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 to 3 years) 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 ⁵NO₃^- 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 ⁵NO₃^-) 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.