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 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.