Soil organic carbon sources exhibit different patterns with stand age in rhizosphere and non-rhizosphere soils

The sources of soil organic carbon (SOC) determine its stability and dynamics. However, in the soil's active zone- rhizosphere, the sources and regulatory mechanisms of SOC are still unclear, and the rhizosphere and nonrhizosphere soils may exhibit variations depending on stand age. We collected rhizosphere and nonrhizosphere soils from a typical subtropical evergreen broadleaf forest ( Castanopsis hystrix plantations) at six distinct stand age in the South Subtropical region, using soil amino sugars and lignin phenols content to characterize microbial-derived carbon and plant-derived carbon, respectively. The results indicated that the SOC sources varied significantly in rhizosphere soils while remained stable in non-rhizosphere soils. Soil amino sugars content in rhizosphere soils was higher than non-rhizosphere soils and increased with stand age, leading to the rhizosphere effect on soil amino sugars content significantly increased with stand age. Soil lignin phenols content in rhizosphere soils was initially lower than that in non-rhizosphere soils at 6---10 stand age, but it increased significantly over time, resulting in a significantly increased in the rhizosphere effect on soil lignin phenols content with stand age. Further analysis revealed that the SOC sources were jointly regulated by stand age and rhizosphere effect during forest restoration, in which the fungal and bacterial phospholipid fatty acids were key factors to drive the variation in soil amino sugars while litter C:N ratio and fine root biomass accounted for the patterns of soil lignin phenols. Therefore, we found significant differences in the patterns and driving factors of SOC sources between rhizosphere and non-rhizosphere soils with stand age, emphasizing the importance of considering the rhizosphere effect when studying soil carbon dynamics during forest restoration.