Microbial community composition regulates SOC decomposition response to forest conversion in a Chinese temperate forest

Forest conversion influences soil organic carbon (SOC) decomposition through cascading effects on forest structure, soil properties, and soil microbial communities. However, interactive effects of these drivers and the key pathways that mediate forest SOC decomposition remain relatively unexplored. In this study, we compared relative importance of variables describing forest structure, soil properties, and soil microbial community on affecting SOC decomposition response to the conversion of a broadleaved Korean pine mixed forest into three other forests in the Changbai Mountains of China. We quantified SOC decomposition rate of these four forest types by measuring incubation soil respiration (SR). We then employed univariate regressions to quantify effect size of individual factor on SOC decomposition rate. A structural equation model (SEM) was developed to analyze pathways, relative importance, and interactive effects of these factors on SR. Our results showed strong marginal effects of dissolved organic carbon (DOC) content, fungal Phospholipid fatty acids (PLFAs) to bacterial PLFAs ratio (F/B), broadleaved to conifer ratio (B/C), and total PLFAs content (TPC) on SR. Measured SOC decomposition rate was most closely related to F/B, which was in turn influenced primarily by soil C/N ratio and fraction of non-oxidized carbon (NOC%). Our study identified “Aboveground forest composition → SOC chemistry → Soil microbial composition → SOC decomposition” as the key pathway by which forest conversion affected SOC decomposition. This research work highlights the critical role of soil microbial community composition in altering SOC decomposition response to forest conversion.