Fate of atmospherically deposited NH 4 + and NO 3 − 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 15 N‐tracer method to investigate both the short‐term (1 week to 3 months) and long‐term (1–3 yr) fates of deposited NH 4 + or NO 3 − by following the recovery of the 15 N 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 NH 4 + and NO 3 − , but their distribution within the ecosystems (plants vs. soil) differed distinctly particularly in the short‐term, with higher 15 NO 3 − recoveries in plants (while lower recoveries in organic layer) than found for 15 NH 4 + . The different short‐term fate was likely related to the higher mobility of 15 NO 3 − than 15 NH 4 + in soils instead of plant uptake preferences for NO 3 − over NH 4 + . In the long‐term, differences between N forms became less prevalent but higher recoveries in trees (particularly in the larch forest) of 15 NO 3 − than 15 NH 4 + tracer persisted, suggesting that incoming NO 3 − may contribute more to plant biomass increment and forest carbon sequestration than incoming NH 4 + . Differences between the two forests in recoveries were largely driven by a higher 15 N recovery in the organic layer (both N forms) and in trees (for 15 NO 3 − ) 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 15 N loss was minor (<1% of the added 15 N) for both N forms and in both forests. Total 15 N recovery averaged 78% in the short‐term and decreased to 55% in the long‐term but with increasing amount of 15 N label (re)‐redistributed into slow turn‐over pools (e.g., trees and mineral soil). The different retention dynamics of deposited NH 4 + and NO 3 − may have implications in environmental policy related to the anthropogenic emissions of the two N forms.