Simulated nitrogen deposition promotes the carbon assimilation of shrubs rather than tree species in an evergreen broad-leaved forest.

Although understory addition of nitrogen (UAN) is commonly used to simulate nitrogen deposition in field studies in forest ecosystems, it ignores the effects of atmospheric nitrogen deposition on the canopy. We studied the effects of nitrogen deposition simulated by UAN and by canopy addition of nitrogen (CAN) on leaf structure, chemical properties, Calvin cycle, and photosynthate distribution strategy of representative woody plant species in a subtropical evergreen broadleaved forest in South China. The results showed that maximum photosynthetic rate (A) of shrub species Blastus cochinchinensis and Ardisia quinquegona under CAN treatments was significantly higher than that of UAN treatments at the same N addition concentration. The concentrations of intermediates (PGK, DPGA and G3P) in Calvin cycle of B. cochinchinensis and A. quinquegona, and Castanea henryi were significantly increased with CAN treatments, but the opposite was true with UAN treatments. CAN25 significantly increased starch concentrations of shrub species Lasianthus chinensis and B. cochinchinensis, and significantly decreased sucrose concentrations of shrub species A. quinquegona and tree species C. henryi. Correlation analyses showed that nitrogen application amount under different modes helped explain the changes in A and Calvin cycle intermediates. In summary, nitrogen deposition may promote the A and Calvin cycle of shrub species, and the adaptability of shrub species to nitrogen deposition is higher than that of tree species, which may help to explain the degradation of subtropical evergreen broad-leaved forest.