frac 3) (7%). Notably, higher passive C–pool in conservation tillage practices over CTTPR–CT suggests that conservation tillage could stabilize the recalcitrant form of carbon that persists longer in the soil. Meantime, zero–till crop establishment treatments had higher (p < 0.05) water stable macro–aggregates, macro–aggregates: micro–aggregates ratio and aggregate carbon content over CTTPR–CT. The treatment NPTPR–ZT significantly increased soil quality parameters over CTTPR–CT. However, the effect was not as prominent as that of ZTTPR–ZT and ZTDSR–ZT. Retention of crop residue increased (p < 0.05) TOC (12%) and soil available nutrients mainly available–P (16%), followed by available–K (12%), DTPA–extractable Zn (11%), and available–S (6%) over residue removal treatment. The constructive changes in soil properties following conservation tillage and crop residue retention led to increased crop productivity over conventional CTTPR–CT. Therefore, conservation tillage (particularly ZTTPR–ZT and ZTDSR–ZT) and crop residue retention could be recommended in tropical rice–based cropping systems for improving soil quality and production sustainability. Highlights • Conservation tillage increased soil aggregation over conventional tillage. • Zero-tillage treatments increased labile carbon stabilization over conventional tillage. • Residue retention increased soil aggregation, soil available P, K and Zn over residue removal. • Zero-tillage with residue retention increased crop yields over conventional tillage practice. [ABSTRACT FROM AUTHOR]

Highlights • Inclusion of grain legumes in rice-wheat rotation increased macro-aggregates and SOC. • In lowland rice-soil, stabilization of carbon was higher in active pool than passive pool. • Integrated nutrient management improved soil aggregation and carbon pools over chemical fertilizer treatment. Abstract Deterioration of soil physical quality and depletion of soil organic carbon (SOC) are widespread challenges in tropical rice–wheat growing regions. Consequently, soil productive capacity, production sustainability, and resource use efficiency have declined in this agro-ecosystem. Ecological engineering approach such as legume inclusion in cropping system offers several ecosystem services, and thus, may serve an important role in the restoration of soil health. Given that, a long-term (2003–2015) field experiment was conducted to assess the impact of four rice-based crop rotation treatments [rice–wheat, rice–chickpea, rice–wheat–mungbean, rice–wheat–rice–chickpea] each with three levels of nutrient management treatments [control, integrated nutrient management, chemical fertilizers] on soil aggregation, soil carbon pools, and carbon stabilization. Legume inclusive rotations increased water stable macro-aggregates (WSMA) over rice–wheat rotation in both surface (0–0.2 m) and subsurface (0.2–0.4 m) soil depths. In surface soil, WSMA was the highest in rice–wheat–mungbean rotation (65%) followed by rice–chickpea rotation (57%) and was the least in rice–wheat rotation (50%). Subsequently, grain legume inclusive rotations had the higher aggregate ratio and mean weight diameter over rice–wheat rotation, being the highest in rice–wheat–mungbean rotation. Rice–wheat–mungbean and rice–chickpea rotations had higher carbon concentration in coarse macro-aggregates, meso-aggregates, and coarse micro-aggregates as compared with rice–wheat rotation. Grain legume inclusive rotations increased active carbon pool (9–18%), SOC (6–17%), and carbon management index (5–7%) over rice–wheat rotation, and the order was rice–wheat–mungbean > rice–chickpea ≥ rice–wheat–rice–chickpea > rice–wheat. Integrated nutrient management treatment resulted in higher macro-aggregate (7%), mean weight diameter (6%), active carbon pool (22%), passive carbon pool (16%), SOC (20%), and carbon management index (22%) over the chemical fertilizer treatment. Thus, it is concluded that inclusion of grain legume in lowland rice-based rotation and integrated nutrient management could improve soil aggregation, carbon concentration in aggregates, and soil carbon pools in the tropical soils. The study highlights the prospects of strategic designing of legume inclusive rotation/s for rice agro-ecosystem health and its sustainability. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) is a combination of materials having different origin and with different stabilization and decomposition processes. To determine the different SOM pools and their turnover rates, a silt loam-textured Luvisol from West Hungary was taken from the 0–20 cm soil depth and incubated for 163 days. Maize residues were added to the soil in order to obtain natural 13C enrichment. Four different SOM fractions—particulate organic matter (POM), sand and stable aggregate (S + A), silt- plus clay-sized (s + c) and chemically resistant soil organic carbon (rSOC) fractions—were separated and analyzed using FT-IR, δ13C, and 14C measurements. The mean residence time (MRT) of the new C and the proportion of maize-derived C in the fractions were calculated. The POM fraction was found to be the most labile C pool, as shown by the easily decomposable chemical structures (e.g., aliphatic, O-alkyl, and polysaccharides), the highest proportion (11.7 ± 2.5%) of maize-derived C, and an MRT of 3.6 years. The results revealed that the most stable fraction was the rSOC fraction which had the smallest proportion of maize-derived C (0.18 ± 2.5%) and the highest MRT (250 years), while it was the only fraction with a negative value of Δ14C (−75.0 ± 2.4‰). Overall, the study confirmed the hypothesis that the SOM associated with finer-sized soil particles decomposes the least, highlighting the significance of the fractionation process for more accurate determination of the decomposition processes of SOM pools.

The aim of this work was to compare the carbon (C) mineralization kinetics of three biochars (Formosan ash (Fraxinus formosana Hayata), ash biochar; Makino bamboo (Phyllostachys makino Hayata), bamboo biochar; and lead tree (Leucaena leucocephala (Lam.) de. Wit), lead tree biochar) applied with two addition rates (2 and 5 wt %) in three excessive compost-fertilized (5 wt %) soils (one Oxisols and two Inceptisols), and to ascertain the increasing or decreasing effect of biochar and soil type in the presence of excessive compost. The study results of 400 days incubation indicated that, in general, the potential of the three biochars for C sequestration is similar in the three studied soils. The presence of excessive compost stimulated the co-mineralization of the more labile components of biochar over the short term (first two months). The potential of biochar addition for neutralizing soil pH and regulating the release of Al from soil for preserving soil organic carbon (SOC) might be the important mechanisms in biochar-compost interactions, especially in the presence of excessive compost. Overall, 5% application rate of three high temperature-pyrolysis biochars showed the less detriments to studied soils. In these incubations of biochar, excessive compost, and soil, it is a decreasing effect overall, that is, the enhanced storage of both biochar-C and SOC, which is expected as a long-term carbon sequestration in soil. The recorded direction and magnitude of effect, both are strongly influenced by biochar and soil type. When co-applied with excessive compost, the negative (reducing CO2 release) effect with increasing biochar application rates was eliminated.