Nitrogen nutrition addition mitigated drought stress by improving carbon exchange and reserves among two temperate trees

Climate change-driven increases in drought and atmospheric nitrogen (N) deposition frequency and severity across the world, and these changes have profound impacts on forest dynamics by affecting tree carbon balance. However, important knowledge gaps persist concerning the interactions between drought and N enrichment on carbon supply and reserve dynamics. We investigated gas exchange and carbohydrate reserve shifts and associated with leaf chemical composition across two temperate tree saplings coping with different levels of drought stress, N nutrition addition and their interactions. Our results showed that drought stress decreased net photosynthetic rate (A) and stomatal conductance (gs), while A and gs were increased by N addition, combined with increased gs under the interactive effects of drought and N addition, indicating that N nutrient availability had suppressing effect on drought stress and improved the drought-induced negative conditions by altering carbon exchange traits. In addition, we found that N addition reduced the concentrations of nonstructural carbohydrate (NSC) and its components in relation to foliar carbon and nitrogen changes and the fast growth of saplings. Under the interaction of drought and N nutrition addition, no variation in carbohydrate concentrations demonstrate that carbon reserves play a critical role in regulating carbon exchange and growth. Our observations provide evidence of N nutrient availability mitigated drought stress by improving carbon exchange and reserves, which is expected to contribute to the predictions of future vegetation dynamics.