Depth-dependent responses of soil organic carbon fractions to three-year warming in subtropical forests

• Soil organic carbon responses to warming were depth-dependent. • Warming reduced topsoil labile organic carbon (LOC) by 19% over three years. • Increased C-degrading hydrolytic enzymes and Gram + /Gram − ratio linked to topsoil LOC loss. • LOC remained stable in subsoil due to increased fine root biomass input. • Recalcitrant organic carbon was unaffected by short-term warming. Climate warming can significantly alter soil organic carbon (SOC) dynamics, thereby influencing the feedbacks between SOC and climate change. However, how different fractions of SOC respond to warming, how their responses vary across soil depths, and the underlying mechanisms remain poorly understood in subtropical forest ecosystems. We conducted a three-year in situ soil warming experiment (+4 ℃) in a Cunninghamia lanceolata plantation using buried resistance cables to investigate how labile and recalcitrant organic C (LOC and ROC, respectively) of topsoil (0–20 cm) and subsoil (20–60 cm) respond to warming. We found that warming significantly reduced LOC content by 19 % in the topsoil, accompanied by enhanced activities of C-degrading hydrolytic enzymes (mainly due to the increase in β-glucosidase activity) and a shift in microbial community structure toward more oligotrophic groups (i.e., higher Gram-positive to Gram-negative bacterial biomass ratio). In contrast, LOC in the subsoil remained unchanged, likely due to increased LOC input from fine root biomass, compensating for decomposition losses. Unlike LOC, ROC content was not affected by warming, indicating its stability under short-term warming. These findings highlight the depth-dependent sensitivity of different SOC fractions to warming, shaped by C-cycling enzyme activities, microbial community structure, and plant fine root biomass, and thereby help address the knowledge gap on the responses and mechanisms of warming on SOC from an important but data-poor region.