Fate of atmospherically deposited NH4+ and NO3？ in two temperate forests in China: temporal pattern and redistribution

The impacts of anthropogenic nitrogen (N) deposition on forest ecosystems depend in large part on its fate. However, our understanding of the fates of different forms of deposited N as well as the redistribution over time within different ecosystems is limited. In this study, we used the 15N‐tracer method to investigate both the short‐term (1 week to 3 months) and long‐term (1–3 yr) fates of deposited NH4+ or NO3？ by following the recovery of the 15N in different ecosystem compartments in a larch plantation forest and a mixed forest located in northeastern China. The results showed similar total ecosystem retention for deposited NH4+ and NO3？, but their distribution within the ecosystems (plants vs. soil) differed distinctly particularly in the short‐term, with higher 15NO3？ recoveries in plants (while lower recoveries in organic layer) than found for 15NH4+. The different short‐term fate was likely related to the higher mobility of 15NO3？ than 15NH4+ in soils instead of plant uptake preferences for NO3？ over NH4+. In the long‐term, differences between N forms became less prevalent but higher recoveries in trees (particularly in the larch forest) of 15NO3？ than 15NH4+ tracer persisted, suggesting that incoming NO3？ may contribute more to plant biomass increment and forest carbon sequestration than incoming NH4+. Differences between the two forests in recoveries were largely driven by a higher 15N recovery in the organic layer (both N forms) and in trees (for 15NO3？) in the larch forest compared to the mixed forest. This was due to a more abundant organic layer and possibly higher tree N demand in the larch forest than in the mixed forest. Leachate 15N loss was minor (<1% of the added 15N) for both N forms and in both forests. Total 15N recovery averaged 78% in the short‐term and decreased to 55% in the long‐term but with increasing amount of 15N label (re)‐redistributed into slow turn‐over pools (e.g., trees and mineral soil). The different retention dynamics of deposited NH4+ and NO3？ may have implications in environmental policy related to the anthropogenic emissions of the two N forms. Data are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.t3k2t2j Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article