Converting Chinese fir plantations into mixed stands: Effects of density on soil organic carbon mineralization and its temperature sensitivity

• Low-density mixed stands increased C fractions and SOC mineralization in topsoil. • The Q10 was higher in monoculture and mixed stands at low and high temperature, respectively. • Available phosphorus becomes increasingly important for SOC mineralization with temperature rises. • The labile C and available nitrogen were the primary factors regulated Q10 in topsoil and subsoil, respectively. • Plant density through substrate quality and microbial-enzyme combined regulated SOC mineralization. Converting monoculture plantations into mixed stands is a key strategy for enhancing soil carbon sequestration. The mechanisms behind soil organic carbon (SOC) turnover and its temperature response at different mixed densities are unclear. This limits the design of targeted carbon management strategies for plantations. In the study we used incubation experiments across temperatures (15, 25, and 35 °C), investigated different mixed density affect SOC fractions, enzyme activities, and mineralization. Results showed that low-density mixed stands significantly increased microbial biomass carbon (MBC) and β-1,4-glucosidase (BG) activity and showed the highest mineralization rate and cumulative release at 25 -35 °C in surface soil (0–20 cm). Potential mineralization of SOC (C 0 ) decreased with depth but increased with temperature, and Chinese fir plantations exhibited the highest C 0 at 25 -35 °C. Moreover, temperature sensitivity (Q 10 ) was higher in Chinese fir plantations at 15–25 °C but greater in mixed stands–especially under high density–at 25–35 °C. Regression analysis indicated Q 10 (15–25°C) and Q 10 (25–35°C) were significantly positively correlated with readily oxidizable organic carbon (ROC) and phenol oxidase (PPO) activity in topsoil (0-20cm). The variance partitioning analysis (VPA) showed that the factors of SOC mineralization shifted from ROC and dissolved organic carbon (DOC) (explaining 81.7% of variance) at lower temperatures to ROC and combined available phosphorus (aP) (explaining 80.8% of variance) under high temperature. Moreover, the labile C (ROC or DOC, explaining 74.2%∼89% of variance) and AN (explaining 47.1%∼87.9% of variance) were the primary factors regulated Q 10 in topsoil and subsoil, respectively. These results provide data on how converting Chinese fir plantations to mixed stands affects soil mineralization when temperature rises, and suggest that low-density mixed stands should be considered for management to enhance soil carbon sequestration under warming.