Nitrogen (N) is one of the major nutrients limiting plant growth in terrestrial ecosystems. To avoid plant-microbe competition, previous studies on plant N uptake preference often used hydroponic experiments on fine roots of seedlings and demonstrated ammonium preference for conifer species; however, we lack information about N uptake and translocation in the field. In this paper, we described a method of in situ paired ¹⁵N labeling and reported rates and the time course of N uptake and translocation by mature trees in situ. We added ¹⁵N-enriched ammonium or nitrate, together with the nitrification inhibitor dicyandiamide, to paired Larix kaempferi (larch) trees from 30-, 40-, and 50-year-old plantations. Fine roots, coarse roots, leaves, and small branches were collected 2, 4, 7, 14 and 30 days after labeling. Nitrate uptake and translocation averaged 1.59 ± 0.16 μg ¹⁵N g⁻¹ d⁻¹, slightly higher than ammonium (1.08 ± 0.10 μg ¹⁵N g⁻¹ d⁻¹), in all tree organs. Nitrate contributed 50% to 78% to N uptake and translocation, indicating efficient nitrate use by larch in situ. We observed no age effect. We suggest that sampling leaves after 4 days of ¹⁵N labeling is sufficient to detect mature tree N uptake preference in situ. Whole tree ¹⁵N-ammonium recovery equaled that of ¹⁵N-nitrate 30 days after ¹⁵N addition, implying the importance of both ammonium and nitrate to mature larch N use in the long run. We conclude that our method is promising for studying mature tree N uptake preference in situ and can be applied to other conifer and broadleaf species. We suggest using highly enriched ¹⁵N tracer to overcome soil dilution and a nitrification inhibitor to minimize ammonium transformation to nitrate. Our study revealed mature tree N preference in situ and demonstrated the strong contribution of nitrate towards mature larch growth on soils rich in nitrate.

中文翻译：

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量化成熟树的氮吸收和转运：原位整树配对 15N 标记方法

氮 (N) 是陆地生态系统中限制植物生长的主要营养素之一。为避免植物与微生物之间的竞争，以往关于植物吸氮偏好的研究通常对幼苗的细根进行水培实验，并证明了针叶树种对铵的偏好；然而，我们缺乏关于田间氮吸收和易位的信息。在本文中，我们描述了一种原位配对¹⁵ N 标记的方法，并报告了成熟树木原位 N 吸收和转运的速率和时间过程。我们在配对落叶松中添加了¹⁵种富含 N 的铵或硝酸盐，以及硝化抑制剂双氰胺（落叶松）来自 30、40 和 50 年历史的种植园的树木。标记后2、4、7、14、30天收集细根、粗根、叶子和小枝。在所有树木器官中，硝酸盐吸收和易位平均为 1.59 ± 0.16 μg ¹⁵ N g ^-1 d ^-1，略高于铵（1.08 ± 0.10 μg ¹⁵ N g ^-1 d ^-1）。硝酸盐对 N 的吸收和转运贡献了 50% 到 78%，表明落叶松原位有效地利用了硝酸盐。我们没有观察到年龄效应。^{我们建议在15} N 标记4 天后对叶子进行取样足以检测原位成熟树 N 吸收偏好。全树¹⁵ N-铵回收率等于^添加15 N 后 30 天添加 15 N-硝酸盐^，这表明铵盐和硝酸盐对于长期使用成熟落叶松 N 的重要性。我们得出的结论是，我们的方法有望用于原位研究成熟树木 N 吸收偏好，并可应用于其他针叶树和阔叶树种。我们建议使用高度富集的¹⁵ N 示踪剂来克服土壤稀释和硝化抑制剂，以尽量减少铵向硝酸盐的转化。我们的研究揭示了原位成熟树木对氮的偏好，并证明了硝酸盐对富含硝酸盐的土壤上成熟落叶松生长的强大贡献。