Abstract: Nitrogen (N) deposition and climate warming threaten plantation soil organic carbon (SOC) stability. Soil respiration (R_s), the primary pathway for SOC decomposition, remains poorly understood in terms of regulatory mechanisms. Biochar may mitigate N deposition impacts. However, the mechanisms by which the interactive effects of N and biochar influence R_s through soil microbial community structure, enzyme activity, and C-N-P cycling processes, as well as the temperature sensitivity (Q₁₀) of Rs under these interactions, remain unclear. This study investigated these issues through a five-year controlled experiment simulating N deposition and biochar addition in a Larix kaempferi plantation, integrating changes in soil C-cycle-related properties and their interactions. The results revealed that low N addition (LN: 50 kg N ha⁻¹ a⁻¹) increased R_s by 7%, while high N addition (HN: 100 kg N ha⁻¹ a⁻¹) reduced it by 32%. Low and high biochar treatments (C₅: 5 t ha⁻¹; C₁₀: 10 t ha⁻¹) increased R_s by 8% and 13%, respectively. N and biochar interactions consistently suppressed R_s, reducing it by 12%−20%. LN, C₅ and C₁₀ enhanced Q_10, whereas HN decreased it. Additionally, N and biochar interactions stabilized Q₁₀. N addition directly or indirectly inhibited microbial biomass and aggregate stability by elevating available phosphorus and NO₃⁻-N content, while biochar’s potential to promote SOC was constrained by its diminishing effects over time. Both factors collectively influenced R_s through a chemical-microbial interaction network. This study elucidates the cascading mechanisms linking soil microbial-physicochemical-R_s under N and biochar additions, providing insights for managing soil C emissions under rising temperatures.

Key words: soil respiration, temperature sensitivity, microbial community, subtropical plantation, biochar amendment