Global assessment of the fate of nitrogen deposition in forest ecosystems: Insights from ¹⁵ N tracer studies

Atmospheric nitrogen (N) deposition is recognized as a pivotal nutrient input in forest ecosystems. However, significant gaps persist in our comprehension of the global-scale fate of N in forest ecosystems. In a pioneering effort, this study analyzed the fluxes and determinants of deposited N by 234 observations from 52 published articles. Our findings indicated that plant uptake, soil retention, and N losses, respectively, accounted for 27.4, 57.9, and 14.6% of the total deposited N. The fate of deposited N was significantly influenced by a suite of factors, including forest type, climatic parameters such as mean annual temperature (MAT) and precipitation (MAP), edaphic characteristics such as soil pH and the carbon to nitrogen ratio (C/N), and experimental factors like nitrogen addition rate (NR), nitrogen forms (NF), plot size (PS) for ¹⁵N studies, and the duration of study. For the uptake of deposited N, MAP emerged as the predominant positive factors, whereas NR was the dominant negative factors; for deposited N soil retention, NR was the key positive factors, while MAT was the key negative factors; for N losses, MAP was the predominant positive factors, with the C/N ratio serving as a significant negative factor. Thus, for a given forest ecosystem with relatively stable climate and soil conditions, NR, NF, and the soil C/N were the main controlling factors regulating the fate of deposited N. These insights significantly advance our grasp of the N cycle in forest ecosystems. Consecutive monitoring of the impact of deposited N on soil N transformations and carbon sequestration is needed in future studies.