Molecular Soil Biology 2024, Vol.15, No.1, 28-36 http://bioscipublisher.com/index.php/msb 29 1 Symbiotic Nitrogen Fixation: An Overview 1.1 Definition and mechanisms of symbiotic nitrogen fixation Symbiotic nitrogen fixation (SNF) is a biological process where certain microorganisms convert atmospheric nitrogen (N2) into ammonia (NH3), a form that plants can readily assimilate. This process is facilitated by the enzyme nitrogenase, which is encoded by the nifH gene, among others. The symbiotic relationship typically involves bacteria colonizing plant roots or tissues, forming specialized structures such as nodules where nitrogen fixation occurs. This mutualistic interaction allows plants to access a vital nutrient without relying on synthetic fertilizers, thereby promoting sustainable agricultural practices (Guo et al., 2020; Singh et al., 2021). 1.2 Key organisms involved Several key organisms are involved in SNF, including rhizobia, endophytic bacteria, and diazotrophs. Rhizobia are well-known for their symbiotic relationship with leguminous plants, forming root nodules where nitrogen fixation takes place. Endophytic bacteria, such as Enterobacter roggenkampii and Gluconacetobacter diazotrophicus, reside within plant tissues and contribute to nitrogen fixation without forming nodules (Guo et al., 2020; Saranraj et al., 2021; Luo et al., 2023). Diazotrophic bacteria like Pantoea dispersa and Enterobacter asburiae have also been identified as significant contributors to nitrogen fixation in sugarcane, enhancing plant growth and stress tolerance (Singh et al., 2021). 1.3 Benefits of symbiotic nitrogen fixation in crop production The benefits of SNF in crop production are manifold. Firstly, it reduces the dependency on synthetic nitrogen fertilizers, which are costly and environmentally damaging due to their contribution to soil and water pollution (Guo et al., 2020; Singh et al., 2021). Secondly, SNF enhances plant growth and yield by providing a consistent and sustainable nitrogen source. For instance, studies have shown that nitrogen-fixing bacteria like Pantoea dispersa and Enterobacter asburiae can significantly improve sugarcane physiological parameters such as plant height, shoot weight, and chlorophyll content (Singh et al., 2021). Additionally, SNF contributes to better disease management and stress tolerance in crops, as evidenced by the increased expression of stress-related genes in sugarcane varieties colonized by nitrogen-fixing bacteria (Guo et al., 2020; Singh et al., 2021). 2 Current Practices in Sugarcane Cultivation 2.1 Conventional methods of nitrogen fertilization in sugarcane farming The traditional approach to sugarcane cultivation involves the extensive use of synthetic nitrogen fertilizers to meet the high nitrogen demands of the crop. These fertilizers are produced from atmospheric nitrogen and natural gas, a process that has been scaled up significantly since the Green Revolution to boost crop yields (Pankievicz et al., 2019). However, the efficiency of nitrogen use in sugarcane is relatively low, leading to substantial nitrogen losses and necessitating frequent applications (Luo et al., 2023). 2.2 Environmental and economic challenges associated with chemical fertilizers The reliance on synthetic nitrogen fertilizers poses several challenges. Environmentally, the excessive use of these fertilizers contributes to greenhouse gas emissions and soil degradation, impacting the sustainability of agricultural practices (Luo et al., 2023). Economically, the production and application of nitrogen fertilizers represent a significant expense for farmers, particularly in developing countries where the cost can be prohibitive (Mus et al., 2016; Junior et al., 2020). These challenges underscore the need for more sustainable and cost-effective nitrogen management strategies. 2.3 Need for sustainable alternatives Given the environmental and economic drawbacks of synthetic nitrogen fertilizers, there is a pressing need for sustainable alternatives. Biological nitrogen fixation (BNF) presents a promising solution, as it involves the natural conversion of atmospheric nitrogen into a form that plants can use, facilitated by symbiotic relationships between plants and nitrogen-fixing bacteria (Mus et al., 2016; Pankievicz et al., 2019). Recent research has focused on extending these symbiotic relationships to non-leguminous crops, including sugarcane, to reduce dependency on chemical fertilizers and enhance nitrogen use efficiency (Pankievicz et al., 2019; Luo et al., 2023).
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