Soil Organic Carbon Content Research Articles

Long-term alfalfa planting mediates the coupling of soil water and organic carbon storage in a semi-arid area of the Loess Plateau, China.

The key to restoring arid and semi-arid ecosystems is maintaining soil water and organic carbon contents. Alfalfa (Medicago sativa L.) is a high-yield perennial forage crop and performs ecological functions as a drought-resistance leguminous herb. It has been widely planted for reconstruction of degraded soils in the Loess Plateau in northwestern China, but long-term planting may affect soil carbon-water coupling and lead to crop yield reduction. To maximize the benefits of reconstructed grassland, this study explored the couplings of soil water, organic carbon, and alfalfa productivity along a reconstruction chronosequence in a semi-arid area of the Loess Plateau. Space-for-time substitution approach was used to select different-aged stands of reconstructed grassland (1, 5, 7, 10, 15, 20, 30 years old). Alfalfa above-ground biomass (AGB), soil water storage (SWS), organic carbon storage (SOCS), and carbon-water coupling coordination degree (D) were measured in the 0-100 cm soil profile. Alfalfa AGB reached a peak in the 7th year, and the degradation began in the 10th year. Both SWS and SOCS varied nonlinearly with stand age. The greatest loss of SWS occurred in the 15th year (80-100 cm depth), whereas the largest increase of SOCS occurred in the 30th year (0-20 cm depth). There was a negative feedback relationship between AGB and SWS over the 30-year study period (Pearson r = -0.835, P = 0.098). AGB and SOCS initially showed a trade-off within the first 10 years (Pearson r = -0.7431, P = 0.2569), in contrast to their positive feedback in the 20-30th years (Pearson r = 0.9978, P = 0.0421). A decoupling between SWS and SOCS (D < 0.6) was observed after 12 years of alfalfa planting. For agricultural production, a greater supply of water and organic fertilizer is required from the 7th year of alfalfa planting, and reseeding may be needed around the 10th year to prolong the life of alfalfa community. Alfalfa should be planted for no more than 12 consecutive years in the study area for ecological protection.

The Potential to Reconstruct 20th Century Soil Organic Carbon Erosion in Rangelands From Small Reservoir Sediments

ABSTRACTSoil erosion and soil organic carbon (SOC) loss are not always linked linearly because SOC‐rich topsoil is eroded at the initial stages of degradation, while horizons with lower SOC content are eroded later, but often at higher rates. Small, silted‐up farm reservoirs potentially document this change during the period of sediment accumulation. This study tests the specific potential of small farm reservoir sediments from the South African Karoo to reconstruct 20th century SOC and total nitrogen (TN) change in rangeland soils. Five reservoir sediment profiles were sampled and texture, total organic carbon (TOC), TN and 137Cs of the samples were analyzed and compared. The results show that there clearly distinguishable flood couplets have been preserved in the sediment, illustrating their suitability for the chronological reconstruction of soil erosion and SOC. With one exception, the older sediments contain more TOC and TN than the younger ones. The TOC changed mostly in earlier than later stages of deposition, which is indicative of soil degradation early after the construction of the dams in the 1920s and 1930s. These distinct changes illustrate that the small reservoir sediments have the potential to reconstruct the impact of land‐use and associated soil erosion on SOC change in rangelands. Their analysis can therefore contribute to a better understanding of the land‐use associated changes of the global carbon cycle during the 20th century.

Organic matter accumulation encouraged K-strategy bacteria increase and metabolism variation in karst vegetation restoration

Vegetation restoration plays a critical role in terrestrial carbon sink increase, especially in subtropic Southwest China which has been the hotspot of vegetation restoration areas in recent decades. However, the variations in microbial community diversity resulting from this intervention remain unclear, and further research is needed to elucidate the complex interactions between microorganisms and soil nutrient elements. Soil samples were collected from three vegetation restoration stages in a karst rocky desertification (KRD) valley of Southwest China, which were named dry cropland (CL), 3–8 years grassland (WL), and afforestation land that recovered more than 20 years (AL), to investigate the soil nutrient and bacterial diversity variation during vegetation restoration course. The soil organic carbon (SOC) and total nitrogen (TN) contents are significantly higher in the afforestation land soil compared to the cropland and grassland soils, while the available nutrient contents decrease during the vegetation restoration course. The soil dominant bacteria genus shifts from R-strategy bacteria in cropland to K-strategy bacteria in afforestation land, and the relative abundances of R-strategy genera remain similar in the grassland and cropland soil. The dual-relative bacteria network evaluates from simple correlation net to very complex ones during the vegetation restoration, of which the core influenced factor alters from available nutrients to organic matter. The results of this study suggest that it will take a relatively longer period for the soil nutrient conditions and bacterial community to recover fully, although vegetation restoration could reduce the impacts of agricultural activities. This study refers to the ecological preservation and restoration strategies under the global climate change background, elucidating the nutrient metabolism variation induced by the accordance variation of organic matter accumulation and soil bacterial life strategy during the ecosystem restoration course.

Reseeding increased plant biomass production and soil fertility, but not plant species diversity in degraded grasslands in China

Reseeding is a primary measure to restore degraded grasslands. Numerous studies have conducted experiments to investigate how the properties of grassland ecosystems respond to reseeding in China. However, there is a lack of summary of the results of these studies. Here, we conducted a hierarchical random-effects meta-analysis on the effects of reseeding on plant, soil, and microbial properties. We collected 19 variables, including plant biomass, species diversity and richness, soil organic carbon content, soil total and available nutrients, soil water content, soil microbial biomass and diversity, and enzyme activity, from a dataset of 1363 paired observations (degraded vs. reseeded) from 75 publications. The results showed that reseeding increased aboveground and belowground plant biomass by 70.2% and 68.0% on average, respectively. Reseeding increased soil organic carbon, phosphorus, and potassium contents, but did not affect soil nitrogen levels. Reseeding increased soil microbial nitrogen under conditions of tillage and fertilization. Reseeding age was found to have a positive correlation with species richness, while planting type, fertilization, and tillage did not have a significant impact on the species richness and diversity. Under the treatments of fertilization, non-tillage, and mix-planting, the response ratio of aboveground biomass to reseeding was positively correlated with the response ratio of species diversity to reseeding. Our results concluded that current reseeding practices can significantly improve plant biomass production and soil fertility but have minor effects on plant species diversity. These findings indicate that the preservation of biodiversity should receive greater attention from both researchers and practitioners in grassland remediation in China.

Impact of organic carbon composition on bacterial taxa assembly upon phosphorus addition in organic and mineral soil layers of a Robinia pseudoaccacia plantation

Increasing phosphorus (P) inputs significantly affect the nutrient cycle of terrestrial ecosystems. The community structure and assembly processes of soil bacteria, which are the primary participants in nutrient cycling within terrestrial ecosystems, are affected by P inputs. However, the assembly processes and driving mechanisms of soil bacterial communities in artificial forest ecosystems in response to P inputs remain unclear. Therefore, in this study, we conducted in situ experimental P additions (0, 0.5, 1, 2, and 4 g P m-2 year-1) to artificial Robinia pseudoacacia forests on the Loess Plateau. We investigated the effects of P addition on the soil organic carbon (SOC) composition, bacterial community characteristics (total, abundant, and rare taxa), and assembly processes in the organic horizon (OH) and mineral horizon (MH) layers of the soil. The results showed that P addition significantly changed soil physicochemical factors, such as available P (AP), SOC content, and SOC composition in terms of the labile (LC) and recalcitrant carbon (RC) fractions. The proportion of the LC fraction increased in OH and decreased in MH, whereas the RC fraction showed the opposite trends. P addition increased the proportion of eutrophic bacteria in OH, whereas it decreased the proportion of oligotrophic bacteria. In contrast to those in OH, the changes in eutrophic and oligotrophic bacteria in MH exhibited the opposite trend. With increasing P levels, the diversity of the bacterial communities in the OH did not change significantly. In the MH, total and rare taxa diversity increased, while the diversity of abundant taxa decreased. During P addition, the total and rare taxa bacterial communities showed stochastic assembly, whereas the assembly of abundant taxa shifted from stochastic to deterministic processes in the OH and from deterministic to stochastic processes in the MH. The structural equation model revealed that during P addition, the assembly of total and rare bacterial communities in the OH and MH was regulated by AP and the LC fraction, respectively. Meanwhile, the assembly of abundant taxa in OH and MH was influenced by the LC fraction and RC fraction, respectively. Our findings highlight the critical role of SOC composition across different bacterial taxa, with abundant bacterial taxa exhibiting heightened sensitivity to environmental filtration and the availability of SOC resources. Our study provides new insights into the community assembly and driving mechanisms of soil bacteria in artificial forest soils during P addition.

Assessing spatiotemporal variations of soil organic carbon and its vulnerability to climate change: A bottom-up machine learning approach

Soil organic carbon (SOC) is a crucial component of the terrestrial carbon cycle and essential for agricultural productivity. Quantifying its sensitivity to future climate change is vital for sustaining agricultural practices and mitigating greenhouse gas emissions. However, this remains a challenge as long-term SOC data are scarce and substantial uncertainties regarding future climate scenarios. This study presents a bottom-up machine learning framework to assess the spatiotemporal variations of SOC and its vulnerability to climate change in the Jinghe River Basin, a typical loess hilly and gully watershed. Firstly, the long-term (2000–2023) dynamics of SOC was estimated by integrating in-situ measurements with machine learning techniques. Results show that the high SOC values are primarily distributed in the farmland of the mountain-loess transition zone, while the low-value areas are mainly found in the loess region. During the study period, the SOC content exhibited a slight increasing trend with a rate of 0.02 ​g ​kg−1 ​yr−1 (p ​= ​0.449). The vulnerability of farmland surface SOC to future climate change was then evaluated by combining a robust machine learning model with the bottom-up framework. To this end, the study explored a wide range of possible future climates to identify critical climate thresholds and their spatial variation across the basin’s farmlands. Based on this analysis, this research found that the farmland in the northern basin is generally more susceptible to changing climate with even marginal rises in temperature could lead to severe loss in SOC. These results highlight the need for proactive climate adaptation strategies to safeguard SOC in vulnerable agricultural landscapes, ensuring soil health and resilience in the face of climate change.

Determinants of rhizospheric organic carbon fractions and accumulation in four different vegetations of coastal saline-alkali soils

Rhizosphere processes are key pathways through which plants influence the carbon cycling of diverse ecosystems and are closely associated with vegetation types. However, the environmental responses of rhizospheric soil organic carbon (SOC) mediated by the interaction between rhizospheric effect and vegetation type remain uncertain, severely limiting our assessment of the carbon sequestration potential of different coastal vegetation. Here, we identified the core factors driving changes in various soil carbon fractions in the rhizosphere of four coastal vegetation types by testing the contents of the reactive, dissolved, particulate, mineral-associated, heavy and light fractions of SOC and their relationships with physicochemical properties and enzyme activities. It was found that the rhizosphere effect significantly influenced SOC, with rhizospheric SOC contents varying significantly among vegetation types, ranging from 2.12 to 9.04 g/kg. Besides, rhizospheric soil pH was found to contribute significantly to SOC fractions (except for light organic carbon) and exert significant inhibitory effects on SOC, reactive organic carbon, mineral-associated organic carbon and heavy organic carbon. Despite the different responses of various SOC fractions to physicochemical properties and enzyme activities, soil available phosphorus and alkaline phosphatase activity emerged as core environmental factors influencing rhizospheric SOC accumulation across different coastal vegetation types. This study presents a promising framework for understanding the determinants of SOC fractions in rhizospheric soil, and their accumulation in the vegetation of coastal saline-alkali soils. In addition to alkalinity, phosphorus acquisition capacity can be a key predictor of coastal SOC fractions and their accumulation.

Tree species selection for optimizing soil carbon storage: Insights from litter decomposition and bacterial community analysis in coastal ecosystems

Coastal wetland ecosystems are critical sinks for atmospheric carbon dioxide, playing a vital role in global carbon cycling and climate regulation. The decomposition of leaf litter plays a crucial role in the formation and stability of soil organic carbon (SOC) in these environments. This study investigated the impact of leaf litter decomposition from five tree species (Populus deltoids, Ligustrum lucidum, Taxodium 'Zhongshanshan', Hibiscus hamabo, and Nerium oleander) on SOC dynamics, humus composition, and soil bacterial community structure in a tidal flat. Litterbags were used to monitor the mass loss and changes in litter chemical composition over 270 days. The results revealed significant differences in decomposition rates among the tree species, with Nerium oleander exhibiting the fastest decomposition and Populus deltoids the slowest. Surprisingly, initial litter chemistry did not correlate with decomposition rates; however, changes in lignin and hemicellulose content during decomposition were significantly related to mass loss. Despite its rapid decomposition, Nerium oleander litter resulted in the highest accumulation of SOC, total humus, and humin compared to the other species, challenging the conventional view that slower decomposition leads to greater SOC storage. The soil microbial community structure was significantly influenced by SOC, humus, and litter components, with distinct microbial assemblages associated with each tree species. A random forest model identified key bacterial taxa, predominantly Proteobacteria, as important predictors of SOC content, highlighting the role of bacterial diversity in regulating SOC dynamics. These findings underscore the importance of considering litter quality, decomposition dynamics, and bacterial community composition in strategies aimed at enhancing soil carbon sequestration. This study suggests that selecting tree species with rapidly decomposing litter, such as Nerium oleander, in coastal plantations can be an effective management tool for optimizing soil carbon storage, offering valuable insights for mitigating climate change impacts.

Goethite introduction strengthens balck soil carbon sequestration under various water management conditions and its microbial mechanisms

Natural and anthropogenic disturbances lead to degradation of (soil organic carbon) SOC content black soil in northeast china. Goethite (α-FeO (OH)) as widely distributed mineral, has a sequestration effect on SOC. With the diversification of crop cultivation in northeast china, various water management are becoming important. However, SOC sequestration mechanism by goethite under varying water management conditions is still unclear. In our study, we set up 3 groups of experiments: 1) 60% water-holding capacity (60% WHC), 60% water-holding capacity with goethite (60% WHCG), 2), waterlogging (W), waterloggin with goethite (WG), 3) alternation of wetting and drying (AWD), alternation of wetting and drying with goethite (AWDG) to elucidate the sequestration mechanism of SOC by goethite under three water management conditions. Results shows introducing goethite can sequester SOC under three water management conditions, with WG SOC sequestration capacity more than AWDG and 60% WHCG. The main sequestration mechanism of SOC has two aspects, on the one hand, goethite introduction increased significantly SOC functional groups, Fe-bound OC, iron molar ratio and Fe-bound OC/SOC, made SOC was bound to goethite by adsorption and co-precipitation. On the other hand, goethite introduction increased the diversity and abundance of bacteria and fungi by affecting soil pH, iron morphology and valence, induced the enrichment of Bryobacter, Gemmatimonas, Conexibacter, Streptomyces, Gaiella, Rubrobacter and Talaromyces potential carbon sequestrating microorganisms, further contributing to SOC sequestration. This study offers a significant theoretical foundation for elucidating the SOC sequestration mechanism of northeastern black soil.

Grazing exclusion promotes soil organic carbon accumulation in Tibetan grasslands with lower temperatures

BackgroundGrazing exclusion is a practical approach to restore vegetation in degraded grasslands and enhance soil organic carbon (SOC) sequestration. However, the dynamics and drivers of SOC in grasslands after grazing exclusion have not been well documented, especially in ecosystems with cold climates.MethodsHere, we established 14 paired treatments (grazing exclusion vs. free-grazing) along a 600-km transect in the northeastern zone of the Qinghai-Tibet Plateau. After six years, we analyzed vegetation biomass dynamics and measured the soil physicochemical properties and organic C concentration across three depths (0–10, 10–20, and 20–30 cm).ResultsGrazing exclusion significantly increased above- and belowground biomass (139.85% and 43.30%, respectively), pH (1.38%), total phosphorus (3.29%), nitrate nitrogen (18.03%), and ammonium nitrogen (17.81%), but significantly decreased soil bulk density (2.43%) and clay content (10.49%), particularly in 0–30 cm. Specifically, SOC concentrations positively responded to grazing exclusion (0–10 cm) in 9 of the 14 sites evaluated. The effects of grazing exclusion on SOC concentrations were significantly higher in areas with a mean annual temperature (MAT) below 0 °C compared to those in sites with a high MAT (> 0 °C). The SOC concentrations significantly correlated with the mean annual precipitation (MAP) in both treatments, but these correlations diminished with increasing soil depth. Ridge regression analysis showed that soil chemical properties (e.g., total nitrogen and phosphorus) positively influenced SOC accumulation, while MAT negatively influenced it after grazing exclusion. Path analysis further revealed that MAT indirectly regulated SOC dynamics via soil chemical properties.ConclusionsOur study highlights that grazing exclusion results in an asynchronous SOC and plant biomass accumulation and may be more beneficial for SOC sequestration in Qinghai-Tibet Plateau grasslands with lower temperatures. Also, humid climates promote SOC concentration in alpine grasslands. These results could help develop management practices and policies that promote sustainable grassland management.

An integrated dataset of ground hydrothermal regimes and soil nutrients monitored in some previously burned areas in hemiboreal forests in Northeast China during 2016–2022

Abstract. Under a warming climate, the occurrence of wildfires has been becoming increasingly more frequent in boreal forests and Arctic tundra over the last few decades. Wildfires can cause radical changes in forest ecosystems and the permafrost environment, such as the irreversible degradation of permafrost, succession of boreal forests, rapid and massive losses of soil carbon stock, and increased periglacial geohazards. Since 2016, we have gradually and more systematically established a network for studying soil nutrients and monitoring the hydrothermal state of the active layer and near-surface permafrost in the northern Da Xing'anling Mountains in Northeast China. Soil moisture content (depth of 0–9.4 m), soil organic carbon content (0–3.6 m), total nitrogen content (0–3.6 m), and total phosphorus and potassium content (0–3.6 m) datasets were obtained in 2016 via field sampling and subsequent laboratory tests. Ground temperature (0–20 m) and active layer thickness (2017–2022) datasets were obtained using thermistor cables that were permanently installed in boreholes or interpolated with these temperatures. The present data can be used to simulate changes in permafrost features under a changing climate and wildfire disturbances and to explore the changing interactive mechanisms of the fire–permafrost–carbon system in hemiboreal forests. Furthermore, they can provide baseline data for studies and action plans to support the carbon neutralization initiative and assessment of the ecological safety and management of the permafrost environment. These datasets can be easily accessed via the National Tibetan Plateau/Third Pole Environment Data Center (https://doi.org/10.11888/Cryos.tpdc.300933, Li and Jin, 2024).

Synergy Between Woody Peat and Bentonite Alters Stability of Soil Organic Carbon in Coarse Soil by Enhancing Capacity for Soil Aggregation and Hydro‐Physical Properties

ABSTRACTCoarse soil has a poor structure and is susceptible to wind and water erosion, thereby making it difficult to maintain the soil organic carbon (SOC) content. Woody peat (WP) is an organic material that can increase the SOC content of the soil, while clay materials can rapidly enhance the capacity for soil aggregate formation. In order to explore the synergistic effects of WP and clay materials (bentonite and red clay) on the aggregate structure and hydro‐physical properties of coarse soil, as well as the mechanism associated with SOC mineralization (ΔSOC), we conducted an incubation study using lou soil (L0) and loessial soil (H0) with three treatments: addition of WP alone (LW, HW), mixture of WP and bentonite (LWB, HWB), and mixture of WP and red clay (LWR, HWR). The three treatments enhanced the proportion of macroaggregate (&gt; 2 mm) and aggregate stability of the two soils, and optimized the water retention and ventilation performance. The highest aggregate stability of LWB and HWB can be attributed to the positive synergistic effect of WP and bentonite, and bentonite was more effective than red clay due to its crystal structure. The results also showed that the ΔSOC values were significantly lower under LWB and HWB than those under WP addition alone and adding the mixture of WP and red clay (p &lt; 0.05). Moreover, partial least squares path modeling analysis showed that the hydro‐physical properties of the two improved soils inhibited SOC mineralization (p &gt; 0.05), whereas particulate organic carbon (POC) content significantly accelerated SOC mineralization (p &lt; 0.01). The synergistic effect of clay materials increased mineral‐associated organic carbon (MAOC), which was beneficial to maintain the long‐term effectiveness of WP. Overall, our results demonstrated that the synergistic use of WP and bentonite enhanced the aggregate stability and hydro‐physical properties of coarse soil and improved SOC storage capacity. These results provide scientific and theoretical guidance to facilitate the rapid improvement of coarse soil through engineering measures in arid and semi‐arid areas with water and fertilizer limitations.

Effect of Himalayan cedar forest longevity on soil properties in the Kumaun Himalaya, Uttarakhand, India

The Himalayan cedar forests in the temperate regions of the Himalaya are crucial for providing ecosystem services such as carbon storage and nutrient maintenance, playing a vital role in combating climate change. This study employs a Dendrochronological approach to examine the impact of Himalayan cedar forest stands on soil properties across five distinct sites in the Kumaun Himalaya. Soil samples were collected from a depth of 0–15 cm and analyzed for total nitrogen (TN), total phosphorus (TP), soil organic carbon (SOC), pH, and C:N ratios. Tree-ring samples were also collected taken from the same sites. Our analysis revealed a positive correlation between tree age and SOC content. Notably, the oldest forests at Jageshwar Dham (480 years) and Hatkalika (413 years) exhibited the highest levels of SOC, TP, and TN, while the youngest forests at Naudunga (135 years) and Patal Bhuwaneshwar (134 years) showed the lowest values. Significant differences in all soil parameters were confirmed through a Tukey post hoc test. Additionally, the relationship between climate and tree growth was analyzed using tree-ring chronologies and meteorological records, indicating that cool and moist spring conditions (February-May) enhance tree growth by increasing soil microbial activity. Our findings emphasize that conserving older natural forests, particularly mature Himalayan cedar forests, is crucial for maintaining soil characteristics, including carbon storage and nutrient levels, to support sustainable soil health and climate change mitigation.

Tree species-dependent effects of afforestation on soil fungal diversity, functional guilds and co-occurrence networks in northern China

Afforestation exerts a profound impact on soil fungal communities, with the nature and extent of these changes significantly influenced by the specific tree species selected. While extensive research has addressed the aboveground ecological outcomes of afforestation, the nuanced interactions between tree species and soil fungal dynamics remain underexplored. This study investigated the effects of afforestation with Caragana microphylla (CMI), Populus simonii (PSI), and Pinus sylvestris var. mongolica (PSY) on soil fungal diversity, functional guilds, and co-occurrence networks, drawing comparisons with neighboring grasslands. Our findings reveal a significant increase in soil fungal Chao1 richness following afforestation, though the degree of enhancement varied across tree species. Specifically, CMI and PSI forests showed notable increases in fungal richness, whereas the response in PSY forests was comparatively modest. Saprotrophic fungal groups, integral to organic matter decomposition, showed a substantial increase across all afforested sites, with CMI forests exhibiting an impressive 205.58% rise. Conversely, pathogenic fungi, which can negatively impact plant health, demonstrated a marked decrease within plantation forests. Symbiotic groups, particularly ectomycorrhizal fungi, were notably enriched solely in PSI forests. Co-occurrence network analysis further indicated that afforestation alters fungal network complexity: CMI forests displayed increased network interactions, while PSI and PSY forests exhibited a reduction in network connectivity. Soil bulk density and organic carbon content emerged as key factors influencing network complexity, whereas tree species identity played a crucial role in shaping soil fungal community composition. Collectively, these results emphasize the importance of adopting a species-specific strategy for afforestation to optimize soil fungal diversity and network structure, ultimately enhancing the ecological resilience and sustainability of forest plantation ecosystems.

The rhizosphere contributes disproportionately to free-living nitrogen fixation in subalpine forest soils

Root activity creates a unique microbial hotspot in the rhizosphere, profoundly regulating free-living nitrogen fixation (FLNF). However, empirical assessments of rhizosphere FLNF and its ecological consequences for nitrogen (N) budgets remain lacking, particularly for different root functional modules. Here, we separately collected rhizosphere soils attached to two root functional modules, absorptive roots and transport roots, and investigated the rates (15N2 incorporation) and regulators of rhizosphere FLNF in a N-limited subalpine coniferous forest. The measured rates were further extrapolated to annual fluxes based on the volume of the rhizosphere and the relationship between FLNF rate and temperature. We found that absorptive roots drove significantly higher rhizosphere FLNF rates than did transport roots (12.2 vs. 7.5 ng N g−1 d−1), with both rhizosphere compartments exhibiting rates well exceeding those in the bulk soil (0.9 ng N g−1 d−1). The FLNF rates were positively correlated with soil organic carbon content but showed no significant relationship with the abundance or composition of the diazotrophic community. Moreover, when extrapolated to the ecosystem level the FLNF fluxes were 2-fold greater in the rhizosphere of absorptive roots than in that of transport roots (0.13 vs. 0.04 kg N ha−1 yr−1). Taken together, despite representing only 6.0% of the soil volume, the two rhizosphere compartments contributed as much as 47.2% to the total soil FLNF fluxes. Overall, we provide empirical evidence that despite its limited volume, the rhizosphere contributes disproportionately to the FLNF in subalpine forest soils. Our findings also underscore the critical role of root functional differentiation in regulating rhizosphere FLNF, which is essential for integrating this process into the ecosystem-level N cycle.

Protective role of reactive aluminum phases to stabilize soil organic matter against long-term cultivation in the humid tropics under volcanic influence

ABSTRACT Cultivation can cause rapid depletion of soil organic matter (SOM), particularly in the humid tropics. Understanding the factors controlling SOM storage is a key step toward effective soil management for sustainable crop production. While clay + silt content is known to be an important factor for soil organic carbon (SOC) storage, recent studies have shown greater importance of oxalate-extracted Al (Alo; roughly corresponds to short-range-order aluminosilicate minerals and organo-Al complexes) and Fe (Feo). However, the extent to which these mineralogical factors control the response of SOM pool to land-use changes remains unclear, especially in the tropics. This study aimed to clarify the factors regulating SOM content in croplands compared to secondary forests or home gardens in Negros Occidental, Philippines. Along two line transects having a wide range of Alo contents, soil samples were collected at a depth of 0 − 10 cm from sugarcane sites (n = 33; long-term continuous cultivation of >70 years) that were paired with either secondary forests (n = 10) or home gardens (n = 20). Sugarcane sites had significantly lower SOC and total nitrogen (N) contents than secondary forest and home garden sites, whereas there was no clear difference between the latter two land uses. Both SOC and total N contents showed significant positive correlations with Alo content and, to a much less extent, with Feo content, but not with clay + silt content. The slope of the regression line between SOC and Alo was indifferent between sugarcane and the other two land uses, while the intercept was significantly lower in the sugarcane sites. These results suggest the protective role of Alo phases in both agricultural and forest soils and the presence of unprotected SOM (e.g. plant litter) in the latter sites. Furthermore, δ13C analysis suggested that the proportion of forest (C3 plant) derived carbon remaining in the sugarcane soils ranged between 10% and 50% and increased with Alo content. Together, our study showed indirect evidence of Alo-induced stabilization of SOM against long-term cultivation under the humid tropical environment.

Organic carbon decomposition temperature sensitivity positively correlates with the relative abundance of copiotrophic microbial taxa in cropland soils

Revealing the relationships between the temperature sensitivity of soil organic matter decomposition (Q10) and microbial communities is critical to predict ecosystem feedbacks to global warming. However, the relationships are still far from well understood, especially within the low latitude regions. To address this knowledge gap, soil samples were collected from >100 cropland sites in Southwest China, a region with highly diverse climatic and environmental conditions. The results showed that Q10 values substantially varied across the region, ranging from 1.85 to 6.81, with an average value of 3.27. They were significantly positively correlated with the contents of soil organic carbon, while negatively correlated with the ratios of soil dissolved organic carbon to total organic carbon content. This indicates that soils with high organic carbon contents are more vulnerable to global warming. Further analysis suggested that Q10 values were positively correlated with soil microbial respiration rates, fungal abundance, prokaryotic diversity, and the relative abundance of copiotrophic microbial lineages, but negatively correlated with the proportions of oligotrophic microbes and microbial co-occurrence network degree. The structural equation modeling analysis suggested that soil organic carbon and its quality, as well as microbial attributes were the main factors explaining the variation in Q10 values. The findings in this study highlight the crucial and complex roles of soil microbiome in determining ecosystem feedbacks to global warming.

Biochar Regulates the Humification of Kitchen Waste and the Effects of the Humic Acid Structure of Products on Black Soil

Kitchen waste is a misplaced resource that is characterized by a high organic matter content, high water content, and a tendency to rot easily. Biochar is a black solid substance produced under high-temperature, anaerobic conditions using agricultural organic wastes as the raw material. It possesses a large specific surface area, a loose and porous structure, and functional groups, which confer high thermal stability and strong adsorption capabilities. However, little is known about how humic products made from biochar affect the composition and structure of soil humus. To solve the above problems, this study carried out a two-year outdoor field experiment by means of element analysis, infrared spectroscopy, and differential thermal analysis (0.4 kg/m2 (W4), 0.8 kg/m2 (W8), 1.2 kg/m2 (W12), 1.6 kg/m2 (W16), and 2.0 kg/m2 (W20)); CK was the blank control (no application). The samples were collected one year and two years after they returned to the field. The results showed that the application of organic materials facilitated the accumulation of soil organic carbon (SOC) and increased the total nitrogen (TN) content. The highest SOC content in the W20 treatment was 12.39 g/kg and 14.67 g/kg in one and two years, respectively. The maximum relative HA content in the W20 treatment was 22.99% one year after returning to the field. The PQ value (the ratio of HA/(fulvic acid (FA) + HA)) for the W20 treatment was 88.21%. The W20 treatment greatly increased the SOC and humus carbon contents. Compared with the CK treatment, all the organic materials applied for one year improved the structure of the humic acid to varying degrees, increased the degree of oxidation, reduced the degree of condensation and thermal stability of the HA in the soil, and gradually simplified the structure of the humic acid; among all the treatments, the W20 treatment had the greatest effect.

Accumulation of soil microbial necromass carbon and its contribution to soil organic carbon after vegetation restoration in the Tibetan Plateau

Vegetation restoration has been proved as an effective strategy to increase soil organic carbon (SOC) sequestration in degraded ecosystems. However, due to different vegetation restoration practices and climate zones, changes of SOC content, dynamics, sources and their controlling mechanisms after vegetation restoration remain inadequately addressed. Taking the advantage of four-year vegetation restoration, we explored the effects of vegetation restoration on SOC content through its fractions, soil aggregates, soil enzymes and microbial necromass contribution to SOC in a desertification region in the Tibetan Plateau (TP). Results showed that vegetation restoration increased SOC contents by 68 % for 0 – 10 cm, 29 % for 10 – 30 cm and 11 % for 30 – 50 cm compared to site without vegetation restoration. Vegetation restoration increased soil macroaggregates (> 0.25 mm) and decreased soil microaggregates (0.53 – 0.25 mm), but increased their associated SOC contents. Meanwhile, vegetation restoration enhanced microbial necromass and its contribution to SOC. Structural equation model revealed that increasing microbial necromass carbon, changing soil aggregates and their associated SOC were the main mechanisms of increasing SOC content after vegetation restoration. Our result further indicated a large potential of SOC sequestration through vegetation restoration in the TP. These findings have important implications for natural based solution for carbon neutrality for China.

Primary productivity regulates rhizosphere soil organic carbon: Evidence from a chronosequence of subtropical Chinese fir (Cunninghamia lanceolata) plantation

Tree plantations worldwide are a large terrestrial carbon sink. Previous studies on the carbon sequestration capacity of plantations mainly focused on tree biomass carbon sequestration, but the importance of soil organic carbon (SOC) was relatively unclear. Living root carbon inputs influence SOC via plant-microbe interactions in the rhizosphere and play an essential role in nutrient cycling. Here, we compared SOC, including its fractions, microbial properties, and major nutrients in rhizosphere and bulk soils, and examined their relationships to net primary productivity (NPP) across three developmental stages of Chinese fir (Cunninghamia lanceolata) plantations (6, 18, and 42 years old) in subtropical China. Although NPP differed among the three plantations, SOC concentration in bulk soils did not vary significantly among them. However, SOC concentration and labile C pool I and recalcitrant C pool in rhizosphere soils were significantly (p < 0.05) higher in the young (6-year) and mature (42-year) plantations, both of which had lower (p < 0.05) NPP (−37.71 % and − 42.67 %) compared to the middle-aged (18-year) plantation, suggesting a decoupling of NPP from rhizosphere SOC in the plantations. The decoupling of NPP from rhizosphere SOC concentrations may be driven by nitrogen (N) and phosphorus (P) tree growth requirements, belowground C allocation, and resultant microbial activity in this highly weathered subtropical soil. Our study provides field-based evidence suggesting that rhizosphere SOC changes are primarily regulated by net primary production in subtropical forest plantations. We propose that accurate predictions of SOC dynamics in forest plantations require an improved understanding of rhizosphere processes during plantation development.