Canopy processing of N deposition increases short‐term leaf N uptake and photosynthesis, but not long‐term N retention for aspen seedlings
Xin Wang, Bin Wang, Chengzhang Wang, Zhenhua Wang, Jing Li, Zhou Jia, Sen Yang, Ping Li, Yuntao Wu, Shengnan Pan, Lingli Liu
Abstract
Summary Forest canopies can retain nitrogen (N) from atmospheric deposition. However, most empirical and modeling studies do not consider the processing of the N deposited in the canopy. To assess whether N deposition through canopy will alter the plant’s N uptake and retention, we conducted a 3‐yr mesocosm experiment by applying ( 15 NH 4 ) 2 SO 4 solution to aspen sapling canopies or directly to the soil. We found that 15 N‐NH 4 + applied to the canopy was directly taken up by leaves. Compared with the soil N application, the canopy N application resulted in higher photosynthesis but lower N retention of the plant–soil system in the first growing season. Plant biomass, N concentration, and leaf N resorption were not significantly different between the canopy and soil N applications. The partitioning of retained 15 N among plant components and soil layers was similar between the two treatments 3 yr after the N application. Our findings indicated that the canopy N processing could alter leaf N supply and photosynthesis in the short term but not N retention in the long term. Under natural conditions, the chronic N deposition could continuously refill the canopy N pool, causing a sustained increase in canopy carbon uptake. Canopy N processing needs to be considered for accurately predicting the impact of N deposition.