Abstract:【Objective】Microbes and their necromass play a key role in the accumulation and long-term sequestration of soil organic carbon (SOC). Moreover, continuous increases in nitrogen (N) and phosphorus (P) inputs can significantly affect microbe-mediated SOC accumulation processes. The microbial necromass accumulation coefficient (NAC), which quantifies the accumulation of microbial necromass per unit of microbial biomass, plays a key role in assessing the efficiency of microbial necromass accumulation. However, the influence of short-term and long-term additions of N and P on this coefficient within meadow ecosystems remains unclear. This study focused on investigating the differential responses of NAC to (1) short-term and long-term N and P additions and (2) additions of N and P across different soil layers. 【Method】To explore the response of NAC to N and P additions, this study analyzed soil samples from the meadow on the Qinghai-Tibet Plateau subjected to 1 year (short-term) and 10 years (long-term) of N and P additions. It was measured the soil microbial necromass carbon (MNC) and the soil microbial biomass carbon (MBC), and calculated the value of NAC. Additionally, considering other environmental factors including soil physical and chemical properties, microbial extracellular enzyme activities, and plant biomass, the main influencing factors of NAC were identified. 【Result】The results showed that after short-term N and P additions, the NAC values in the 0-10 cm and 20-30 cm soil layers were 31.33±2.97 (mean±SE) and 38.12±3.90, respectively, and N and P additions had no significant effect on NAC (P>0.05). After long-term additions of N and P, the NAC values in the 0-10 cm and 20-30 cm soil layers were 14.46±1.12 and 17.49±3.22, respectively; and the additions of N and P significantly reduced the NAC in the 20-30 cm layer (P<0.05). The results of the Random Forest indicated that pH was the most important factor affecting NAC, and the correlation analysis revealed a significant positive relationship between soil pH and NAC. Moreover, the long-term N addition, P addition and simultaneous addition of N and P significantly reduced the pH of the 20-30 cm soil layer. These findings suggest that the decrease in soil pH due to long-term N and P supplementation is the main cause of the reduction in NAC. The lowered soil pH may lead to the dissolution of minerals, thereby reducing the mineral protection of MNC, making it more susceptible to decomposition, ultimately decreasing the NAC of microorganisms. 【Conclusion】In summary, changes in pH resulting from long-term nutrient additions dominated the changes in NAC. In the context of ongoing increases in N and P deposition, it is advisable to closely monitor changes in soil pH and implement timely measures to maintain the stability of SOC. This study explores the differential responses of NAC to N and P additions and their influencing factors, providing data support for understanding microbial-mediated carbon accumulation under the context of increasing N and P deposition.

Abstract:【Objective】Soil organic carbon (SOC) plays an important role in the global carbon cycle, and extremely small changes in SOC could cause dramatic changes in atmospheric CO2 concentration. Accurately grasping the spatial distribution characteristics of SOC and its main controlling factors is an important requirement for improving soil carbon sequestration potential and coping with climate change. Therefore, this study aimed to analyze the spatial distribution of SOC in the topsoil layer (A genetic horizon), subsoil layer (B genetic horizon), and parent material layer (Parent material) in Anhui Province from the perspective of the soil profile occurrence layer and explore the factors controlling the changes of SOC in different profile occurrence layers.【Method】 In this study, a total of 451 sites were distributed in the study area using the systematic distribution method combined with the judgmental distribution method. The basic soil parameters, such as SOC content, pH, soil texture, and bulk density, were obtained from 451 sites through wild sampling and indoor experiments. Meanwhile, the related environmental variables, such as climatic factors, topographic factors, and normalized difference vegetation index, were also collected. Also, we used geostatistical methods to obtain the best half-variance function model and spatial distribution characteristics of SOC content at different soil profile levels, as well as correlation analysis and random forest regression analysis to explore the influencing factors of spatial differences in SOC content.【Result】The results showed that the average organic carbon content of the soil profile in Anhui Province was 8.47 g?kg-1 and there was a phenomenon of surface aggregation of SOC, whose occurrence in the layer was as follows: A genetic horizon: 15.86 g?kg-1 ＞ B genetic horizon: 5.80 g?kg-1 ＞ Parent material: 3.74 g?kg-1 and all of them had moderate spatial variability. The spatial distribution map of SOC showed that the spatial distribution of organic carbon content in each occurrence layer was generally increasing from north to south. We also found that there were some differences in the driving factors of SOC content in different profiles of the occurrence layer. In the A genetic horizon, soil texture, and bulk density were the most important factors affecting SOC content; as the depth of the soil layer increased, the influence of topographic factors and soil texture gradually strengthened on the accumulation of SOC content in the B genetic horizon. For the Parent material, the influence of soil texture, topographic factors, and bulk density were all more influential on the SOC content.【Conclusion】Soil texture is the main factor driving the spatial distribution characteristics of SOC in Anhui Province, but the effects of topographic factors and bulk density should also be fully considered in the subsequent development of SOC control measures, to provide theoretical support for improving soil quality and coping with climate change.

Abstract:【Objective】 Soil organic carbon (SOC) is an essential indicator of soil health. It not only provides a carbon source for plant growth and maintains the physical structure of soil, but also releases carbon into the atmosphere in the form of greenhouse gases, such as carbon dioxide. Therefore, it plays a critical role in the global carbon balance. Currently, the world is experiencing climate change characterized predominantly by warming and increasing frequency and intensity of extreme weather events. However, the impacts of the changing climate, including long-term warming and extreme weather events on SOC are not entirely the same. Distinguishing and quantifying the effects of extremely high temperatures (EH) and global warming (GW) on SOC is the key to formulating adaptive strategies.【Method】 In this study, we focused on paddy soils in Zhangzhou of Fujian Province, a typical subtropical region of China. Based on a 1:50,000 detailed soil database, we employed the biogeochemical process model (DeNitrification-DeComposition, DNDC) to simulate SOC dynamics under four climate scenarios: de-trended climate base state (CTRL), extreme high temperatures (EXP_EH), long-term warming (EXP_GW), and measured temperatures (EXP_obs).【Result】 The results revealed that the total amount of carbon sequestered by paddy fields in Zhangzhou from 1980 to 2016 under the four different climate scenarios (CTRL, EXP_EH, EXP_GW, and EXP_obs) was 1,032.17, 952.15, 1,045.98 and 966.03 Gg, with the corresponding average annual sequestration rates of 93.98, 86.70, 95.24, and 87.96 kg·hm-2, respectively. The long-term warming led to a net increase of 13.81 Gg of SOC in paddy fields across Zhangzhou, while extremely high temperatures resulted in a net decrease of 80.02 Gg. The combined effect of these two factors was -66.14 Gg in SOC, indicating that long-term warming promoted the sequestration of organic carbon in paddy soils, while extremely high temperatures reduced the soil carbon sink capacity, with extremely high temperatures exerting a dominant negative effect. Also, the variations in annual carbon sequestration rates between different climate scenarios indicated that extremely high temperatures throughout the years from 1980 to 2016 had a negative effect on carbon sequestration in the paddy soils of Zhangzhou, but the long-term warming effect on SOC turned from positive to negative around the year of 2000. This may be related to the diminishing effect of warming on plant growth over time. The results of grey relational analysis-structural equation modeling also indicated that the clay content, bulk density, and organic fertilizer application rate were most closely associated with the carbon sequestration rate in rice fields of Zhangzhou, followed by the annual average temperature, precipitation, and pH levels. At the county level, climate change had the greatest impact on the carbon sequestration of Nanjing County. Additionally, the extremely high temperatures and long-term warming caused -26.23% and 7.27% impacts on its carbon sequestration rate, respectively. Among subclasses of rice soils, acid sulfate paddy soils were most affected, with -23.05% and 8.10% changes in carbon sequestration rate caused by warming and extremely high temperatures, respectively. Furthermore, among different terrain and topographical areas, the carbon sequestration rate of hilly and mountainous areas was significantly affected by extremely high temperatures and long-term warming, with -8.84% and 1.98% changes, respectively. 【Conclusion】 In conclusion, while the paddy soils in Zhangzhou still maintain a strong carbon sequestration capacity in the context of climate change, the increasing extreme high-temperature events in the future may potentially contribute to greater carbon losses to some extent.

Abstract:【Objective】The soil carbon-to-nitrogen ratio (C/N) reflects not only soil quality but also the nutrient balance of soil carbon and nitrogen elements. Thus, rapid and accurate determination of this ratio and the grade is crucial for guiding real-time scientific fertilization and improvement of soil quality. 【Method】This study used visible-near infrared (VNIR) and mid-infrared (MIR) spectroscopic data, along with total organic carbon (TOC), total nitrogen (TN), and C/N data from 501 typical tobacco-corn rotation farmland topsoil samples (0~20 cm) in Guizhou Province for characterization. After processing the spectra with Savitzky-Golay (SG) smoothing and standard normalization, three modeling methods were applied: partial least squares regression (PLSR), random forest (RF), and Cubist. Models for predicting soil C/N were constructed using both direct prediction of C/N and indirect prediction (first predicting TOC and TN, then calculating C/N), and the precision of C/N value and grade predictions was analyzed. 【Result】The results revealed that: (1) For C/N value prediction, the optimal prediction strategy was direct prediction using MIR-PLSR, which had a prediction precision (relative standard error, RPD) of 1.20; (2) C/N grade could be accurately predicted, with the optimal strategy being direct prediction using the MIR-PLSR model, achieving a grade determination accuracy of 0.71; (3) The main reasons for the low prediction accuracy of C/N values are twofold. First, the uniform stringent fertilization measures in the tobacco fields have reduced the spatial variation in the carbon and nitrogen content of the plow layer soil, thereby also reducing the spatial variation of C/N (the coefficient of variation is 17.15%, indicating moderate variation). Second, the correlation between C/N and both VNIR and MIR spectra was relatively low. 【Conclusion】Therefore, the MIR-PLSR model can be used for direct prediction of C/N grades.

Abstract:【Objective】 Soil particulate organic carbon (POC) is a key player in the transformation and sequestration of soil carbon pools. However, POC content is significantly regulated by changes in soil environment. Therefore, this study was aimed to clarify the change of POC and its influencing factors with vegetation degradation of alpine wetlands, in an attempt to provide certain basic data for further understanding the responses of soil carbon pool dynamics to climate change and human activities in alpine wetland. 【Methods】 In this study, the swampy meadow of Gahai wetland in the northeastern edge of the Qinghai-Tibet Plateau (QTP) was taken as the study area. In the typical vegetation growth area around Gahai Lake, the spatial instead of temporal method was used to characterize the degree of degradation. Sample plots were set up by selecting lots with gentle terrain and consistent slope orientation. Different vegetation degradation levels were classified according to the indicators of plant species composition, aboveground biomass, community height and cover. Soil samples were collected from four vegetation degradation levels, including non-degraded (ND), slightly degraded (SD), moderately degraded (MD), and heavily degraded (HD) in swampy meadow at Gahai wetland. The contents of soil POC were investigated in in the growing seasons of 2016-2017 by field sampling and laboratory analysis. Three-factor analysis of variance was used to analyze the effects of vegetation degradation, soil layer, sampling time and their interactions on soil moisture, soil organic carbon (SOC) and POC contents. Redundancy analysis was performed to determine the dominant factors affecting the change of SOC components in each vegetation degradation levels. 【Results】 The results showed that vegetation degradation significantly decreased the amount of POC at soil surface layers (0-10 and 10-20 cm), but there was no significant effect on the deep layers (20-40, 40-60, 60-80 and 80-100 cm). As the growing season progresses, the contents of POC at 0-10 and 10-20 cm layers decreased first and then increased in four vegetation degradation levels. However, the contents of POC at the other deep layers did not change significantly. In terms of inter-annual variation, soil POC levels and fluctuations were higher in 2016 than in 2017. Analysis of variance (ANOVA) showed that the sampling time, the vegetation degradation and soil layer had significant effects on the POC content, respectively. Meanwhile, the interaction of sampling time, vegetation degradation and soil layer had a significant effect on soil POC content. To further identify the intrinsic factors affecting changes in POC content. Redundancy analysis was utilized to reveal the differences between the studied factors. The results showed that total nitrogen and below-ground biomass were the main factors driving changes in soil organic carbon fractions. 【Conclusion】 In summary, the process of vegetation degradation in alpine wetlands may impair the accumulation of surface soil carbon pools in the wetlands of the QTP. The original POC accumulation is gradually lost with the increasing degree of vegetation degradation. This phenomenon suggests that vegetation degradation may have transformed the QTP wetlands into a new potential carbon source.

Abstract:【Objective】 With global climate change and overgrazing, shrub encroachment is extensively occurring in global grasslands. However, relatively little is known about how the structure of bacterial communities shifts with shrub encroachment. Thus, considering the aboveground plant community, soil carbon chemical composition, soil bacterial community structure and network beneath the canopies of three typical shrub species (Potentilla fruticosa, Spiraea alpina, and Caragana microphylla) as well as in adjacent grassland (as a control), the effects of shrub encroachment on the structure of soil bacterial communities and soil carbon pools were explored.【Method】 16S rRNA gene sequencing was used to investigate the bacterial communities and co-occurrence features among bacterial taxa while Fourier transform infrared spectroscopy (FTIR) was conducted to assess the soil organic carbon (SOC) chemical composition.【Result】 Shrub encroached grasslands (Potentilla fruticosa and Caragana microphylla) showed significant changes in aboveground plant community composition (P < 0.01) while the aboveground plant community diversity and richness remained constant (P > 0.05). The biomass of the three shrub plots was significantly higher than that of grassland (P < 0.05) whereas underground biomass showed no significant difference (P > 0.05). Shrub encroachment had no significant effects on SOC and total nitrogen (TN) contents, but weakened the differences of SOC contents between top- and subsoils, as shown by significantly higher SOC contents in the topsoil of the grassland than in its subsoil (P < 0.05), with no such trend in the three shrub plots.The SOC chemical composition in both top- and subsoils of the three shrublands and grassland was dominated by aromatics(except for deep soil in Caragana microphylla plots), with no significant difference in aromatic content between shrub and grassland plots (P > 0.05). However, the Caragana microphylla plots exhibited a surface-aggregated distribution of aromatics (P < 0.05). Random forest model analysis revealed that the distribution of Acidobacteria and Actinobacteria was the most important predictor of shrub encroachment in top and subsoils (P < 0.01). According to Non-metric multidimensional scaling (NMDS) analysis, the bacterial community composition of alpine grassland was significantly altered by shrub encroachment. Moreover, plant community composition and SOC chemical compositions were the main explanatory factors affecting bacteria community composition in both depths. Functional prediction analysis identified four biological metabolic pathways, including cellular processes, environmental information processing, metabolism, and genetic information processing, with metabolism being enriched in shrub plots (P < 0.05). Based on topological parameters of total links, complexity, and natural connectivity, the results showed that the soil bacterial network of shrublands was more complicated and stabilized than that in grasslands, and mutualism or commensalism may play an important role in establishing the bacterial community structure. 【Conclusion】 In summary, the results of this study suggest that shrub encroachment had an important regulatory effect on soil bacterial community structure and soil carbon pool. The results enrich the literature on soil microbial community in alpine grassland and provide a theoretical basis for the effect of soil carbon source and sink in alpine grassland.

Abstract:【Objective】 Soil CO2 concentration is often higher than that of the atmosphere. Current studies on soil organic carbon mineralization are mostly conducted under conditions of increasing atmospheric or simulated atmospheric CO2 concentration. This may lead to deviation of the results from the actual organic carbon mineralization process in the soil profile or impose some bias on indoor mineralization incubation experiments towards the “mineralization potential” rather than the actual mineralization rate. How and to what extent soil organic carbon mineralization is affected by high CO2 concentrations in the soil profile? The lack of a clear answer to this question limits the comprehensive understanding of soil organic carbon stability. 【Method】 In this paper, an indoor mineralization incubation test was conducted with six CO2concentration gradients of CK (400 µmol·mol–1, atmospheric level), 800, 2 000, 4 000, 6 000, and 8 000 µmol·mol–1, and three replicates were set for each treatment. The effects of different concentrations of CO2 on the rate of soil organic carbon mineralization, cumulative mineralization, and active organic carbon fractions were investigated, and the extent to which CO2 concentration and other influencing factors explained the cumulative mineralization was analyzed. 【Result】 The results showed that: 1) High concentration of CO2(2 000-8 000 µmol·mol–1) in soil significantly inhibited the mineralization of soil organic carbon, with the mineralization rate decreasing by 6.27%-45.61%, and the cumulative amount of mineralization decreased by 1.72%-40.82%; 2) Lower concentration of CO2(800 µmol·mol–1) in soil significantly promoted the mineralization of soil organic carbon, the mineralization rate increased by 4.38%-12.65%, and the cumulative mineralized amount increased by 17.37%-48.43%; 3) The CO2 concentration in the soil effected the content of active organic carbon fractions. At a range of CO2 concentrations, soil microbial biomass carbon (MBC) content increased significantly and dissolved organic carbon (DOC) content decreased significantly compared to CK. However, the content of easily oxidizable organic carbon (EOC) was not significantly changed; 4) The mineralization characteristics of organic carbon showed a significant negative correlation with CO2 concentration, a significant positive correlation with DOC, a negative correlation with EOC, and no significant correlation with MBC; 5) Under the appropriate conditions of temperature and humidity, the contribution of CO2 concentration to the cumulative mineralization of soil organic carbon reached 22.93%. 【Conclusion】 High CO2 concentration significantly inhibited soil organic carbon mineralization by affecting the soil organic carbon readily available carbon source, which may be one of the important factors to maintain soil organic carbon stability.

Abstract:【Objective】 Rape multiple-cropping is an important planting mode to promote grain stabilization and rapeseed increase in South China rice growing area. We explored the influence of soil organic carbon (SOC) accumulation and its stability characteristics under different rice–rape rotation measures with whole-straw returning, which is of great significance for in-depth analysis of soil carbon cycle in paddy fields by making full use of winter fallow fields to plant rape. 【Method】 This study is based on an 8-year yield localization experiment. In contrast with rice–rice–winter fallow, we explored the characteristics of SOC and its fraction accumulation under three rice–rape rotation treatments: rice–rice–rape, rice–rape tillage, and rice–rape no tillage. 【Result】 The results indicated that the content of SOC in 0–20 cm soil layer was increased by 5.28%–25.12% under the three rice–rape rotation treatments, especially under the rice–rice–rape treatment. Also, the increasing rate of SOC in 20–40 cm soil layer was 18.48%—43.97%, among which the rice–rape tillage and the rice–rape no tillage treatment reached a significant level.The content of mineral-associated organic carbon (MAOC) from all the rice–rape rotation measures was increased significantly in different soil layers. At the same time, the ratio of particulate organic carbon (POC) to SOC was significantly decreased while the ratio of MAOC to SOC increased in each treatment from both 0–20 cm and 20–40 cm soil layer. The increasing rate of MAOC/SOC were 2.31%–7.49% and 1.56 %–2.66% in the two soil layers, respectively. Possible causes of these results may be that rice–rape rotation increased the activity of organic carbon invertase enzyme (β-glucosidase、β-1, 4-glucanase and Laccase) as well as microbial biomass carbon in 0–20 cm soil layer to varying degrees, thereby promoting the conversion of POC to MAOC. 【Conclusion】 In summary, rape multiple-cropping in winter fallow not only promoted the accumulation of SOC in paddy field, but also increased the ratio of MAOC/SOC, ultimately enhancing the stability of soil carbon pool.

Abstract:【Objective】 Soil organic carbon (SOC) forms the basis of soil fertility, food production, and soil health, and plays a key role in climate change via mediating greenhouse gas emissions. Consequently, accurate characterization of SOC spatiotemporal dynamics is extremely important for the sustainable management of soil resources, ecosystem stability maintenance, and mitigation and adaptation to climate change. 【Method】 A total of 399, 413, and 407 cropland topsoil (0 ~ 20 cm) SOC data in 1980, 2000, and 2015 were collected from the southern Jiangsu Province of China, respectively, and the microbial-explicit SOC model MIMICS (Microbial-Mineral Carbon Stabilization) was used to model the spatiotemporal dynamics of SOC. The Sobol global sensitivity analysis was applied to identify the sensitive parameters of the MIMICS model, and then, two-parameter optimization schemes, one batch (using all SOC observations in a batch mode to optimize the parameters) and site-by-site (using SOC observations at individual sites to optimize the parameters site by site), were used to optimize the sensitive parameters of the MIMICS model through Markov Chain Monte Carlo (MCMC) approach, respectively. The coefficient of determination(R2), root mean squared error (RMSE), and mean absolute error (MAE) that were calculated from the independent validation of SOC in 2000 and 2015 were used to compare the performance of different parameter optimization schemes.【Result】Results show: (1) The net increment of SOC density between 1980 and 2000 was 0.89 kg·m-2, while the net decrement was 0.44 kg·m-2 between 2000 and 2015, representing a net increment of 0.45 kg·m-2 over the period of 1980-2015; (2) The MIMICS model with parameters optimized by either One batch or site-by-site method can represent the overall trends in topsoil SOC dynamics during the period of 1980-2015, but the model with parameters optimized by the site-by-site method presents more local details on the variability of the SOC change rate; (3) Compared with the default parameter values and the One-batch optimized parameter values, the MIMICS model with site-by-site optimized parameter values had the best performance in modeling the spatiotemporal dynamics of SOC in the study area, with the RMSE decreasing by 22.2% (the independent validation in 2000) and 14.7% (the independent validation in 2015) in comparison with the MIMICS model with default parameter values. Yet, its prediction accuracy in 2015 was still relatively low (R2 = 0.13, RMSE = 1.22 kg·m-2). 【Conclusion】The optimization of sensitive parameters can improve the space-time SOC prediction accuracy of the MIMICS model, and the representation of local details on the spatiotemporal patterns of SOC dynamics. Although the MIMICS model with the spatially heterogeneous parameter values optimized by the site-by-site method had the best performance, its prediction accuracy in 2015 was still relatively low, which indicated that the MIMICS model still has limitations in representing the responses of SOC to anthropogenic activities such as changes in land use and agricultural management practices. Thus, further improvement of the MIMICS model structure and enhancing the spatiotemporal resolution of model input data are still significant challenges for regional scale modeling of SOC spatiotemporal dynamics through microbial-explicit SOC models.

Abstract:【Objective】Masson pine forest (Pinus massoniana Lamb.) is a typical ectomycorrhizal (ECM) dominant forest. However, in recent years, the ecological service function of the Masson pine forest decreased due to pine wood nematode disease, and the Masson pine forest was gradually replaced by an Arbuscular mycorrhizal (AM) dominant broadleaved forest. However, it remains unclear what influence could be exerted by the changes of dominant mycorrhizal types on soil organic carbon accumulation during the conversion of Masson pine forest to broadleaf forest in the subtropical region. 【Method】In this study, the biomass of ECM fungi and AM fungi were determined by high-performance liquid chromatography (HPLC) and neutral lipid fatty acids (NLFA), respectively. At the same time, phospholipid fatty acids (PLFAs) technology was used to study the characteristics of the microbial community. The content of glomalin-related soil protein (GRSP) and the activities of soil extracellular enzymes was also determined in Masson pine and broadleaved forests in Jiande County, Zhejiang Province.【Result】The results showed that: AM fungi-dominated (AMD) broadleaved forest replaced ECM fungi-dominated (ECMD) Masson pine forest, soil organic carbon in AM fungi dominated broadleaved forest was significantly enhanced by 36.81%, microbial carbon use efficiency (CUE) significantly increased by 53.85%, and AM fungal biomass significantly increased by 25.57%. Moreover, compared with ECM fungi-dominated forests, the biomass of ECM fungi in AM fungi-dominated forests decreased significantly by 45.04%. The Masson pine forest, which was dominated by ECM fungi, was subjected to more severe microbial nitrogen limitation. Phospholipid fatty acids analysis showed that the gram-positive bacteria (G+) and the ratio of gram-positive bacteria to gram-negative bacteria (G+/G-) in Masson pine forest dominated by ECM fungi compared with the broadleaved forest dominated by AM fungi were significantly decreased by 21.47% and 6.46%, respectively. Redundancy analysis (RDA) results showed that there were significant differences in microbial community structure between forests dominated by AM fungi and ECM fungi (P<0.05), in which AM fungal biomass (R2=0.48, P=0.002) and soil organic carbon content (R2=0.47, P=0.003) were significantly correlated with the variation of microbial community structure (P<0.05).【Conclusion】The decrease of GRSP and the different recruit of microbial groups by different mycorrhizal fungi types were important reasons for the reduction of soil organic carbon content in forests dominated by ECM fungi compared to AM fungi dominated forests. Therefore, the substitution of broadleaved forest for Masson pine forest in the subtropical region increased the content of forest soil organic carbon and improved the function of the forest carbon sink.