Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 118 5.2 Restoration of degraded soil functions Soil degradation (such as organic matter depletion, acidification, salinization, etc.) is an important issue threatening agricultural sustainability. The use of legume-microorganism symbiosis can restore the ecological function of degraded soil to a certain extent. First of all, for nutrient-deficient degraded land, planting leguminous plants and accumulating soil nitrogen through symbiotic nitrogen fixation is one of the effective means to restore soil fertility. In ecological restoration projects, leguminous shrubs or forages with strong nitrogen fixation ability are often planted first to improve barren soil. For example, planting alfalfa, clover, etc. in abandoned mining areas and desert margins will significantly increase the total nitrogen and organic matter content of the soil after a few years, and the productivity and stability of the plant community will also increase. Bakhoum et al. (2018) pointed out that legumes can even survive in poor sandy or nutrient-deficient environments, thanks to the nutrient support provided by their rhizobia and mycorrhizal fungi symbionts, which makes legumes a key species in the ecosystem and can be used for vegetation restoration in fragile habitats. Secondly, legumes and their symbiotic microorganisms can improve the biodiversity and food web structure of degraded soils. Degraded soils often have impaired microbial community functions, and the roots of legumes continuously transport organic carbon and nitrogen, which is conducive to the reconstruction of soil microbial flora. In particular, after rhizobia and mycorrhizal fungi colonize in degraded soils, they not only benefit from legumes, but also promote the reproduction of other beneficial microorganisms through joint action, gradually restoring the balance of soil microecology. Chen et al. (2020) believe that the reconstruction of soil microbial communities is the key to ecological restoration of degraded land, and planting symbiotic nitrogen-fixing plants can help to rebuild this microbial community as soon as possible. Leguminosae are important hosts for many indigenous mycorrhizal fungi. Their introduction can restore the soil mycorrhizal network, thereby promoting the settlement of other plants and improving vegetation diversity. Therefore, in the management of degraded grasslands, sandy lands and saline-alkali lands, leguminous plants are often mixed with other plants to play the role of "soil improvement" and "microbial restoration". Experiments have compared the differences between annual and perennial leguminous forage in terms of overproduction and diversity effects on degraded grasslands, and found that the introduction of leguminous plants can significantly improve grassland productivity and biodiversity, and enable degraded grassland vegetation to recover faster. Thirdly, for specific degraded types of soils such as salinization and acidification, the leguminous plant symbiotic microbial system also has a certain corrective effect. As mentioned above, rhizobium symbiosis can partially neutralize soil acid and increase pH; the combination of salt-tolerant growth-promoting bacteria and leguminous plants can reduce the effective concentration of soil salt (Wu and Yan, 2024). 5.3 Enhancing agroecosystem stability A stable and efficient agricultural ecosystem requires diverse nutrient cycle pathways and robust biological networks. The introduction of legume crop-microorganism symbiosis increases the biological pathways and food web complexity of farmland nutrient cycles, thereby helping to improve the stability and resilience of the entire agricultural ecosystem. First, as a part of the nitrogen cycle, legume symbiotic nitrogen fixation complements fertilizer nitrogen, making farmland nitrogen supply more continuous and diversified. When fertilizer supply fluctuates or decreases, symbiotic nitrogen fixation can partially fill the gap and ensure crop growth. This "dual-track" nitrogen supply mechanism improves the buffering capacity of the production system against external dependence. Studies have shown that in years with low nitrogen fertilizer supply, the crop yield fluctuations of plots with legumes in rotation are small, while the yield of plots with pure nitrogen-consuming crops in rotation declines significantly. This shows that legume symbiotic nitrogen fixation provides a stabilizer for the system. In addition, the nitrogen fixed by the symbiotic system is gradually released in an organic form, reducing the accumulation of inorganic nitrogen and the risk of sudden nitrogen loss caused by heavy rain erosion, which is also conducive to the stability of nutrient supply.
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