Abstract: To reveal the microbial mechanisms of soil organic carbon (SOC) accumulation in continuous peanut cropping fields under organic substitution of chemical fertilizers, we carried out a five-year experiment with the treatments of vermicompost combined with chemical fertilizer (VM), bio-organic fertilizer combined with chemical fertilizer (BF), total-chemical fertilizer (CF) and no-fertilizer control (CK) in a continuous peanut cropping field. The composition and diversity of soil microbial community were tested by the phospholipid fatty acid method (PLFA). The contents of SOC and its components, peanut yields as well as the relationships between organic substitution, SOC and soil microbial community were analyzed. The results showed that the continuous peanut cropping seriously deteriorated soil environment and peanut yield, whereas the partial replacement of chemical fertilizers with organic fertilizers significantly increased the contents of SOC and its components and soil microbial biomass, changed the composition and diversity of soil microbial community, and enhanced peanut yield, with the most significant effect in the VM treatment (P<0.05). Compared with CK, the peanut yield of VM treatment increased by 56%, the SOC content and the total PLFA content increased by 11.4% and 149.4%, respectively. The results of redundancy analysis and stepwise regression analysis showed that VM and BF treatments were positively associated with the contents of SOC and its components and the PLFA contents of various soil microbial groups, especially in the VM treatment (P<0.05). The CK and CF treatments were negatively associated with all response variables except G⁺/G^- ratio. The contents of SOC components were significantly correlated with the PLFA contents of various soil microbial groups. In conclusion, the organic substitution of chemical fertilizers induced increase of exogenous organic carbon and subsequent changes in soil microbial community which were the driving factors for SOC accumulation, and soil microbial activity was the key factor affecting SOC accumulation. Partial replacement of chemical fertilizers by organic fertilizers, especially vermicompost, is an effective strategy to improve soil quality in continuous peanut cropping fields and alleviate the obstacles in continuous cropping.

Key words: organic fertilizer, soil organic carbon, phospholipid fatty acid method, soil microbial community, continuous peanut cropping.