Biochar effects on soil multifunctionality depend on nitrogen deposition levels in Larix kaempferi plantations

Nitrogen deposition is a key global change driver that can alter soil processes and ecosystem functioning in forest ecosystems. Biochar is widely used as a soil amendment to improve soil properties and ecosystem functioning, yet how biochar regulates soil multifunctionality (SMF) across different nitrogen deposition levels remains poorly understood, particularly in plantation forests. Here, we conducted a field experiment in mature Larix kaempferi plantations to examine how biochar addition influences SMF across simulated nitrogen deposition levels and to explore the microbial mechanisms underlying these responses. The effects of biochar on SMF were strongly dependent on nitrogen addition levels. Biochar increased SMF under both ambient and high nitrogen additions, but reduced SMF under low nitrogen addition when applied at high biochar rates, indicating context-dependent responses of SMF to biochar under varying nitrogen deposition conditions. SMF was more strongly associated with fungal richness and community composition, whereas bacterial community properties and microbial network attributes showed only weak associations with SMF. Structural equation modeling revealed that fungal community composition directly influenced SMF and acted as a major pathway through which biochar application affected SMF. Although biochar significantly altered microbial network structure, these changes were not strongly associated with variation in SMF. Overall, our results indicate that biochar regulates SMF under different nitrogen deposition levels primarily through shifts in fungal community rather than through changes in bacterial community and microbial interaction networks. These findings highlight the importance of accounting for nitrogen deposition context when applying biochar in subtropical plantation forests.