Assessment of nitrogen (N) saturation of forests is critical to evaluate how ecosystems will respond to current and future global changes such as N deposition. However, quantifying N saturation remains a challenge. We developed a conceptual model of N saturation stages in forest ecosystems based on i) a hypothetical relative rate of ammonification, nitrification, and denitrification, ii) concentrations of ammonium and nitrate in the soil, and iii) ¹⁵N enrichment pattern of bulk soil N, ammonium, and nitrate in the soil profile. We tested the hypotheses using the data from the five forests located at five sites across eastern Asia, including one boreal forest an dtwo temperate forests in northeastern China, one temperate forest in Japan, and one subtropical forest in south China. The fraction of nitrate in total inorganic N (TIN) indicated that the sites represent an N saturation gradient with one boreal forest being least saturated, three temperate forests moderately saturated, and the tropical forest most saturated. The δ¹⁵N of bulk soil N increased from topsoil to subsoil more sharply at N-limited sites than at the N-rich sites along the N deposition gradient. We also found distinct ¹⁵N enrichment patterns of bulk soil N, ammonium, and nitrate in the soil profile across the study sites. At the least saturated forest, nitrate was more ¹⁵N-depleted than ammonium only in the organic soil horizon indicating limited nitrification while the ¹⁵N depletion of nitrate than ammonium was observed in the deeper mineral soil in the moderatley and most saturated forests. Furthermore, ammonium was more ¹⁵N-depleted than bulk soil N in the least and moderately saturated forests but more ¹⁵N-enriched than bulk soil N in the most saturated forest. Our study suggests that soil profile patterns of δ¹⁵N of bulk soil N, ammonium, and nitrate provide information about the relative rates of mineralization, nitrification, and denitrification, thus can be an additional measure of N saturation of forest ecosystem across broad environmental gradients.