Molecular Soil Biology 2024, Vol.15, No.2, 74-86 http://bioscipublisher.com/index.php/msb 81 8.2 Economic and ecological benefits in specific regions The economic and ecological benefits of legume-rhizobia symbiosis are particularly evident in regions where sustainable agriculture practices are prioritized. For example, the use of legume crops that engage in symbiotic nitrogen fixation can significantly reduce the need for chemical nitrogen fertilizers, which are associated with greenhouse gas emissions and nitrogen pollution. This makes legume production systems more efficient and environmentally friendly, contributing to climate preservation and sustainable agriculture (Goyal et al., 2021). In temperate grasslands, the biological nitrogen fixation by legume-rhizobia symbiosis is a crucial source of soil nitrogen. Studies on Trifolium fragiferum, a crop wild relative legume species, have shown that inoculation with native rhizobia can prevent nitrogen deficiency and enhance plant growth. This not only supports plant health but also improves soil fertility, which is vital for the sustainability of agricultural ecosystems in these regions (Figure 3) (Jēkabsone et al., 2022). Furthermore, the compatibility between legumes and rhizobia plays a significant role in the success of nitrogen-fixing symbiosis. Understanding the molecular mechanisms by which legumes recognize and select their symbiotic partners can lead to biotechnological advances that improve the efficiency of this symbiosis in agronomic systems. This has the potential to enhance crop yields and reduce dependency on synthetic fertilizers, providing both economic and ecological benefits (Clúa et al., 2018). 8.3 Case study: enhancing soybean production in tropical soils Soybean (Glycine max) is a crucial crop for many tropical regions, where it serves as a significant source of protein and oil. However, the productivity of soybean in these areas is often limited by poor soil fertility, particularly nitrogen deficiency. The symbiotic relationship between soybean and rhizobia, nitrogen-fixing bacteria, offers a sustainable solution to this problem by enhancing nitrogen availability in the soil. In tropical soils, environmental factors such as high temperatures, drought, and soil acidity further complicate the establishment and effectiveness of rhizobial inoculants. Research in Brazil has shown that these conditions can severely limit rhizobial growth and survival, thereby reducing nodulation and nitrogen fixation. Selecting stress-tolerant rhizobial strains and implementing appropriate soil management practices, such as liming to reduce soil acidity, have been effective strategies to overcome these challenges and improve soybean yields (Hungriaa and Vargasb, 2000). The genetic diversity and ecological adaptability of rhizobia also play a crucial role in their symbiotic efficiency. Studies have highlighted the importance of understanding the genetic and environmental factors that influence the localization and dominance of soybean-nodulating rhizobia. For instance, different rhizobial species and strains exhibit varying levels of nitrogen fixation efficiency and plant growth-promoting functions, which can be leveraged to optimize soybean production in specific tropical environments (Rodríguez-Navarro et al., 2011; Nakei et al., 2022). Moreover, the interaction between rhizobia and other soil microbes can significantly influence the symbiotic efficiency. Research has shown that certain soil bacteria, such as Bacillus species, can either promote or inhibit the growth of rhizobia, thereby affecting nodulation and nitrogen fixation. For example, Bacillus cereus has been found to promote the growth of Sinorhizobium while suppressing Bradyrhizobium, highlighting the need for a holistic approach to managing soil microbial communities to enhance soybean production (Han et al., 2020). 9 Challenges and Future Directions 9.1 Limitations and challenges in harnessing legume-rhizobia symbiosis One of the primary challenges in harnessing the legume-rhizobia symbiosis is the "rhizobial competition problem," where applied rhizobial inoculants often fail to compete effectively with native soil rhizobia, leading to suboptimal nitrogen fixation and reduced crop yields (Mendoza-Suárez et al., 2021; Quides and Atamian, 2021). This competition is exacerbated by the genetic diversity and adaptability of native rhizobia, which can outcompete inoculant strains for nodule occupancy. Additionally, the persistence of inoculant strains in the field is often limited due to the transfer of symbiotic genes to native populations, further reducing their effectiveness over time (Mendoza-Suárez et al., 2021).
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