Differential responses of fine root biomass to nitrogen deposition mediated by functional diversity

Fine root functional diversity can help explain and predict ecosystem functioning by integrating multiple fine root traits. However, because different functional diversity metrics emphasize distinct aspects of trait variation, their explanatory and predictive power may vary. Under ongoing atmospheric nitrogen (N) deposition, the responses of fine root functional diversity to N inputs and its contribution to community-level fine root biomass remain insufficiently understood.

We conducted coordinated field experiments in warm-temperate and subtropical forests, applying three N addition levels (0, 25, and 50 kg ha^-1 yr^-1) and two N addition ways (canopy addition N, CAN; and understory addition N, UAN). During both the growing and non-growing seasons, we quantified community-level fine root biomass and evaluated the relative contributions of the community-weighted mean (CWM) and functional diversity (FD) of dominant species to biomass variation.

Our results showed that N addition increased community-level fine root biomass in the warm-temperate forest during the growing season, whereas no clear pattern emerged in the non-growing season; by contrast, fine root biomass in the subtropical forest decreased in both seasons. In both forest types, fine root biomass during the growing season was primarily explained by CWM, supporting the mass-ratio hypothesis, whereas biomass patterns in the non-growing season were better explained by FD, consistent with the niche complementarity hypothesis.

These findings highlight the pivotal role of climatic differences in mediating the effects of N addition on fine root biomass and trait expression, providing empirical evidence for understanding the applicability of the mass-ratio and niche complementarity hypotheses to belowground forest responses to N deposition.