Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 119 Secondly, legumes play a role in enriching biodiversity in crop rotation and intercropping. Yu et al. (2021) concluded that intercropping/rotation between Gramineae and Legumes can reduce competition between crops and inhibit pests and diseases through temporal and spatial niche dislocation and root system interactions, thereby improving the stability of the entire farmland ecosystem. On the one hand, the addition of legumes breaks the cycle host of pests and diseases and reduces the obstacles of continuous cropping; on the other hand, due to its nitrogen fixation and promotion of microorganisms, it improves the growth environment of subsequent crops. This positive feedback improves the self-regulation ability of the system and reduces the interannual and spatial variation of yield. Many long-term experiments have shown that multi-crop rotations containing legumes have more stable yields, slower soil fertility decline, and stronger resistance to extreme climate than single-crop systems. For example, in North American experiments, two-year corn-soybean rotations have lower N2O emissions and lower yield variation coefficients than continuous corn planting systems, and are considered a model that both reduces emissions and stabilizes yields. Symbiotic microorganisms give legumes a certain degree of stress resistance and disease resistance, which will also enhance the overall resilience of agricultural ecosystems. When drought, high temperature or pest and disease epidemics occur, legume crops with microbial symbiosis protection may show stronger adaptability and avoid being wiped out, thus playing a role in stabilizing yields in multi-crop systems. For example, in mixed grasslands, grasslands containing symbiotic nitrogen-fixing legumes have a smaller decline in yield in drought years than pure grass grasslands and recover faster. For example, in the rice-legume green manure rotation, when the supply of chemical fertilizer is insufficient in a certain year, the nitrogen-fixing nutrients provided by the previous crop of legume green manure ensure that the rice yield does not decrease. This reflects the contribution of the symbiotic system to the stability of the system. 6 Case Studies: Field Applications of Symbiotic Relationships 6.1 Long-term trials on soybean-rhizobium systems The symbiotic nitrogen fixation system of soybean (soybean) and rhizobium is one of the most extensively studied and applied legume symbiotic models. In the Northeast Black Soil and other regions, long-term positioning experiments have been used to evaluate the role of soybean-rhizobium symbiosis in sustainable production. A typical experiment compared the nutrient absorption and yield changes of soybean continuous cropping under different fertilization treatments. The results showed that: soybeans without nitrogen fertilizer but inoculated with rhizobia every season, their nitrogen absorption and yield can be maintained at a high level, and the nitrogen absorption is 30.5% to 100% higher than that of the non-inoculated treatment; the number of soybean nodules inoculated with rhizobia also increased significantly (an average increase of 9.3% to 53.8%), and the nitrogenase activity remained stable. These data show that in the soybean continuous cropping system, rhizobium symbiotic nitrogen fixation effectively compensates for the lack of exogenous nitrogen and supports the long-term stability of soybean yield. More interestingly, microbiome analysis found that continuous soybean cultivation without nitrogen fertilizer will gradually shape a unique rhizosphere microbial community: some functional bacteria related to nitrogen fixation (such as Bradyrhizobium, Azospirillum, etc.) are enriched, while the abundance of such beneficial bacteria is lower under excessive nitrogen fertilizer treatment (Figure 2) (Wei et al., 2023; Huang, 2024). The rhizosphere bacterial diversity of soybean plots without nitrogen application for a long time is higher, and the community structure tends to be more beneficial functional types such as nitrogen fixation and phosphorus solubilization, which is considered to be one of the reasons why soybeans can still maintain yield under low nutrient input. However, the long-term treatment without phosphorus fertilizer has observed a decrease in the abundance of rhizosphere microorganisms and hindered community development, indicating that the symbiotic system also requires other nutrient combinations. 6.2 Peanut-microbial inoculant synergy for yield enhancement Peanut is an important leguminous oil crop, and its symbiotic nitrogen fixation also occupies a place in agricultural production. In recent years, research on improving peanut yield and resource utilization by inoculating high-efficiency bacterial agents has made considerable progress. Ding et al. (2024) reported a field plot
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