Effects of Slope Positions on Soil Organic Carbon and Its Labile Fractions in Chinese Fir Plantations

Soil samples were collected from two layers at various slope positions (upper slope, middle slope, and lower slope) of a Chinese fir plantation in Qingyuan District, Ji'an City, Jiangxi Province. Soil organic carbon (SOC), permanganate oxidizable carbon (POXC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) contents were analyzed. The results indicate that slope position had a significant effect on SOC and its active fractions (P < 0.05), displaying a trend of lower slope >middle slope >upper slope. The SOC, POXC and MBC were accumulated in soil surface. SOC and its labile fractions were closely related to environmental factors.There were extremely significant positive correlation between available nutrients and SOC, POXC and MBC. And pH showed a positive correlation with DOC. The RDA results suggest that total nitrogen and pH are the crucial factors responsible for the variability of SOC components of different slope positions. Overall, slope position dramatically affects the distribution of SOC and its active fraction in Chinese fir plantations, with the carbon being more unstable and susceptible to erosion on the upper slopes. In contrast, the organic carbon pool in the lower slopes is more stable and more conducive to the accumulation of SOC and nutrients. The results of this study provide a reference for the reconstruction of the SOC pool in degraded forest land and the rational restoration of Chinese fir plantations in this region.