Ten-year ¹⁵ N labeling reveals the nitrogen retention threshold for maximizing carbon sequestration in a temperate forest

Anthropogenic nitrogen (N) deposition comprising reduced (NH_x) and oxidized (NO_y) forms, differentially influences ecosystem processes. However, their long-term fates and collective impacts on ecosystem nitrogen use efficiency (NUE) and carbon (C) sequestration remain unresolved. Here, using a decade-long paired labeling (¹⁵NH₄⁺ and ¹⁵NO₃^-) experiment in a temperate forest, we demonstrate that initially distinct fates of different N forms converged within 2 y due to low initial loss rates prior to organic incorporation. After 10 y, overall ecosystem retention was remarkably similar (55.3 to 65.6%) for both N forms. A process -based N-cycling model was built and parameterized by our ¹⁵N recovery data and the model predicted declining ecosystem NUE with increasing N deposition level. We further identified a deposition-induced C sequestration potential of 41.41 kg C kg^-1 N under an incremental deposition of 10 kg N ha^-1 y^-1, exceeding prior estimates at comparable loads. This study provides mechanistic, high-resolution temporal data critical for refining N-cycle models and reveals an optimal N deposition range (20 to 30 kg N ha^-1 y^-1) that maximizes C sequestration while minimizing environmental risks (e.g., nutrient imbalance, eutrophication) in temperate forests.