Spatial Variation Of Soil C And N And Their Determinants In A Subtropical Evergreen And Deciduous Broad-leaved Mixed Forest In Mt. Badagongshan

As the largest carbon (C) pool and nitrogen (N) pool in terrestrial ecosystems, soil C and N pools could have profound influences on global carbon and nitrogen cycles, even with slight fluctuations. Sequestration of carbon into soil is generally regarded as one of the most important ways to mitigate elevated CO2 concentration in the atmosphere and to alleviate the associated climate change. Due to the large amount of spatial variation in soil C and N, there still exist great uncertainties in quantifying soil C and N pools, thus limit our ability to evaluate the magnitude of soil sequestered carbon. To investigate the spatial variation of forest soil C and N and their determinants at local scale can provide reliable information to accurately predict soil C and N pools and their dynamics, and improve our understanding of the mechanisms of soil C and N cycles in forest landscapes. Furthermore, we examined the effects of different forest community types on soil C and N dynamics to provide a theoretical reference to develop reasonable forest management measures.We collected soil samples (n=972) in the 25 ha Badagongshan (BDGS) forest dynamics plot to explore the spatial variation of SOC, TN and soil ?13C and their determinants using semivariance analysis and variation partitioning.In addition, two different vegetation types (Schima parviflora and Fagus lucida) were selected within the BDGS plot to investigate their effects on SOC and TN concentrations and spatial distributions. The main findings are:SOC and TN in both surface soil (0-10 cm) and subsurface soil (10-30 cm) exhibited moderate spatial autocorrelation with explainable fractions ranged from 31% to 47%. There existed a large amount of variations at fine scales. Fine-scale environmental factors including topographical variations and the associated microbial communities may play a key role in fine-scale (<2 m) spatial variation of SOC and TN.In our study site, soil variables are the main determinants of SOC and TN patterns. Soil pH, available phosphorous (AP), clay content had significant relationships with SOC and TN. The effect of vegetation variables on SOC and TN was scale-dependent. The explanation power of vegetation variables at 20 m scale on SOC was higher than that at 5 m and 10 m scale. Although the effect of topography was weak, mantel's test results still showed a significant relationship between topography and SOC. Spatial structure had important effect on SOC and TN variation. In addition, there were complicated interaction among environmental factors.The range of soil ?13C value in BDGS was relatively small. Both surface soil and subsurface soil exhibited moderate spatial autocorrelation in soil 813C. There was still some variation at the fine scale (<2 m), and this may be resulted from isotopic fractionations in microbial metabolism and other spatial processes occurring at fine scales.Similar explanation power of environmental factors to soil ?13C variation were found between surface and subsurface soil. However, explanation power of spatial structure was higher in subsurface soil than that in surface soil. This indicated that various spatial processes might play key roles in soil carbon stable isotope composition in subsurface soil. Low explanation power of vegetation variables indicated that differences of vegetation attributes included in this study among sampling points had weak effects on soil carbon stable isotope composition. Soil variables may affect soil ?13C spatial variation through regulating the isotopic fractionation in microbial related processes. The effects of topography on soil ?13C variation is mainly occurring through the interactions between topography and spatial structure.Plant community types had significant effects on the spatial distribution of SOC and TN. Only surface soil showed a significant difference in SOC and TN concentration. This may be related to the carbon input from litter and strong soil leaching in this region. For SOC and TN stocks, large differences were only found in deep soil, C/N ratio was consistent among plant communities and soil depths. There were significant differences in soil ?13C value between the two vegetation types. This difference is likely inherented from the differences in litter and fine roots ?13C values of the dominant tree species.