Natural 15N abundance of bulk soil N, ammonium, and nitrate in soil profiles

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 ..., ,