Source Of Soil Organic Carbon Research Articles

Ridge with no-tillage facilitates microbial N2 fixation associated with methane oxidation in rice soil

Aerobic methane-oxidizing bacteria (MOB) play a crucial role in mitigating the greenhouse gas methane emission, particularly prevalent in flooded wetlands. The implementation of ridge with no-tillage practices within a rice-rape rotation system proves effective in overcoming the restrictive redox conditions associated with waterlogging. This approach enhances capillary water availability from furrows, especially during periods of low rainfall, thereby supporting plant growth on the ridges. However, the microbe-mediated accumulation of soil organic carbon and nitrogen remains insufficiently understood under this agricultural practice, particularly concerning methane oxidation, which holds ecological and agricultural significance in the rice fields. In this study, the ridge and ditch soils from a 28-year-old ridge with no-tillage rice field experiment were utilized for incubation with 13C-CH4 and 15NN2 to estimate the methane-oxidizing and N2-fixing potentials. Our findings reveal a significantly higher net production of fresh soil organic carbon in the ridge compared to the ditch soil during methane oxidation, with values of 626 and 543 μg 13C g−1 dry weight soil, respectively. Additionally, the fixed 15N exhibited a twofold increase in the ridge soil (14.1 μg 15N g−1 dry weight soil) compared to the ditch soil. Interestingly, the result of DNA-based stable isotope probing indicated no significant differences in active MOB and N2 fixers between ridge and ditch soils. Both Methylocystis-like type II and Methylosarcina/Methylomonas-like type I MOB catalyzed methane into organic biomass carbon pools. Soil N2-fixing activity was associated with the 15N-labeling of methane oxidizers and non-MOB, such as methanol oxidizers (Hyphomicrobium) and conventional N2 fixers (Burkholderia). Methane oxidation also fostered microbial interactions, as evidenced by co-occurrence patterns. These results underscore the dual role of microbial methane oxidation – not only as a recognized sink for the potent greenhouse gas methane but also as a source of soil organic carbon and bioavailable nitrogen. This emphasizes the pivotal role of microbial methane metabolism in contributing to soil carbon and nitrogen accumulation in ridge with no-tillage systems.

Distribution characteristics of microbial necromass carbon along soil profiles in different restoration periods of Caragana korshinskii in mountainous areas of Southern Ningxia, China

Microbial necromass, an important and stable source of soil organic carbon (SOC), is an important index to evaluate the contribution of microorganisms to SOC transformation and accumulation. It is not clear about the accumulation of microbial necromass in deep soil layer and its contribution to SOC during the restoration process of Caragana korshinskii forests. Combined with the biomarker method, we investigated the carbon contents of bacte-rial, fungal, and microbial necromass in the soil profiles (0-100 cm) of C. korshinskii forests in 16, 28, and 38 years of restoration, with natural grassland as control. We further examined the contribution of microbial necromass to soil organic carbon. The results showed that: 1) Along the soil profile (0-100 cm), the contents of fungal necromass carbon (FNC), bacterial necromass carbon (BNC), and microbial necromass carbon (MNC) significantly decreased with increasing soil depth in natural grassland and C. korshinskii forests. Except for the significant decrease in FNC/SOC, BNC/SOC, and MNC/SOC in the soil of C. korshinskii forests in 38 years of restoration, FNC/SOC and MNC/SOC generally showed an increasing trend followed by a decreasing trend in other plots, while BNC/SOC gradually decreased. 2) With the increases of restoration years, the contents of FNC, BNC, and MNC significantly decreased in C. korshinskii forests. FNC/SOC and MNC/SOC showed an overall increasing trend followed by a decreasing trend, while BNC/SOC gradually decreased. 3) The average contribution of microbial necromass carbon to SOC was highest in C. korshinskii forests in 28 years of restoration (35.0%), followed by C. korshinskii forests in 16 years of restoration (33.5%), natural grassland (31.0%), and C. korshinskii forests in 38 years of restoration (28.6%). In conclusion, when the restoration years of C. korshinskii forests are 16, the contents of microbial necromass carbon and their contributions to SOC are higher compared to natural grassland, which are beneficial for SOC sequestration.

Effect of forest planting patterns on the formation of soil organic carbon during litter lignocellulose degradation from a microbial perspective.

Litter decomposition is an important source of soil organic carbon, and it plays a key role in maintaining the stability of forest ecosystems. The microbial mechanism of soil organic carbon (SOC) formation in different urban forest planting patterns during litter lignocellulose degradation is still unclear. The key genes, microbes, and metabolites in the process of lignocellulose degradation and SOC formation were determined by metagenomics and metabolomics in different litter decomposition layers and soil layers in different urban forest planting patterns, including three types of broadleaf forests (BP forests), three types of coniferous forests (CP forests), and two types of mixed coniferous and broadleaf forests (MCBP forests). The results indicated that the cellulose, hemicellulose, and lignin concentrations from the undecomposed layer to the totally decomposed layer decreased by 70.07, 86.83, and 73.04% for CP litter; 74.30, 93.80, and 77.55% for BP litter; and 62.51, 48.58, and 90.61% for MCBP litter, respectively. The soil organic carbon of the BP forests and MCBP forests was higher than that of the CP forests by 38.06 and 94.43% for the 0-10 cm soil layer and by 38.55 and 20.87% for the 10-20 cm soil layer, respectively. Additionally, the gene abundances of glycoside hydrolases (GHs) and polysaccharide lyases (PLs) in the BP forests were higher than those in the MCBP forests and CP forests. Amino acid metabolism, sugar metabolism, TCA metabolism, and cAMP signaling metabolism were mainly between the CP forests and BP forests, while the TCA cycle, pyruvate metabolism, phenylalanine metabolism, and tyrosine metabolism were mainly between the BP forests and MCBP forests during litter decomposition. Additionally, ammonia nitrogen and hemicellulose were key factors driving SOC formation in the CP forests, while ammonia nitrogen, hemicellulose, and lignocellulose-degrading genes were key factors driving SOC formation in the BP forests. For the MCBP forests, cellulose, pH, ammonia nitrogen, and lignin were key factors driving SOC formation. Our findings revealed that the BP forests and MCBP forests had stronger lignocellulose degradation performance in the formation of SOC. This study provided a theoretical basis for the flow and transformation of nutrients in different urban forest management patterns.

Roots exert greater influence on soil respiration than aboveground litter in a subalpine Cambisol

Aboveground plant litter and roots are primary sources of soil organic carbon (SOC), profoundly affecting soil respiration and SOC dynamics. With global change, the quantities of aboveground plant litter and root inputs into soil are changing in the subalpine region; however, the impacts of these changes on soil respiration and the quantity and quality of SOC remain unclear. Here, a three-year detritus input and removal treatment (DIRT) experiment that included six treatments (namely, control, CK; double litter, DL; litter removal, NL; root exclusion, NR; root exclusion plus double litter, NRDL; and root exclusion plus litter removal, NRNL) was conducted in a subalpine Cambisol on the eastern edge of the Tibetan Plateau to assess the different effects of aboveground litter and roots on soil respiration and the quantity and quality of SOC dynamics. Among the treatments, the mean soil respiration in NL, NR and NRNL treatments significantly decreased by 21.7%, 31.9% and 40.5%, respectively, compared to the CK treatment. When with intact roots, double litter (DL) had a positive impact on soil respiration, resulting in a 13.2% increase in soil respiration compared to the CK treatment. In contrast, when with root exclusion (in the absence of roots), combined root exclusion and litter addition (NRDL) hardly influence soil respiration compared with the NR treatment, resulting in a 30.5% decrease in soil respiration. The soil respiration, labile SOC content and the abundance of soil microbes exhibited similar patterns among treatments. Roots excluded (NR, NRDL, NRNL) exerted a stronger effects on soil respiration, temperature sensitivity of soil respiration (Q10), labile SOC content and the recalcitrance index of carbon (RIC) than aboveground plant litter manipulation (CK, DL, NL). Correlation analysis and redundancy analysis revealed that soil respiration and its Q10 were regulated by the quantity and quality of SOC substrates and the abundance of soil microbes. Our results highlighted that roots exerted greater influence on the quantity and quality of SOC, the abundance of soil microbes, soil respiration and its Q10 than aboveground litter manipulation in subalpine Cambisol in response to future climate scenarios, especially the mediating effect of the presence of roots on the function of aboveground plant litter.

Chemical-microbial effects of acetic acid, oxalic acid and citric acid on arsenic transformation and migration in the rhizosphere of paddy soil

Low-molecular-weight organic acids (LMWOAs) are essential components of rice roots exudates and an important source of soil organic carbon. The chemical-microbial pathway by which LMWOA affects arsenic (As) cycling in the rhizosphere of paddy soils is still unclear. In this study, three typical LMWOAs (acetic acid (AA), oxalic acid (OA), and citric acid (CA)) in rice root exudates were added to As-contaminated soil at a concentration of 10 mM, mimicking the rhizosphere environment. The results showed that the addition of AA and OA inhibited the mobilization of As in the rhizosphere soil. After 14 days of incubation, the content of As in the porewater of AA and OA decreased by 40% and 22%, respectively, compared with the control. AA hindered the mobilization of As in soil via promoting the formation of secondary minerals. The addition of OA inhibits the mobilization of As via increasing the proportion of As (V) in porewater and promoting the formation of secondary minerals in soil. In addition, OA addition not only significantly increased the aioA gene abundance but also notably enriched the microorganisms containing As (III) methylation functional genes (arsM). The addition of CA greatly expedited the release of As from the soil solid phase through the solubilization of Fe/Mn minerals via the effects of both soil chemistry and microbial action. Furthermore, linear discriminant analysis effect size (LEfSe) revealed the possibility that bacteria such as Burkholderia, Magnetospirillum, and Mycobacterium were involved in the reduction or methylation of As in the rhizosphere of paddy soil. This study revealed the internal causes of LMWOAs regulating As transformation and mobilization in flooded paddy soil and provided theoretical support for reducing As accumulation in rice by breeding rice varieties with high AA and OA secretions.

Effects of acid rain and root exclusion on soil organic carbon in Cunninghamia lanceolata and Michelia macclurei plantations.

Acid rain is one of most serious environment problems in China. The types of acid rain have gradually transformed from sulfuric acid rain (SAR) to mixed acid rain (MAR) and nitric acid rain (NAR) in recent years. Roots are one source of soil organic carbon and play an important role in the formation of soil aggregates. However, the changes in acid rain type and the effect of root removal on soil organic carbon in forest ecosystems are poorly understood. In this study, we removed roots and simulated acid rain with different types (SO42-:NO3- ratio of 4:1, 1:1, and 1:4) for three years in Cunninghamia lanceolata (CP) and Michelia macclurei (MP) plantations to analyze the changes of soil organic carbon and physical properties and to measure the size and mean weight diameter (MWD) of aggregates. Results showed that root removal in C. lanceolata and M. macclurei remarkably reduced soil organic carbon pool by 16.7 % and 21.5%, and soil recalcitrant carbon by 13.5% and 20.0%, respectively. Root removal substantially decreased the MWD and proportion and organic carbon content of soil macroaggregates in M. macclurei, but not in C. lanceolata. Acid rain did not affect soil organic carbon pool and soil aggregate structures. Our results indicated that roots promote soil organic carbon stabilization and that their contribution to the stability of soil organic carbon varies with forest types. Moreover, soil organic carbon stabilization is not affected by different types of acid rain in the short term.

Predicting Fine Root Decomposition from Functional Traits in 10 Temperate Tree Species

Decomposition of plant roots and their related fungal mutualists is a fundamental process of ecosystem material cycles. Despite the fact that fine roots are the dominant source of soil organic carbon (SOC) storage, our understanding of the functional traits controlling fine root decomposition is still far from clear. In the present study, the decomposition of fine roots among four arbuscular mycorrhizal (AM) and six ectomycorrhizal (EM) species was studied in a temperate forest after 570 days of exposure. Our results showed that fine roots among AM species decomposed faster than EM species. Our findings further suggested that initial aluminum (Al) and manganese (Mn) concentrations were the best predictors for decomposition of fine roots among the traits that we measured. Initial cellulose concentration, carbon:nitrogen ratio (C:N), and lignin:N ratio were closely related to decomposition among AM species. In contrast, among EM species, initial phosphorus (P), calcium (Ca), and non–structural carbohydrate (NSC) concentrations were the best predictors of fine root decomposition. The initial concentrations of Na, Fe, NSC, cellulose, and hemicellulose were useful predictors of fine root decomposition across the 10 studied tree species.

Changes in Soil Organic C Fractions and C Pool Stability Are Mediated by C-Degrading Enzymes in Litter Decomposition of Robinia pseudoacacia Plantations.

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.

Soil carbon sequestration by root exudates.

Root exudates are well-known 'labile' sources of soil carbon that can prime microbial activity. Recent investigations suggest that the stability of labile carbon inputs in soil mostly depends upon the physical, chemical, and biological properties of the surroundings. Here, we propose that, in some ecosystems, such as forests and grasslands, root exudates can function as a source of soil organic carbon (SOC) that can be stabilized through various mechanisms leading to long-term sequestration. Increasing soil carbon sequestration is important for capturing atmospheric CO2 and combating climate change issues. Thus, there is an urgent need to preserve existing ecosystems and to adopt strategies such as afforestation, reforestation, and establishment of artificial grasslands to foster carbon sequestration through higher root exudate inputs in the soil.

Plant-derived lipids play a crucial role in forest soil carbon accumulation

Plant and microbial residues are two main sources of soil organic carbon (SOC). While recent studies have extensively examined the distribution of microbial necromass in different ecosystems, how plant residues (in particular, non-lignin components) contribute to SOC accumulation is less clear, especially in forests which make up 50% of the global soil carbon storage. Filling this knowledge gap will help us better understand SOC accumulation patterns and their response to land-use changes. Here, we analyze plant- and microbial-derived biomarkers (including lignin phenols, amino sugars, free and hydrolysable lipids) in the topsoil of major forest types in China and compare their distribution patterns together with the existing data (for lignin phenols and amino sugars) in forests and grasslands distributed globally. At the global scale, forests contain significantly less microbial necromass in SOC compared with grasslands, suggesting higher contribution of plant-derived components to forest SOC. However, plant-derived lignin phenols do not seem to play a major role in SOC accumulation, given their negative relationship with SOC contents. Instead, leaf- and root-derived hydrolysable lipids constitute a much higher proportion of SOC than lignin phenols in the investigated forests of China, even compared to grassland soils. Moreover, in contrast to lignin phenols, both SOC contents and the relative abundance of hydrolysable plant lipids in SOC increase with decreasing soil pH, increasing reactive iron and aluminum contents and with increasing lignin oxidation (indicated by acid-to-aldehyde ratios) in these forest soils. These results suggest that with increasing lignin decomposition, plant lipids and SOC accumulated via (oxyhydr)oxide protection. Collectively, our results demonstrate differential importance of plant-derived components in SOC accumulation in forests versus grasslands and highlight that plant lipids play a more important part than lignin in forest SOC accumulation. Quantitative investigations on the distribution of plant-derived lipids in addition to lignin in forest soils may help to elucidate pathways and hotspots of plant component-dominated SOC accrual.

Mowing Facilitated Shoot and Root Litter Decomposition Compared with Grazing

Shoot and root litter are two major sources of soil organic carbon, and their decomposition is a crucial nutrient cycling process in the ecosystem. Altitude and land use could affect litter decomposition by changing the environment in mountain grassland ecosystems. However, few studies have investigated the effects of land use on litter decomposition in different altitudes. We examined how land-use type (mowing vs. grazing) affected shoot and root litter decomposition of a dominant grass (Bromus inermis) in mountain grasslands with two different altitudes in northwest China. Litterbags with 6 g of shoot or root were fixed in the plots to decompose for one year. The mass loss rate of the litter, and the environmental attributes related to decomposition, were measured. Litter decomposed faster in mowing than grazing plots, resulting from the higher plant cover and soil moisture but lower bulk density, which might promote soil microbial activities. Increased altitude promoted litter decomposition, and was positively correlated with soil moisture, soil organic carbon (SOC), and β-xylosidase activity. Our results highlight the diverse influences of land-use type on litter decomposition in different altitudes. The positive effects of mowing on shoot decomposition were stronger in lower than higher altitude compared to grazing due to the stronger responses of the plant (e.g., litter and aboveground biomass) and soil (e.g., soil moisture, soil bulk density, and SOC). Soil nutrients (e.g., SOC and soil total nitrogen) seemed to play essential roles in root decomposition, which was increased in mowing plots at lower altitude and vice versa at higher altitude. Therefore, grazing significantly decreased root mass loss at higher altitude, but slightly increased at lower altitude compared to mowing. Our results indicated that the land use might variously regulate the innate differences of the plant and edaphic conditions along an altitude gradient, exerting complex impacts in litter decomposition and further influencing carbon and nutrient cycling in mountain grasslands.

Microbial necromass as the source of soil organic carbon in global ecosystems

Despite the recognized importance of the contribution of microbial necromass to soil organic carbon (SOC) sequestration, at a global scale, there has been no quantification for cropland, grassland, and forest ecosystems. To address this knowledge gap, the contents of fungal and bacterial necromass were estimated based on glucosamine and muramic acid contents in cropland (986 samples), grassland (278 samples), and forest (452 samples) soils. On an average, microbial necromass C contributed 51%, 47%, and 35% to the SOC in cropland, grassland, and forest soils, respectively, in the first 20 cm of topsoil. The contribution of microbial necromass to SOC increased with soil depth in grasslands (from 47% to 54%) and forests (from 34% to 44%), while it decreased in croplands (from 51% to 24%). The microbial necromass accumulation coefficient (the ratio between necromass and living microbial biomass C) was higher in soil from croplands (41) and grasslands (33) than in forest (20) soils. These results suggest that the turnover of living microbial biomass is faster in grassland and cropland soils than in forest soils, where the latter contains more partially decomposed plant residues. Fungal necromass C (>65% of total necromass) had consistently higher contributions to SOC than bacterial necromass C (32–36%) in all soils due to i) a larger living fungal biomass than bacterial biomass, and ii) fungal cell compounds being decomposed slowly and, thus able to persist longer in soil. The ratio of fungal:bacterial necromass C increased from 2.4 to 2.9 in the order of croplands < grasslands < forests, because fungi are the principal decomposers of complex substrates dominant in grassland and, especially, in forest soils. The ratios of bacterial:microbial necromass and bacterial:fungal necromass in cropland soils are larger than those in grassland and forest soils. This result indicates that the relative contribution of fungal necromass to total microbial necromass is lowest in cropland among the three land uses. Moreover, fungal and bacterial necromass increased with the total living microbial C and N contents. Lower temperatures and soil pH (e.g., in temperate and boreal ecosystems) stimulate fungal and bacterial necromass accumulation. These findings highlight the fact that shifts in the bacterial:fungal necromass ratio and the microbial necromass contribution to SOC are ecosystem-specific and depend on climate. In conclusion, microbial necromass contributes to approximately half of the SOC in cropland and grassland soils, and only 35% in forest soils; whereas, two-thirds of microbial necromass are of fungal origin.

Growth of Legume Cover Crops under Cassava and Its Effect on Soil Properties

Background: Low soil organic carbon is a constraint to cassava tuber formation. Some legume cover crops could be an alternative to provide organic matter on the cassava field as a source of soil organic carbon. The study was aimed to evaluate the growth of some legume cover crops under cassava and their effects on soil properties. Methods: During September 2017-July 2018 legume cover crops (Calopogonium mucunoides, Centrosema pubescens, Pueraria javanica and the mixed) were planted under cassava variety of Mangu and UJ-5. The land coverage by the legume cover crops was measured monthly from 2 to 10 months after planting. Cassava growth was observed weekly from 8 to 32 weeks after planting. Soil properties were analyzed before planting and at harvesting of cassava. Result: Pueraria javanica was tolerant toward cassava shading. The land coverage was linearly increased along with the growth of cassava. At the end of cassava growth, the land area coverage by this legume cover crop was 98.08%. It produced more organic matter and could maintain soil moisture content than other legume cover crops. P. javanica could consider being a suitable legume cover crop under cassava to improve soil quality.

Climate change drivers alter root controls over litter decomposition in a semi-arid grassland

Plant roots are the primary source of soil organic carbon (C) and critically support the growth and activities of microbes in the rhizosphere. Climate change factors may, however, modify root-microbial interactions and impact C dynamics in the rhizosphere. Yet, the direction and magnitude of interactive climate change effects, as well as the underlying mechanisms, remain unclear. Here we show evidence from a field experiment demonstrating that warming and precipitation changes strengthen root controls over litter decomposition in a semi-arid grassland. While warming and precipitation reduction suppressed microbial decomposition of root litter regardless of the root presence, precipitation increase stimulated litter decomposition only in the absence of roots, suggesting that plant competition for water constraints the activities of saprophytic microbes. Root presence increased microbial biomass but reduced microbial activities such as respiration, C cycling enzymes and litter decomposition, indicating that roots exert differential effects on microbes through altering C or water availability. In addition, nitrogen (N) input significantly reduced microbial biomass and microbial activities (respiration). Together, these results showed that alterations in soil moisture induced by climate change drivers critically modulate root controls over microbial decomposition in soil. Our findings suggest that warming-enhanced plant water utilization, combined with N-induced suppression of microbes, may provide a unique mechanism through which moderate increases in precipitation, warming and N input interactively suppress microbial decomposition, thereby facilitating short-term soil C sequestration in the arid and semi-arid grasslands.

Changes in organic carbon fractions and sources in deltaic topsoil and subsoil layers: autochthonous and allochthonous inputs

AbstractRiver deltas are often considered as hotspots of atmospheric carbon regulation, but few studies have paid attention to the differences between topsoil and subsoil for carbon sequestration. Here physical and chemical stabilization mechanisms and sources of soil organic carbon (OC) accumulation in different soil layers of a major river delta (Yellow River delta) were studied by fractionation and isotopic analysis. We found that in topsoil, conversion from tidal flat to wetland substantially increases OC by 84%, whereas cotton cultivation decreases OC by 66%. Wetland with a high soil OC content is characterized by substantial accumulation of plant‐derived OC and OC in mineral fractions. In subsoil, the OC variation is controlled by the yellow silt layer (YSL)–red clay layer (RCL) sequence. Although the weathering intensity of the RCL is much higher than that of the YSL, the OC age is younger in the RCL, indicating that the highly reactive, secondary minerals and iron oxides are capable of stabilizing younger organic carbon in transportation, deposition and post‐pedogenic alteration. 14C activity demonstrates that modern autochthonous OC is the main component of OC in wetland topsoil. In contrast, millennia‐old allochthonous OC is the main component of OC in the subsoil layers, contributing 4–24% of total bulk soil OC. The higher δ13C values of carbon fractions (−24.3 to −21.4‰ for the intra‐aggregate particulate organic matter fraction, intra‐microaggregate silt and clay fraction, and free silt and clay faction) and bulk samples (~ −22‰) are likely to imply the contribution of aged OC in the Yellow River delta. This study offers direct confirmation that soil OC fractions and sources are influenced by different land‐use types and pedological properties in characteristic soil layers. These findings suggest that biogeochemical alteration of the soil matrix controls OC stabilization mechanisms in the river sediment–tidal flat–wetland–farmland continuum.Highlights Land‐use types and pedological features control OC storage in topsoil and subsoil, respectively. Reclamation strongly decreases content of all OC fractions and the proportion of fSC and rOC. Soil layers with distinct texture and mineral compositions lead to OC stratification in subsoil. The red clay layer in subsoil is capable of stabilizing both fresh and aged OC.

Tracking the Deposition and Sources of Soil Carbon and Nitrogen in Highly Eroded Hilly-Gully Watershed in Northeastern China.

Understanding the deposition and tracking the source of soil organic carbon (C) and nitrogen (N) within agricultural watersheds are critical for assessing soil C and N budgets and developing watershed-specific best management practices. Few studies have been conducted and reported on highly eroded hilly-gully watersheds. In this field study, a constructed dam-controlled hilly-gully watershed in northeastern China was selected to identify the sources of soil C and N losses. Soils at various land uses and landscape positions, and sediments near the constructed dam, were collected and analyzed for selected physiochemical properties, total organic carbon (TOC), total nitrogen (TN), and stable isotopes (13C and 15N). Soil C and N loss and deposition in the watershed were assessed and the relative contributions of each source quantified by a stable isotope mixing model (SIAR). Results indicated that soil C loss was primarily from cropland, accounting for 58.75%, followed by gully (25.49%), forest (9.2%), and grassland (6.49%). Soil N loss was similar to soil C, with cropland contribution of 80.58%, gully of 10.30%, grassland of 7.54%, and forest of 1.59%. The C and N deposition gradually decreased along the direction of the runoff pathway near the constructed dam, and the deposited C and N from cropland and gullies showed an order: middle-dam > bottom-dam > upper-dam and upper-dam > bottom-dam > middle-dam, respectively. A high correlation between soil TOC or TN and the sediment properties suggested that the deposition conditions could be the major factors affecting the C and N pools in the sedimentary zones. This study would provide a scientific insight to develop effective management practices for soil erosion and nutrient loss control in highly eroded agriculture watersheds.

Transformation of litter carbon to stable soil organic matter is facilitated by ungulate trampling

Plant litter is an important source of soil organic carbon (SOC) in terrestrial ecosystem. The formation of SOC from plant litter and SOC mineralization to atmosphere is a critical determinant of long-term net ecosystem C balance. While it is generally understood that grazing plays a role in SOC cycling, what mechanisms are involved in driving SOC dynamic are not clear. Trampling enhances microbial processes by incorporating litter into the soil, likely influencing the fate of litter C and SOC mineralization. We conducted a controlled microcosm experiment to assess the role of trampling on the fate of litter C, litter-derived SOC mineralization (priming effect), and net C balance through incubation of 13C isotopically labelled Stipa krilovii litter placed on the soil surface, or incorporated into soil via simulated trampling, for 6 months. Litter C transferred into the SOC pool was further fractionated into mineral-associated soil organic carbon pool (MASOC, <53 μm) and particulate organic carbon pool (POC, >53 μm). We analyzed soil enzyme activities and litter-derived microbial biomass carbon (MBC) to determine microbial activities. We found trampling increased overall losses in litter mass (+16%) and litter C (+14%), with proportionally more decomposed litter C transferred to SOC pools (MASOC + 47% and POC + 157%), compared to absence of trampling. The litter-derived MBC was positively correlated with SOC formation, which was increased (+40.78%) by trampling, indicting a stronger microbial contribution to SOC formation. The disturbance of trampling did not induce significant positive priming effects, consistent with invariant soil total MBC and soil enzyme activities following trampling. As a result, trampling induced an increase in SOC formation and invariant priming effect, which contributed to positive net soil C balance (-230.74 ± 89.44 vs 100.33 ± 32.65 mg C/kg soil). Our results show trampling incorporates litter into soil and promotes microbial utilization of litter C and physiochemical stabilization of decomposed litter C, suggesting trampling is an important mechanism of SOC storage with litter C efficiently transferred into SOC pool. We demonstrate the importance of trampling in SOC formation and stabilization. Our findings indicate SOC formation efficiency from C input should be included in SOC predictive models in managed ecosystems.

Soil organic carbon controls dissimilatory nitrate reduction to ammonium along a freshwater-oligohaline gradient of Min River Estuary, Southeast China

Quality and source of soil organic carbon controlling DNRA in estuarine wetlands have not been well understood. We used a 15N isotopic tracer approach to investigate DNRA rates and determined the effects of organic carbon quality and source on magnitudes and distribution of DNRA along a freshwater-oligohaline gradient of Min River estuary, Southeast China. DNRA rates ranged from 0.45 to 2.92 nmol g−1 h−1 and were significantly higher in summer than in winter. DNRA was well predicted by dissolved organic carbon (DOC), organic carbon isotope (δ13Corg), total OC as the main predictor variables, which explained 63%, 6.0% and 7.0% of DNRA variances, respectively. Water content and temperature were crucial for DOC availability with important implications on DNRA. Soil δ13Corg increased from freshwater to oligohaline wetlands and showed a positive correlation with DNRA. We therefore conclude that high DOC availability enhances DNRA and can best predict spatial distribution in subtropical estuary.

Higher soil organic carbon sequestration potential at a rehabilitated mangrove comprised of Aegiceras corniculatum compared to Kandelia obovata

The present study compared the ecosystem organic carbon (OC) stocks and soil OC sources between two 12-year-old monospecific mangrove sites comprised of different species (Kandelia obovata in tree form and Aegiceras corniculatum in shrub form). We tested whether the carbon sequestration performance following rehabilitation varied with plantation of species in different forms and whether mangrove vegetation sequestrate OC more rapidly than soil pool. The results showed that mangrove rehabilitation increased the ecosystem OC stock relative to that of a non-vegetated bare flat. The accumulation of soil carbon was accompanied by increased soil total nitrogen contents and decreased δ13C values of soil OC, indicating that the increases in OC and TN contents were a function of accumulation of 13C-depleted mangrove materials in the soil. The sequestrated OC over the 12 years was considerably less in soil than in biomass at each mangrove site, suggesting that mangrove vegetation contributes more rapidly than the soil to ecosystem OC sequestration following rehabilitation before the vegetation has reached maturity. Compilation of the carbon stocks from worldwide rehabilitated mangrove forests with various ages further supports this finding. The K. obovata site had an apparently higher biomass OC stock but less OC in the soil than those at the A. corniculatum site. There was a higher standing leaf litter stock on the forest floor and more mangrove materials incorporated into the top 15 cm soil at the A. corniculatum site. These results suggested that the two rehabilitated mangrove sites had different development trajectories of both biomass and soil OC sequestration. Moreover, the performance of ecosystem carbon sequestration was related to plantation of different mangrove species. These carbon sequestration feature of rehabilitated mangrove forests therefore deserve attention in future rehabilitation programs to promote carbon sequestration performance.

Enhanced root exudation of mature broadleaf and conifer trees in a Mediterranean forest during the dry season.

Root exudates are part of the rhizodeposition process, which is the major source of soil organic carbon (C) released by plant roots. This flux of C is believed to have profound effects on C and nutrient cycling in ecosystems. The quantity of root exudates depends on the plant species, the period throughout the year, and external biotic and abiotic factors. Since root exudates of mature trees are difficult to collect in field conditions, very little is known about their flux, especially in water-limited ecosystems, such as the seasonally hot and dry Mediterranean maquis. Here, we collected exudates from DNA-identified roots in the forest from the gymnosperm Cupressus sempervirens L. and the evergreen angiosperm Pistacia lentiscus L. by 48-h incubations on a monthly temporal resolution throughout the year. We examined relationships of the root exudate C flux to abiotic parameters of the soil (water content, water potential, temperature) and atmosphere (vapor pressure deficit, temperature). We also studied relationships to C fluxes through the leaves as indicators of tree C balance. Root exudation rates varied significantly along the year, increasing from 6μg C cm -2 root day-1 in both species in the wet season to 4- and 11-fold rates in Pistacia and Cupressus, respectively, in the dry season. A stepwise linear mixed-effects model showed that the three soil parameters were the most influential on exudation rates. Among biotic factors, there was a significant negative correlation of exudation rate with leaf assimilation in Cupressus and a significant negative correlation with leaf respiration in Pistacia. Our observation of enhanced exudation flux during the dry season indicates that exudation dynamics in the field are less sensitive to the low tree C availability in the dry season. The two key Mediterranean forest species seem to respond to seasonal changes in the rhizosphere such as drying and warming, and therefore invest C in the rhizosphere under seasonal drought.