The role of mixed-species forests in post-fire soil organic carbon restoration: Mechanisms and microbial-mediated pathways

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Wildfire-induced soil organic C (SOC) loss threatens global C sequestration. However, the mechanisms underlying SOC recovery during reforestation, particularly the roles of tree diversity and microbial-mediated pathways, remain poorly understood. In this study, we aimed to quantify SOC accumulation patterns across restoration methods and identify microbial mechanisms underlying SOC storage. To this end, we compared monoculture, two-species mixture, and five-species mixture plantations in a subtropical forest 14 years post-fire by integrating SOC fractionation (particulate organic C and mineral-associated organic C [MAOC]), microbial physiological traits (C use efficiency [CUE] and turnover), necromass biomarkers (amino sugars), and soil physicochemical properties. Adjacent undisturbed secondary forests with analogous edaphic and topographic conditions served as reference ecosystems. The five-species mixture restored 77.97 % of the reference SOC levels, significantly outperforming the monoculture (48.76 %) and two-species mixture (65.23 %), primarily through MAOC accumulation (76.55 % of the reference). Microbial necromass C (bacteria + fungi) was the dominant predictor of MAOC (36.1 % of explained variance), whereas CUE showed no significant relationship with SOC. Rapidly available N promoted necromass–MAOC coupling by alleviating microbial stoichiometric constraints. These results demonstrate that reforestation enhances post-fire SOC stabilization by diversifying microbial necromass inputs and optimizing organo–mineral interactions, rather than through metabolic efficiency. Our findings challenge the universal role of CUE in SOC models and advocate for biodiversity-driven strategies to restore fire-affected landscapes. Schematic diagram of SOC accumulation patterns across restoration methods • Biodiversity-driven strategies can effectively restore fire-disturbed soils. • Mixed-species plantations accelerate post-fire soil organic C recovery. • Microbial necromass accumulation and stabilization are key mechanisms. • Microbial C-use efficiency is not as useful in disturbance-prone ecosystems.