Responses of plant carbon and nitrogen assimilations to nitrogen addition in a subtropical forest: Canopy addition vs. understory addition.

The global atmospheric nitrogen (N) deposition has intensified in recent years, resulting in a complex impact on forest ecosystems. This study investigated the effects of canopy (CAN) and understory additions of N (UAN) on leaf carbon (C) and N assimilations, as well as growth parameters of representative woody plant species in an evergreen broad-leaved forest, i.e. Castanea henryi, Schefflera heptaphylla, Blastus cochinchinensis, and Lasianthus chinensis. The results showed that leaf N assimilation key enzyme nitrate reductase (NR) activities of B. cochinchinensis and S. heptaphylla were significantly decreased by UAN, and were significantly decreased by CAN for C. henryi. CAN significantly decreased the nitrite reductase activity of C. henryi, while significantly increased that of L. chinensis. However, the Amax values of each woody species were not significantly different among control (CK), CAN, and UAN. Community surveys demonstrated that CAN and UAN inhibited the growth (diameter at breast height, height, or crown width) of the representative large tree, C. henryi, while promoting the growths of understory woody species (B. cochinchinensis and L. chinensis). Overall, N addition was found to change the physiological processes of N and C metabolisms of the dominant woody species in an evergreen broad-leaved forest. The community of subtropical evergreen broad-leaved forests may further decline and its C fixation capacity may be detrimentally changed under N deposition in the future.