Direct belowground nitrogen (N) transfer has often been reported where plants with contrasting nutrients acquisition strategies (N2-fixing and non-fixing) co-occur, and there is still a gap in the knowledge of the extent of this transfer in the top soil under the field conditions. However, assessment under field conditions is challenging. We hypothesized a practical application of the analysis of natural abundance of δ15N supplemented with an isotopic mixing model ‘IsoSource’ to understand the relative direct contribution of N2-fixing Alnus acuminata to wheat intercrop (non-fixing) N isotopic signatures. A field experiment was conducted in an andic soil of high lands in northern Rwanda to quantitatively determine the proportional contribution of nitrogen by Alnus acuminata to wheat vegetative tissue isotope signatures at different distances from the trees (1 m, 3 m, 5 m, and 7 m). The study involved the measurements and analyses of natural abundance of stable isotopes δ15N and isotopic mixing modeling by IsoSource. Leaf samples were collected from twigs of 10 years old Alnus acuminata grown on the terrace-risers, along with soil samples (0–20 cm) and wheat flag leaf samples across terrace at 1 m, 3 m, 5 m, and 7 m from trees for isotopic measurement. The chlorophyll content index of wheat flag leaf at the four points across terrace was estimated by means of SPAD meter 502. The δ15N proportional contribution by Alnus acuminata to wheat was obtained through IsoSource analysis. We noted a significant (p < 0·01) gradient in depletion of wheat δ15N signatures moving further away from the tree line of Alnus acuminata. The wheat at 1 m from the trees exhibited the δ15N values closer to that of the tree, while at 7 m, the crop δ15N signature was significantly different from that of the tree. An isotopic mixing model ‘IsoSource’ indicated that the tree N may have provided 33·6 ± 4·3 % of the wheat intercrop N at 1 m distance from the trees. Therefore, this study shows that the understanding of field-based crop N and nutrient transfer in agroforestry may be enhanced by analysis of the physiological basis of stable isotopes signatures.

经常报告直接地下氮 (N) 转移，其中植物具有不同的养分获取策略（N 2固定和非固定）同时发生，并且在顶部这种转移程度的知识方面仍然存在差距田间条件下的土壤。然而，现场条件下的评估具有挑战性。我们假设分析 δ 15 N 的自然丰度并辅以同位素混合模型“IsoSource”的实际应用，以了解 N 2固定Alnus acuminata的相对直接贡献小麦间作（非固定）N 同位素特征。在卢旺达北部高地的荒地土壤中进行了一项田间试验，以定量确定Alnus acuminata氮对距树木不同距离（1 m、3 m、5 m 和 7米）。该研究涉及稳定同位素δ 15 N的自然丰度的测量和分析以及 IsoSource 的同位素混合建模。叶子样品是从梯田立管上生长的 10 年生桤木的树枝上采集的，连同s油样 (0–20 cm) 和小麦旗叶样品在距离树木 1 m、3 m、5 m 和 7 m 处的梯田进行同位素测量。小麦旗叶跨阶地4个点的叶绿素含量指数采用SPAD仪502估算。通过IsoSource分析得到赤杨对小麦的δ 15 N比例贡献。我们注意到小麦 δ 15 N 特征的耗竭梯度显着 ( p < 0·01) 逐渐远离Alnus acuminata的树线。距离树木 1 m 处小麦的 δ 15 N 值更接近于树木，而在 7 m 处，作物 δ 15N 签名与树的签名显着不同。同位素混合模型“IsoSource”表明，在距树木 1 m 处，树木 N 可能提供了 33·6 ± 4·3 % 的小麦间作 N。因此，本研究表明，通过分析稳定同位素特征的生理基础，可以增强对农林业中基于田间作物氮和养分转移的理解。