基于15N示踪法的双根大豆系统氮素吸收和分配特性研究

【目的】施氮可以促进大豆生长并提高产量，同时会抑制根瘤生长和固氮。因此研究大豆对不同形态氮素的吸收、分配及再分配特点，可以为解析大豆氮的转运特性及施氮对根瘤的系统性抑制提供参考。【方法】利用嫁接方法，制备具有两个根部和一个地上部的双根大豆植株，在砂培条件下分别以NO₃–和NH₄+为氮源设置两种试验处理。试验Ⅰ，一侧施50 mg/L的15NO₃– 或15NH₄+(A侧)，另一侧不施氮 (B侧)；试验Ⅱ，一侧施50 mg/L的15NO₃– 或15NH₄+(A侧)，另一侧施同形态的50 mg/L的NO₃– 或NH₄+(B侧)。于始花期 (R1) 和始粒期 (R5) 取样两次，将植株分为A根、B根、A侧根瘤、B侧根瘤、茎、叶片、叶柄、荚等部位，用于测定15N丰度、干重和氮含量等指标。【结果】试验Ⅰ和试验Ⅱ结果发现，大豆A和B两侧根瘤的15N丰度均高于自然丰度 (0.365%)，说明根瘤的生长发育过程中，所需要的氮不是全部来自自身固氮，还需要从根中吸取氮。与试验Ⅰ相比，试验Ⅱ的根瘤固氮率明显下降，表明大豆植株优先吸收利用肥料氮。NO₃–与NH₄+处理相比，各器官15N丰度均没有显著性差异，说明在50 mg/L的氮浓度下，NO₃–和NH₄+对大豆的氮营养没有显著差异。试验Ⅰ和试验Ⅱ均发现大豆B侧根及根瘤的15N丰度高于自然丰度 (0.365%)，且小于施加的肥料氮的15N丰度 (3.63%)，表明A侧根吸收的氮会经地上部转移到B侧的根及根瘤中，即根吸收的肥料氮会以一定的比例运输到地上部，随后会再次重新分配回根及根瘤中。本试验将双根大豆系统中地上部和B侧根及根瘤看成一个氮转移系统，利用15N丰度的差异，构建了R1～R5期地上部向根及根瘤转移氮量的计算方法。经计算发现，当施氮浓度为50 mg/L时，在始花期至始粒期，根来自地上部转移的氮占根部氮积累量的28.4%～40.8%，根瘤来自地上部转移的氮占其氮积累量的14.4%～17.2%。【结论】根瘤生长所需要的氮不是全部来源于自身固氮，有一部分来源于根系吸收的氮。在有肥料氮存在时，大豆植株优先吸收肥料氮。根系吸收的肥料氮以及根瘤固氮被运输到地上部后，会再次重新分配回根及根瘤中。在50 mg/L的氮浓度下，氮素形态 (NO₃–和NH₄+) 不会影响大豆植株对氮的吸收及分配。

Study on absorption and distribution characteristics of nitrogen in soybeans with dual root systems based on 15N tracing technique

【Objectives】Nitrogen application promote soybean growth and increase yields, but inhibit nodule formation and nitrogen fixation. The resource of nitrogen in root and nodules were studied, and the effect of supplying NO₃– and NH₄+ on the nitrogen absorption and distribution of soybeans was systematically investigated.【Methods】The grafting method was used to generate soybean plants with dual root systems, in which two modulated roots shared one symbiotic shoot. Two experimental treatments were conducted with NO₃– and NH₄+ as nitrogen sources under sand culture conditions. Experiment I, supplying one side of root with 50 mg/L of 15NO₃– or 15NH₄+ (side A), and no nitrogen on the other side (side B). Experiment II, supplying one side of root with 50 mg/L 15NO₃– or 15NH₄+ (side A), and supplying the other side with 50 mg/L NO₃– or NH₄+. At R1 (initial flowering) and R5 (initial seeding) stages of soybean, the plant samples were collected and divided into separate parts for the analysis of N contents.【Results】The 15N abundance in the nodules on both the side A and side B were higher than natural (0.365%), which indicated that the nitrogen in the nodules was derived from both self-nitrogen fixation and root absorption. The rates of nodule-fixed N in both side A and B of experiment Ⅱ were significantly lower than those in experiment Ⅰ, indicating that the fertilizer N was preferentially absorbed and used by soybeans. No significant differences were observed in both the 15N abundance and N accumulation in all parts of soybean when supplied with NO₃– or NH₄+, which indicated that soybean was not sensitive to N forms under the experimental N level of 50 mg/L. In experiment Ⅰ, the 15N abundance of root and nodules on the side B was higher than the natural but lower than that in the tested fertilizers (3.63%), suggesting that the N absorbed from root of side A was transferred to the root and nodules in side B via the shoot. Considering the shoots, root and nodule in the dual root systems as a system, we proposed a method for calculating the amount of N translocation from shoots to roots and nodules during the R1–R5 stages based on the difference in the 15N abundance. When adding 50 mg/L of N, the translocated N from the shoots accounted for 28.4%–40.8% of the N accumulation in roots and 14.4%–17.2% of that in nodules of soybeans.【Conclusions】The N required for nodule growth and development is derived from both self-nitrogen fixation and root absorption. The fertilizer N will be preferentially absorbed and used by soybeans in the presence of fertilizer N. N forms, namely NO₃– and NH₄+, will not affect the N absorption and distribution of soybean plants under the tested N supply concentration (50 mg/L). All the N acquired by the roots and nodules will be transported to the shoots, and a portion of them is then redistributed to the roots and nodules.