Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 113 3.2 Promoting soil enzyme activity and aggregate structure Another prominent role of leguminous plants in promoting soil microecological functions is to enhance soil enzyme activity and improve soil aggregate structure. Leguminous crop root secretions and residues provide rich carbon and nitrogen sources for soil microorganisms, activate soil microbial metabolism, and thus enhance the activity of a variety of soil enzymes. Studies have shown that in the rhizosphere of leguminous plants, the activity of key enzymes such as carbohydrate activating enzymes, ureases, and phosphatases is usually higher than that of non-leguminous plants, which is conducive to the recycling of nutrients. For example, Chang Dana et al. found that compared with the control without green manure, planting astragalus green manure can increase soil dissolved organic carbon by 29%, dissolved organic nitrogen by 257%, and significantly increase soil urease and sucrase activities. In the legume/grass intercropping system, the activities of multiple hydrolases in the rhizosphere of legumes are more uniform and higher than those of gramineous plants. This may be related to the continuous secretion of soluble organic materials and the supply of microbial activity by the legume roots. Yang et al. (2020) also found through high-throughput sequencing and functional prediction that inoculation of rhizobia and AM fungi can increase the abundance of nitrogen-fixing bacteria and phosphate-solubilizing bacteria in the rhizosphere of alfalfa, and the abundance of enzyme genes related to soil nitrogen cycle increases, which is beneficial to plant nitrogen and phosphorus nutrition acquisition. Therefore, the legume crop symbiotic system can stimulate the activity of soil enzymes and promote the mineralization and circulation of organic nutrients by enriching the rhizosphere nutrient supply and microbial community. The improvement of soil structure by legumes is reflected in promoting aggregate formation and improving aggregate stability. The large amount of polysaccharide mucilage and hyphae network produced by leguminous roots and their symbiotic microorganisms are natural "soil cementing agents" that can cement soil particles into larger aggregates. In particular, the "glomerular protein" (i.e., mycorrhizal mycocolloid, GRSP) secreted by arbuscular mycorrhizal fungi is called the "adhesive" of soil structure. It can significantly increase the content of water-stable aggregates and improve the aggregate structure and water holding capacity of the soil through hyphae entanglement and cementation. The orchard experiment of Yan et al. (2024) showed that planting leguminous green manure significantly increased the proportion of large aggregates in citrus orchard soil, improved aggregate stability and organic carbon content. Similarly, the introduction of leguminous crops into farmland soil can also help form a more loose and porous soil structure, improve soil ventilation and water retention. This is not only beneficial to the growth of crop roots, but also provides a more suitable habitat for soil microorganisms. It is worth mentioning that mycorrhizal fungi play a unique role in improving soil structure. Al-Arjani et al. (2020) pointed out that inoculation of AM fungi increased soil water holding capacity and stabilized soil structure by promoting the formation of large aggregates. Therefore, in the legume-microorganism symbiotic system, plants, bacteria and fungi work together to promote the development of soil biological structure: plants provide organic matter, rhizobia contribute polysaccharide mucus, and mycorrhizal fungi provide mycelial networks and adhesive substances. Under the synergy of the three, soil aggregates are generated in large quantities and become more stable. This is of great significance for preventing soil erosion and maintaining soil health. In general, legume crops enhance the ecological function and environmental resistance of soil by increasing soil enzyme activity and improving aggregate structure. 3.3 Improving soil physicochemical properties The planting and management of legume crops can also cause a series of positive changes in soil physical and chemical properties. First, in terms of soil pH, the nitrogen fixation of legumes can partially neutralize soil acidity. Generally, the application of ammonium nitrogen fertilizer will release H⁺ in the nitrification process, leading to soil acidification. The nitrogen provided by symbiotic nitrogen fixation is slowly released in the form of organic nitrogen and ammonium nitrogen, which is relatively mild and not easy to significantly reduce soil pH. Studies have observed that in fields where leguminous green manures are planted for consecutive years, the downward trend of soil pH is buffered, and the degree of acidification is lighter than that of fields where only chemical fertilizers are applied. This may be because leguminous plants absorb and utilize part of the nitrate in the soil, and the nitrogen fixation process consumes H+, which regulates the soil. In addition, some leguminous forage grasses
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