Twelve-year biochar application alters soil labile organic carbon, nitrogen pools and microbial diversity in a subtropical forest

Biochar application is recognized as a potential measure for enhancing soil carbon sequestration and improving ecological functions. However, the long-term impacts of a single biochar application on the dynamics of labile soil carbon and nitrogen and on microbial communities in subtropical forests remain unclear. In this study, a 12-year field experiment was conducted with three biochar application rates: 0 (CK), 10 (B10), and 30 (B30) t·ha⁻¹, to systematically investigate the long-term effects of biochar on soil easily labile organic carbon and nitrogen contents, and microbial communities. The results showed that biochar application significantly increased soil extractable organic carbon and nitrogen contents, indicators of labile organic carbon and nitrogen, with a more pronounced effect observed under the high application rate. Microbial community analysis revealed that the Shannon index of bacterial diversity significantly increased only under the low application rate (B10), while no significant change was detected under the high rate (B30). At the phylum level, biochar application increased the relative abundance of Pseudomonadota and decreased that of Acidobacteriota and Actinomycetota. In contrast, fungal diversity showed a decreasing trend, primarily characterized by a reduction in Basidiomycota and an increase in Mucoromycota. Co-occurrence network analysis further demonstrated that the complexity and stability of bacterial networks increased with higher biochar application rates, whereas fungal networks maintained relatively higher structural and functional stability only under the low application rate. From the perspective of microbial community regulation and carbon-nitrogen transformation, this study elucidates the key pathways through which long-term biochar input influences forest soil carbon and nitrogen cycling. It provides long-term observational evidence and mechanistic insights for biochar-based strategies aimed at enhancing forest soil carbon sequestration and regulating microbial functions.