Shifts in genes encoding enzymes that degrade plant- and microbial-derived carbon affect soil organic carbon pools in different subtropical forests of China

Abstract. Microbial transformation of plant- and microbial-derived carbon plays a central role in soil organic carbon (SOC) formation and stabilization, yet how microbial carbon-degrading potential links forest type to SOC accumulation and persistence remains poorly understood. Here, we investigated three representative subtropical forest types—broadleaf, coniferous, and bamboo forests—and integrated litter quality, soil physicochemical properties, SOC fractions, and metagenomic data. Using the CAZyme database, we quantified genes encoding enzymes involved in the degradation of plant- and microbial-derived carbon and evaluated their relationships with environmental factors and SOC fractions. Forest type significantly altered both the abundance and diversity of genes encoding carbon-degrading enzymes, with a clear decoupling between these two metrics. Litter quality and soil environmental conditions jointly regulated these functional genes and directly influenced SOC, mineral-associated organic carbon (MAOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC), but not particulate organic carbon (POC). Instead, POC was significantly associated with the abundance of genes encoding enzymes involved in the degradation of plant- and microbial-derived carbon. SOC and MAOC were more strongly linked to genes encoding enzymes that degrade fungal-derived carbon, whereas MBC was more closely associated with genes encoding enzymes involved in bacterial peptidoglycan degradation. Broadleaf forests showed significantly higher SOC and MAOC contents and greater carbon stability than coniferous and bamboo forests. These findings indicate that microbial carbon-degrading potential represents an important functional pathway linking forest type to SOC accumulation and stabilization, highlighting the role of microbial functional traits in regulating soil carbon persistence in subtropical forest ecosystems.