Determining Nitrogen Accumulation and Nitrogen Use Efficiency of Mung Bean with Organic Nitrogen and 15N-labeled Chemical Fertilizer

The study investigated the dynamics of nitrogen accumulation in mung been under different nitrogen levels and different nitrogen forms. A pot experiment was conducted, including six treatments: no fertilizer applied (N0), 233 g 15N.plant −1 (N1), 292 g 15N. plant −1 (N2), 350 g 15N.plant −1 (N3), 146 g 15N+146 g organic nitrogen (N organic), plant −1 (ON1), and 292 N organic. Plant −1 (ON2). The organic nitrogen was provided by the fermented soybean cake. The 15N was provided in chemical nitrogen form. The yield in N2 was the highest among N0-N3. With 15N increasing, Rhizobia fresh weight showed downward trend. Compared with N2, both ON1 and ON2 improved the 1000-grain weight, yield and grain crude protein content. According to 15N-labeled method, mung bean plants got more nitrogen from soil before R1 stage than other growth periods, later the absorbed nitrogen would be dynamically exchanged out of plant through root-Rhizobia interaction, but mung bean again accumulated more nitrogen from soil at R8. N2 had the largest 15N translocation rate from roots to grains in the experiment; The nitrogen use efficiency increased with chemical nitrogen increasing, the value being 14.0% in N1, 22.5% in N2 and 26.1% in N3. The nitrogen use efficiency of ON1 was 23.0%. The repart coefficient of 15N ranged from 50.4–56.7% in grains, 3.0–3.1% in shells, 18.7–27.2% in leaves, 14.3–15.4% in stems and 4.9–8.3% in roots. The nitrogen use efficiency calculated based on nitrogen difference was 6.6–31.3% higher than that based on 15N tracer technique. In sum, appropriate nitrogen fertilizer (N2) is beneficial to the increase of mung bean yield. The application of organic nitrogen fertilizer could alleviate the inhibition effect of nitrogen fertilizer on Rhizobia, reduce the loss of soil nitrogen, and further improve the yield of mung bean.