Soil microbes benefit enhancing stability of soil organic carbon pools under mixed-species plantations

Soil organic carbon (SOC) plays a pivotal role in climate regulation and ecosystem resilience. Mixed-species plantations are expected to increase SOC stocks more effectively than monoculture plantations, with microbially mediated processes potentially driving this improvement. In southern China, Cunninghamia lanceolata and Cryptomeria fortunei are the primary species used for afforestation. However, the mechanisms by which soil microbes influence SOC stocks in these mixed-species plantations remain poorly understood, particularly at different soil depths. In this study, monoculture plantations of C. lanceolata and C. fortunei were compared with their mixed-species plantations to investigate the effects of afforestation patterns and soil depths (30 cm, 60 cm, and 90 cm) on SOC and its components, and their relationships with soil microbes. Results revealed that mixed-species plantations increased SOC and its component stocks, as well as the proportion of mineral-associated organic carbon (MAOC) within SOC, across soil depths. Furthermore, SOC and its component stocks decreased with depth across all afforestation patterns studied. Mixed-species plantations also increased soil microbial α-diversity, relative abundance of Proteobacteria, and network complexity. Shifts in microbial α-diversity were primarily driven by soil nitrogen and available phosphorus content. Notably, increased soil microbial α-diversity, especially for bacteria, tended to increase the contribution of MAOC to SOC rather than particulate organic carbon. This indicates enhanced microbial-mediated stabilization of soil carbon stocks. However, this beneficial effect decreased with increasing soil depth. Collectively, our findings demonstrate that mixed-species plantations of C. lanceolata and C. fortunei represent a more effective strategy than monoculture plantations for improving the stability of SOC pools, with implications for sustainable forest management under global change scenarios.