Assessment of nitrogen (N) saturation of forests is critical to the evaluation of the manner in which 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 (1) a hypothetical relative rate of ammonification, nitrification, and denitrification, (2) concentrations of ammonium and nitrate in the soil, and (3) ¹⁵N enrichment pattern of bulk soil N, ammonium, and nitrate in the soil profile. We tested the hypotheses using data from forests located at five sites across eastern Asia. The fraction of nitrate in total inorganic N indicated that the sites represent an N saturation gradient with one boreal forest at stage 1 (least saturated), three temperate forests at stage 2, and one tropical forest at stage 3 (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 stage 1, nitrate was more ¹⁵N-depleted than ammonium only in the organic soil horizon, indicating limited nitrification, while the ¹⁵N depletion of nitrate to ammonium was observed in the deeper mineral soil at stages 2 and 3. Furthermore, ammonium was more ¹⁵N-depleted than bulk soil N at stages 1 and 2 but more ¹⁵N-enriched than bulk soil N at stage 3. 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 and thus can be an additional measure of N saturation of forest ecosystems across broad environmental gradients.

中文翻译：

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土壤剖面中铵和硝酸盐的天然 15N 丰度：对森林生态系统氮饱和度的新见解

评估森林的氮 (N) 饱和度对于评估生态系统对当前和未来全球变化（例如 N 沉降）的反应方式至关重要。然而，量化 N 饱和度仍然是一个挑战。我们基于 (1) 氨化、硝化和反硝化的假设相对速率，(2) 土壤中铵和硝酸盐的浓度，以及 (3) ^{15 ，开发了森林生态系统中 N 饱和阶段的概念模型。}土壤剖面中散装土壤 N、铵和硝酸盐的 N 富集模式。我们使用来自东亚五个地点的森林数据测试了这些假设。总无机氮中硝酸盐的比例表明，这些地点代表了一个 N 饱和梯度，在第 1 阶段有一个北方森林（最不饱和），在第 2 阶段有三个温带森林，在第 3 阶段有一个热带森林（最饱和）。沿 N 沉降梯度，在 N 有限的地点，与富含 N 的地点相比，大块土壤 N的 δ ^{15 N 从表土到底土的增加更剧烈。}我们还在研究地点的土壤剖面中发现了不同的¹⁵ N 富集模式，即散装土壤 N、铵和硝酸盐。在第 1 阶段，硝酸盐高于¹⁵仅在有机土壤层中 N 比铵贫化，表明硝化作用有限，而在第 2 和第 3 阶段在较深的矿质土壤中观察到¹⁵ N 的硝酸盐消耗为铵。此外，铵比大块土壤更¹⁵ N 贫N 在第 1 阶段和第 2 阶段，但比在第 3 阶段的大块土壤 N 富集更多^{15 N。我们的研究表明，大块土壤 N、铵和硝酸盐的 δ 15} N 土壤剖面模式提供了有关矿化、硝化的相对速率的信息和反硝化作用，因此可以作为在广泛环境梯度下森林生态系统氮饱和度的额外衡量标准。