Application of biochar to soil may stabilize soil organic carbon (SOC), concomitantly increasing nutrient retention. However, the interactive effect of biochar and nutrients on SOC and the underlying microbial mechanisms remain poorly understood, particularly in intensively managed forests where decarbonization is substantial after converting from natural forests. This 80-day incubation experiment aimed to quantify native SOC mineralization as affected by biochar (B) and nutrients [nitrogen (N) or phosphorus (P)], linking to the chemical composition of SOC, soil microbial community composition, and enzyme activities within a subtropical Moso bamboo (Phyllostachys edulis) forest soil. Results presented that compared to the control (nil-nutrient), nutrients (N, P, and NP) significantly destabilized native SOC [positive priming effect (PE); 20–98% increase in SOC mineralization], whereas such destabilization effect was significantly reduced by biochar (6.0–19%). The positive PE by nutrient was due to the increases in O-alkyl C, microbial biomass C, available mineral N, soil pH, β-glucosidase, and invertase activities. Meanwhile, the greater PE by N than P could be attributed to (i) decreases in diversity of bacterial and fungal communities; and (ii) increases in the relative abundances of microbial taxa such as Bacilli, Planctomycetes, and Alphaproteobacteria. Importantly, biochar’s stabilization effect was because biochar not only lowered NH4+-N and NO3−-N and β-glucosidase activity, but also increased the activity of C-fixing enzyme (RubisCO) and polyphenol oxidase activity. Furthermore, biochar significantly decreased soil O-alkyl C that possibly resulted in less labile SOC mineralization, but increased aromatic C resulting in lower fungal diversity. We conclude that the biochar significantly reduces the destabilization effects of nutrients on SOC, highlighting that the biochar application is an effective approach to mitigate soil CO2 emissions within subtropical forest.