Objective Unreasonable fertilization leads to soil degradation, low nutrient utilization efficiency, and reduced crop yield and quality. This study investigated the effects of different application methods of bio-organic fertilizers and micronutrient fertilizers on the nitrogen fixation efficiency of mung bean root nodules, the impacts on mung bean yield and quality, aiming to provide key technical support for improving mung bean production and quality in China and to promote the sustainable development of the mung bean industry.

Methods A two-year field experiment was conducted from 2023 to 2024 at the Maozao experimental base in Huairen County, Shanxi Province. The tested mung bean cultivar was 'Tonglv 6'. The experiment adopted a completely random group design, and set up seven treatments: no n fertilization control (CK), local fertilizer practice (FM), optimized fertilization(OF), antibacterial agent (GPB), biological organic fertilizer (BOF), biological organic fertilizer and boron fertilizer (BOFB), biological organic fertilizer and molybdenum fertilizer (BOFMo). The activity of nitrogen metabolism-related enzymes and the number of nodules were determined at then podding (R3) and expending stage (R6); The yield and quality of mung beans were analyzed at harvest.

Results Compared with OF treatment, BOFB treatment significantly increased the flavonoid content, nitrogen-fixing enzyme activity, nitric acid reductase activity and glutamine synthase activity, with increment in flavonoid content by 89.32% and 64.13% at R3 and R6 stage, in nitrogen-fixing enzyme activity by 61.82% and 69.92%, in nitrate reductase activity by 44.31% and 96.11%, and in glutamine synthase activity by 70.35% and 98.16%, respectively. The nitrogen accumulation, nitrogen utilization rate, nitrogen agronomy efficiency, partial factor productivity of nitrogen fertilizer, and nitrogen fertilizer contribution rate of BOFB treatment were increased by 142.10 kg/hm², 64.00%, 6.05 kg/kg, 16.15 kg/kg, respectively. Compared with OF treatment, BOFMo treatment significantly increased the protein content of mung bean seeds by 9.90%; BOFB treatment increased the protein content of mung bean by 5.33%, the dietary fiber content by 36.81%, and the yield of mung beans by 31.98%. The average two-year net output of BOFB treatment was significantly higher than that of the other treatments, reaching 4449.15 CNY/hm².

Conclusion The combined application of Pseudomonas, bio-organic fertilizer, and boron fertilizer at the base of chemical fertilizer promotes nitrogen fixation of mung bean and the activities of nitrogenase, nitrate reductase, and glutamine synthetase, thus improves nitrogen use efficiency and mung bean yield and quality, demonstrating an efficient nutrient management strategy for mung bean production.