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.