Microbial carbon use efficiency governs the accumulation of microbial-derived carbon in restored mangroves

Abstract While mangrove restoration has a great potential for enhancing soil organic carbon (SOC) sequestration in coastal wetlands, microbial-mediated SOC decomposition introduces huge uncertainty to this process. Microbial carbon use efficiency (CUE) is a crucial trait for microorganisms controlling SOC turnover, but how mangrove restoration could affect microbial CUE remain unclear. Here, we investigated the effects of mangrove restoration on microbial CUE in a typical restored mangrove wetland and further explored its connection to microbial necromass carbon (MNC) content. We found that mangrove restoration increased microbial CUE by 37.84%–56.76% due to an increase in organic carbon quality and a shift in microbial community structure from fast-growing r-strategist (bacteria-dominated including Proteobacteria and Bacteroidota) to slow-growing K-strategist (fungal taxa and bacterial phyla such as Actinobacteriota, Acidobacteriota, and Chloroflexi). Microbial CUE was also positively correlated with MNC, explaining 73% and 69% variations in fungal and bacterial necromass C, respectively. These findings indicate that mangrove restoration enhances SOC sequestration not only through increased plant-derived carbon input but also by elevating microbial CUE and promoting MNC accumulation. Although bacterial necromass carbon showed a higher percentage increase, fungal necromass constituted the dominant portion of the accrued microbial-derived carbon pool, underscoring the critical role of fungal communities in the formation of stable SOC. Our study highlights the significant role of microbial processes in promoting SOC accumulation during mangrove restoration. These results emphasize the importance of incorporating microbial processes into coastal wetland restoration strategies to maximize C sequestration.