Combined Application of Bamboo Biochar and Lime Alleviates Soil Acidification and Enhances Organic Carbon Stability in an Intensively Managed Acid Bamboo Plantation

Liming is widely used to alleviate soil acidification, but its potential to stimulate soil organic carbon (SOC) decomposition may compromise SOC stability. Whether lime–biochar co-application can mitigate the associated increase in soil CO2 release remains unclear. A one-year field experiment in an intensively managed acid bamboo plantation compared an unamended control (CK), lime alone (L), bamboo biochar alone (B), and lime–biochar co-application (LB). Soils collected at 3, 6, and 12 months after application were analyzed for SOC fractions, extracellular enzyme activities, and microbial gene abundance and incubated for 90 days to measure CO2 release. At 3 and 6 months, L increased cumulative CO2-C release by 45% and 24%, respectively, relative to CK, accompanied by increased labile C and microbial C limitation inferred from enzyme stoichiometry. Relative to L, LB reduced mean CO2-C release rates by 18% and 13%, respectively; this response was associated with higher non-hydrolysable (recalcitrant) C, lower geometric mean enzyme activity, and weaker microbial C limitation. After 12 months, the stimulatory effect of L was no longer evident, whereas B and LB increased cumulative CO2-C release by 31% and 29%, respectively, relative to CK. Structural equation modeling linked the lime predictor to greater microbial abundance and C limitation, and the biochar predictor to higher recalcitrant C concentrations and lower C limitation. Because CO2 sources were not partitioned, these responses cannot be attributed exclusively to native SOC decomposition. Lime–biochar co-application thus moderated the short-term increase in soil CO2 release associated with liming, but this effect weakened over time, highlighting the need for longer-term assessment of SOC stability.