Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 121 The successful case of this experiment shows that microbial agents (such as rhizobia agents) have great potential for field application. By scientifically selecting excellent strains and coordinating with fertilization systems, the symbiotic system of leguminous crops can achieve greater output under low input conditions. This is particularly important for modern agriculture that focuses on input-output ratio and environmental benefits. For example, in some peanut production areas, due to continuous planting, soil nutrients are unbalanced and nutrient utilization rate decreases. After the introduction of rhizobia agents, not only biological nitrogen sources are provided, but also the absorption rate of peanuts to residual nutrients (such as phosphorus) in fertilizers is improved, and the yield increases under the combined effect. The peanut case also reflects a key point: the symbiotic nitrogen fixation system does not operate in isolation, and it has a synergistic effect and critical balance with chemical nutrient input. Moderate nitrogen reduction can encourage plants to rely more on symbiotic nitrogen fixation, thereby giving full play to the role of rhizobia, but excessive nitrogen deficiency signals may inhibit plant growth and are not conducive to the start of symbiosis. In the experiment, the 40% nitrogen reduction treatment obviously found a "sweet spot", which is conducive to symbiotic nitrogen fixation and guarantees the basic nitrogen supply, achieving the complementarity of the two. This suggests that when promoting symbiotic bacteria agents, soil nutrient conditions and reasonable fertilization formulas must also be considered to achieve the best results. 6.3 Soil microbial community restructuring via legume-based rotation The introduction of leguminous crops into the farmland rotation system not only improves the balance of nutrient income and expenditure, but also has a profound impact on the soil microbial community, playing a role in "reconstructing" the soil microecology. A research review by Yu et al. (2021) showed that in long-term monoculture systems, soil microbial diversity tends to decline and functions tend to be single. However, by rotating legumes with other crops, rhizosphere carbon and nitrogen inputs can be increased, microbial food sources can be enriched, and the originally unbalanced soil microbial community can be revitalized. In particular, in legume/grass rotations, the organic nitrogen and organic matter increased by legume nitrogen fixation in one season provide a "nutritional bedding" for the next season of gramineous crops. At the same time, the microbial groups involved in the carbon and nitrogen cycle in the soil have also changed significantly. Studies have found that in legume rotation systems, the richness of functional bacteria such as nitrogen-fixing bacteria, phosphate-solubilizing bacteria, and cellulose-decomposing bacteria in the soil is often higher than that in monoculture systems, while the ratio of some pathogens and antagonistic bacteria is more balanced, which makes the soil food web more complex and stable. For example, an experiment compared the differences in soil microorganisms between corn-soybean rotation and continuous corn cropping. The results showed that beneficial bacteria such as actinomycetes and Trichoderma increased significantly in the rotation soil, the density of pathogenic fungi of corn damping-off decreased, and the diversity of corn rhizosphere microbial communities increased. This shows that legumes play a positive role in regulating soil microbial communities in crop rotation, which is conducive to forming an ecological environment with a relative balance between beneficial bacteria and potentially harmful bacteria, thereby reducing the risk of diseases and improving the production stability of the entire system. Another case comes from the green manure system in southern rice fields. After long-term implementation of rice-legume green manure (such as astragalus) rotation, the community structure of facultative anaerobic nitrogen-fixing bacteria and denitrifying bacteria in the soil has changed, and the potential for N2O production in the soil nitrogen cycle pathway has decreased. This means that legume green manure can also reduce nitrogen loss and greenhouse gas emissions in rice fields through microbial community regulation. Legume residues promote the development of some heterotrophic nitrogen-fixing bacteria, which can continue to fix nitrogen in the flooded and anoxic environment of rice fields, bringing additional value to the rice field ecosystem (Wang et al., 2021). At the same time, legume symbiotic bacteria do not completely disappear after a growth period. Studies have found that some rhizobia can survive in a free state in the soil, waiting to be re-infected when the next legume crop is sown. Therefore, in the legume rotation system, over time, the "seed pool" of symbiotic bacteria in the soil is continuously enriched, and the resources of native rhizobia increase, which also has a cumulative effect on the efficiency of symbiotic nitrogen fixation.
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