Molecular Soil Biology 2024, Vol.15, No.2, 74-86 http://bioscipublisher.com/index.php/msb 80 nitrogen-fixing rhizobia, which can significantly improve nitrogen fixation and plant growth. Studies have shown that combined inoculation with arbuscular mycorrhizal fungi (AMF) and rhizobia can enhance nodulation and nitrogen fixation efficiency in legumes such as common beans (Phaseolus vulgaris L.). Additionally, the use of commercial inoculants like Rhizobium tropici CIAT899 has been demonstrated to improve phosphorus use efficiency, which is crucial for optimal nitrogen fixation (Tajini et al., 2012). However, the effectiveness of these inoculants can be influenced by various factors, including soil conditions and the presence of other microbial communities (Nandasena et al., 2007). 7.2 Crop rotation and intercropping with legumes Crop rotation and intercropping with legumes are sustainable agricultural practices that can enhance nitrogen fixation and improve soil health. Intercropping legumes with cereals, for example, has been shown to increase the proportion of nitrogen derived from biological nitrogen fixation (BNF) and improve the overall nitrogen use efficiency in agroecosystems (Rodriguez et al., 2020). A meta-analysis of field-scale studies revealed that intercropping grain legumes with cereals not only enhances nitrogen fixation but also increases soil nitrogen acquisition by cereals, thereby reducing the need for external nitrogen inputs. This practice can lead to more resilient cropping systems and contribute to the sustainability of agriculture by diversifying cropping systems and reducing greenhouse gas emissions (Rodriguez et al., 2020). 7.3 Management practices to optimize nitrogen fixation Optimizing nitrogen fixation in legumes involves a combination of management practices that address both biotic and abiotic factors. One key aspect is the regulation of soil nitrogen levels, as excessive nitrate can inhibit nitrogen fixation by rhizobia. Research has shown that moderate soil nitrate levels, irrespective of herbivory, result in minimal fixed nitrogen allocation, whereas non-supplemented soils can enhance aboveground nitrogen allocation following herbivore damage (Thompson and Lamp, 2021). Additionally, systemic signaling mechanisms within the plant play a crucial role in adjusting nodule formation and function based on the plant's nitrogen demand (Lepetit and Brouquisse, 2023). Understanding these regulatory circuits can help in developing strategies to optimize nitrogen fixation under varying environmental conditions. Furthermore, the compatibility between legumes and rhizobia is essential for successful symbiosis. Plants must recognize and select efficient rhizobial partners from a diverse soil microbiome, which can be influenced by metagenomic studies and advances in understanding plant-microbe interactions (Clúa et al., 2018). By integrating these agricultural practices, farmers can enhance the symbiotic relationship between legumes and rhizobia, leading to improved nitrogen fixation, better soil health, and more sustainable agricultural systems. 8 Case Studies and Practical Applications 8.1 Successful examples of legume-rhizobia symbiosis in different agricultural systems The symbiotic relationship between legumes and rhizobia has been extensively studied and applied in various agricultural systems, demonstrating significant benefits in terms of nitrogen fixation and plant growth. For instance, Medicago sativa (alfalfa) has shown increased aboveground allocation of biologically fixed nitrogen in response to herbivory, particularly in soils without additional nitrate supplementation. This indicates that herbivory can enhance the function of legume-rhizobia symbioses, leading to better nitrogen fixation and plant growth in certain conditions (Thompson and Lamp, 2021). Another successful example is the symbiosis between lima bean (Phaseolus lunatus) and rhizobia, which not only promotes plant growth but also enhances plant defense against herbivores. This dual benefit underscores the importance of rhizobia in improving both the productivity and resilience of leguminous plants in agricultural systems (Thamer et al., 2011). In Western Australia, the introduction and inoculation of Biserrula pelecinus with Mesorhizobium ciceri bv. biserrulae have led to the rapid evolution of diverse competitive strains of mesorhizobia. Despite some strains being suboptimal in nitrogen fixation, the overall success of this symbiosis highlights the adaptability and potential of legume-rhizobia interactions in different environmental contexts (Nandasena et al., 2007).
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