Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 114 (such as alfalfa) have the effect of improving acidic soil. Their deep roots can bring alkaline nutrients from the deep layer to the surface, and secrete organic anions to chelate with aluminum ions, reducing aluminum toxicity, thereby improving the suitability of acidic soil. Secondly, leguminous plants also significantly improve the physical and chemical properties of soil such as aeration and water permeability by improving soil structure and increasing organic matter. The pore network formed by the deep roots of leguminous plants and mycorrhizal hyphae in the soil is conducive to soil aeration and water infiltration. This is particularly important for heavy or compacted soils. Studies have shown that in orchard soils with long-term monoculture and heavy fertilization, the introduction of leguminous green manure intercropping can reduce soil bulk density, increase porosity, and significantly improve ventilation conditions. The experiment of Zhang et al. (2021) also showed that intercropping leguminous green manure in kiwifruit orchards can enhance soil permeability and water holding capacity, improve soil aggregate structure and water content, and thus create a good growth environment for fruit tree roots. At the same time, the microclimate of soil temperature and humidity is also regulated by leguminous mulching: green manure mulching can reduce the daily difference in surface temperature, reduce water evaporation, and achieve the effect of "cooling and conserving moisture in summer, and keeping warm and preventing freezing in winter". These changes also have a positive impact on soil microbial flora. Microbial diversity and activity will increase under more stable and suitable physical and chemical conditions, thus forming a virtuous circle. Leguminous crops can also reduce the risk of soil salinization. Introducing leguminous forage (such as alfalfa) in some saline-alkali lands can adapt to and improve saline-alkali soils through their deep root system and apoplast barrier mechanism. Leguminosae absorb a large amount of cationic nitrogen (ammonium) and calcium and magnesium, which helps to reduce the proportion of sodium ions in the soil and regulates the balance of soil salt ions. Although leguminous plants themselves are not tolerant to extreme salinity, the combined use of salt-tolerant growth-promoting bacteria (such as inoculation of salt-tolerant rhizobia or salt-tolerant PGPR) can improve the saline soil environment and increase plant survival rate to a certain extent. 4 Microbial Support for Nutrient Uptake and Stress Tolerance 4.1 Nitrogen fixation and nitrogen use efficiency The most significant effect of the symbiosis between soil microorganisms and legumes is to provide nitrogen nutrients to host plants, significantly improving the nitrogen utilization efficiency of legumes. Rhizobia reduce the inert N2 in the atmosphere to ammonium ions in the symbiotic nodules, providing tens to hundreds of kilograms of nitrogen per hectare per year for legumes, which is equivalent to a certain amount of nitrogen fertilizer input. For major legume crops such as soybeans, the nitrogen provided by biological nitrogen fixation during the growth period can account for more than 50% of their total nitrogen demand. Some studies have estimated that this proportion can reach 60%~70% for soybeans. This means that through the rhizobium symbiotic system, legumes have greatly improved their self-sufficiency in nitrogen sources, thereby reducing their dependence on exogenous nitrogen fertilizers. Under the action of symbiotic nitrogen fixation, legumes often show higher nitrogen utilization efficiency (NUE). Compared with the application of chemical fertilizers, legumes have less loss and more complete conversion of symbiotic fixed nitrogen. Studies have shown that when leguminous crops are grown without nitrogen fertilizer, their N2O greenhouse gas emissions are significantly lower than those of non-leguminous crops that rely on nitrogen fertilizer. Model simulations also show that replacing chemical fertilizers with leguminous crops will not reduce yields, but can effectively reduce field N2O emissions. Therefore, from an agronomic and environmental perspective, the nitrogen provided by rhizobium symbiotic nitrogen fixation is more "efficient" (Savvas et al., 2017; Goyal et al., 2021). On the one hand, it supplies the needs of plants in real time, avoiding the waste of excessive fertilizer application and leaching; on the other hand, the nitrogen fixation process is accompanied by carbohydrate consumption, which helps more carbon to be fixed in the soil to form humus and improve soil fertility. Long-term positioning test data support the conclusion that symbiotic nitrogen fixation improves NUE: in soybean fields without nitrogen fertilizer but inoculated with excellent rhizobia, soybean yield and nitrogen
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