Tree-mycorrhizal types differ in their biomass response to nitrogen addition

Abstract

Increasing nitrogen (N) deposition can stimulate forest productivity and carbon (C) sequestration in woody biomass, but the magnitude and global importance of this effect remain poorly quantified. By synthesizing 123 N addition experiments globally, we show that woody biomass C gain per unit N applied (hereafter “C_perN”) was best explained by tree-mycorrhizal association (i.e., trees associated with arbuscular mycorrhizal [AM] vs. ectomycorrhizal [ECM] fungi) and latitude. Overall, C_perN increased with latitude, and was ∼6-fold greater in AM than ECM trees due to their distinct N-acquisition strategies. Using a global map of tree-mycorrhizal distributions, we estimated that N-induced tree C sequestration was 12% lower globally and 17% lower in temperate forests when accounting for the divergent mycorrhizal-tree C_perN, compared to estimates that ignored these effects. This reduction was largely due to the predominance of ECM trees in many temperate forests. Our results suggest that in areas receiving high N loading, trees with more acquisitive nutrient use strategies (such as AM tree sepcies) may be better positioned to sequester more C than trees with more conservative nutrient use strategies (such as ECM tree species). Therefore, shifts in the relative abundance of AM versus ECM trees could be a critical determinant of the future forest C sink under continued N enrichment.