Field Crop 2024, Vol.7, No.4, 191-200 http://cropscipublisher.com/index.php/fc 193 Aasfar et al. (2021) illustrates the non-symbiotic nitrogen fixation process carried out by free-living nitrogen-fixing bacteria, specifically Azotobacter species, in soil. The process begins with the conversion of atmospheric nitrogen (N₂) into ammonium (NH₄⁺) via the nitrogenase enzyme complex. The key components of this enzyme, including the Fe-protein (4Fe–4S cluster) and MoFe-protein (P-cluster and FeMo-co), are essential for electron transfer and ATP hydrolysis, which drive the reduction of nitrogen. The NH₄⁺ produced is subsequently taken up by plants and converted into nitrates (NO₃⁻) and amino acids, supporting plant growth. This mechanism underscores the critical role of non-symbiotic nitrogen fixation in enhancing soil nitrogen availability and reducing dependency on synthetic fertilizers. 2.3 Benefits of BNF in agriculture The benefits of BNF in agriculture are manifold. Primarily, BNF reduces the dependency on synthetic nitrogen fertilizers, which are associated with environmental issues such as nitrate pollution and greenhouse gas emissions (Mahmud et al., 2020; Soumare et al., 2020). In sugarcane, BNF has been shown to contribute significantly to the plant's nitrogen needs, with some studies reporting that over 60% of the nitrogen in sugarcane can be derived from BNF (Martins et al., 2020). This not only enhances the sustainability of sugarcane cultivation but also improves soil health by maintaining a natural nitrogen cycle (Mahmud et al., 2020). Furthermore, BNF-associated bacteria can promote plant growth through other mechanisms, such as phytohormone production and protection against phytopathogens (Aasfar et al., 2021). 2.4 Factors influencing BNF efficiency in sugarcane Several factors influence the efficiency of BNF in sugarcane. Nutrient availability, particularly nitrogen and phosphorus, plays a critical role. Excessive nitrogen fertilization can inhibit BNF, while phosphorus addition can have variable effects depending on the type of nitrogen fixation (symbiotic or free-living) (Santachiara et al., 2019; Zheng et al., 2019). Environmental conditions such as water stress, temperature, and flooding also significantly impact BNF efficiency. For instance, water stress and flooding have been shown to reduce BNF activity, especially during the vegetative stage of plant growth (Santachiara et al., 2019). Additionally, genetic factors, such as the specific sugarcane genotype, can influence BNF efficiency. Different sugarcane genotypes exhibit varying levels of BNF, which can be attributed to differences in nitrogen metabolism, hormone regulation, and microbial recognition pathways (Carvalho et al., 2022; Luo et al., 2023). In conclusion, optimizing BNF in sugarcane involves understanding the complex interactions between microbial players, plant genetics, and environmental factors. This optimization is crucial for enhancing the sustainability of sugarcane cultivation and reducing the reliance on synthetic fertilizers. 3 Impact on Reducing Fertilizer Use 3.1 Current fertilizer practices in sugarcane cultivation Sugarcane cultivation traditionally relies heavily on synthetic nitrogen (N) fertilizers to achieve high yields. However, the efficiency of nitrogen use in sugarcane is relatively low, leading to significant environmental concerns such as nitrate leaching and ammonia volatilization (Castro et al., 2019; Junior et al., 2023). Current practices often involve the application of high rates of N fertilizer, sometimes exceeding 150 kg N ha⁻¹, which can result in diminishing returns and increased production costs (Castro et al., 2019). Additionally, the timing of fertilizer application plays a crucial role in optimizing yields, with studies showing that earlier applications can significantly enhance both stalk and sugar yields (Castro et al., 2019). 3.2 Role of BNF in reducing chemical fertilizer dependency Biological nitrogen fixation (BNF) offers a promising alternative to reduce dependency on synthetic N fertilizers in sugarcane cultivation. BNF involves the conversion of atmospheric nitrogen into a form that plants can use, facilitated by nitrogen-fixing bacteria. Research has shown that endophytic diazotrophs, such as those found in various Saccharum species, can significantly contribute to the nitrogen needs of sugarcane (Figure 2) (Singh et al., 2022; Soumare et al., 2022). For instance, the use of legume cover crops has been demonstrated to increase soil nitrogen storage and microbial biomass carbon, thereby reducing the need for additional inorganic fertilizers (Tenelli et al., 2021). Moreover, certain sugarcane varieties and their wild progenitors exhibit high BNF capacity, which remains resilient even under varying nitrogen conditions (Luo et al., 2023).
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