Molecular Soil Biology 2025, Vol.16, No.4, 188-198 http://bioscipublisher.com/index.php/msb 1 88 Research Report Open Access Engineering Rhizobium Strains for Enhanced Nitrogen Fixation in Soybean Weiliang Shen, Yuping Huang, Hangming Lin Tropical Legume Research Center, Hainan Institute of Tropical Agricultural Resources, Sanya, 572025, Hainan, China Corresponding email: hangming.lin@hitar.org Molecular Soil Biology, 2025, Vol.16, No.4 doi: 10.5376/msb.2024.15.0018 Received: 06 Jun., 2025 Accepted: 10 Jul., 2025 Published: 27 Jul., 2025 Copyright © 2025 Shen et al., This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Shen W.L., Huang Y.P., and Lin H.M., 2025, Engineering rhizobium strains for enhanced nitrogen fixation in soybean, Molecular Soil Biology, 16(4): 188-198 (doi: 10.5376/msb.2024.15.0018) Abstract Soybean (Glycine max) is an important food and oil crop with a high demand for nitrogen. Long-term reliance on chemical nitrogen fertilizers not only raises production costs but also causes environmental pollution. To address this problem, several engineered rhizobium strains with strong nitrogen-fixing capacity, good stress tolerance, and plant growth-promoting ability were obtained through genetic modification and selection. Field trials were conducted in temperate, subtropical, and semi-arid climate zones, and in various soil types including acidic soil, gray terrace soil, loam, and sandy loam. The results showed that these strains could stably attach to soybean roots and form many effective nodules, maintaining high nitrogen fixation even under adverse conditions such as high temperature, drought, and low pH. Data from the trials indicated that inoculated soybeans yielded 15%~40% more than controls, and even with a 50% reduction in nitrogen fertilizer, high yield and good quality were maintained; seed protein and oil content also increased. In some trials, co-inoculation with phosphate-solubilizing bacteria further reduced nitrogen and phosphorus fertilizer use. Farmers involved in the trials generally found the technology easy to apply and economically beneficial. The study suggests that promoting these rhizobium inoculants can help reduce fertilizer use, lower environmental pressure, and improve the efficiency and sustainability of soybean production. Keywords Soybean (Glycine max); Engineered rhizobium; Nitrogen fixation; Sustainable agriculture; Biofertilizer 1 Introduction Biological nitrogen fixation (BNF) is the conversion of atmospheric N2 to plant-available ammonia by specific microorganisms. BNF is a core step in the nitrogen cycle and supplies crops with nitrogen while lowering the use of synthetic fertilizers (Shome et al., 2022; Abd-Alla et al., 2023; Qin et al., 2023). In legumes, rhizobia form a symbiosis with the host to carry out this process. With this partnership, legumes grow in nitrogen-poor soils, improve soil fertility, and reduce energy use and greenhouse gas emissions. Soybean (Glycine max) is one of the most important legume crops in the world. It can be used as food, feed and industrial raw materials. It needs a lot of nitrogen, which directly determines the yield, protein content and quality. In the past, farmers used to apply large amounts of chemical nitrogen fertilizer in order to ensure the harvest, but this not only costs high, but also may lead to water pollution and soil degradation. With the help of biological nitrogen fixation, soybean can "produce" most of the required nitrogen by itself, thereby reducing the amount of external fertilization (Douka et al., 1986; Danso et al., 1987; Alam et al., 2015; Shome et al., 2022; Hu et al., 2023). During symbiosis, rhizobia will enter the root of soybean and form nodules. Nitrogen fixation is carried out inside the nodule to convert nitrogen in the air into ammonia for soybean absorption. After inoculation with high-efficiency strains, the number of nodules and nitrogenase activity will be significantly improved, and the nitrogen accumulated by plants will also be more (Thuita et al., 2011; Alam et al., 2015; damanhuri et al., 2020; Shome et al., 2022). This can not only reduce the amount of nitrogen fertilizer, but also protect the soil and promote the sustainable development of agriculture (Damanhuri et al., 2020; Shome et al., 2022; Abd-Alla et al., 2023). Under different soil environment and management methods, the coordination degree of rhizobia and soybean will be different, so it is very important to select the suitable strains (Douka et al., 1986; Thuita et al., 2011; Alam et al., 2015).
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