13-year nitrogen addition increases nonstructural carbon pools in subtropical forest trees in southern China

Nonstructural carbohydrates (NSC) are important for plant growth and defense, and represent an important carbon pool that contributes to carbon (C) cycling and balance, but we know little about the variation of NSC in trees under global change scenarios, particularly nitrogen (N) deposition. Here, we investigated the responses of NSC (sugar and starch) pools of two dominant tree species (Castanopsis chinensis (broadleaf) and Pinus massoniana (coniferous)) to 13-year N treatments in different seasons in southern Chinese forests. Nitrogen addition increased NSC pools of the two tree species at the whole-tree and organ levels. We observed the larger starch pool (only in the wet season) and the highest increased NSC in the branches (3.8-fold) of C. chinensis following N addition. We also found the larger sugar pools (both in the dry and wet season) and the highest increased NSC in the roots (2.2-fold) of P. massoniana after N addition. Despite the increases in NSC of trees, we found no changes in chloroplast and photosynthetic capacity in response to N addition; however, the increased NSC may support prime defense of trees, as evidenced by significant correlations between NSC pools with volatile organic compounds (VOCs) emission (P < 0.05, R2 in the range of 0.5–0.8). Our findings indicate that long-term N deposition induced carbon distribution and adaptation strategies of different subtropical trees, with increased NSC storage at the whole-tree and organ levels and more NSC being allocated to the branches of broadleaf tree species and to the roots of coniferous tree species, respectively. This study provides a new insight into understanding the mechanisms of subtropical forest trees adapting and surviving under long-term N deposition, and is also important for exploring the ecosystem C-N coupling relationships under global environment changes.