Molecular Soil Biology 2024, Vol.15, No.4, 183-192 http://bioscipublisher.com/index.php/msb 185 significant implications for nutrient cycling and soil health. Additionally, long-term nitrogen fertilization, often used in Bt crop cultivation, has been shown to decrease bacterial diversity and favor the growth of Actinobacteria and Proteobacteria (Dai et al., 2018). These shifts in microbial community structure highlight the need for ongoing monitoring to understand the long-term ecological impacts of Bt crop cultivation. 3.3 Comparative effects of Bt crops vs. chemical pesticides When comparing the effects of Bt crops to chemical pesticides, studies have generally found that Bt crops have a less pronounced impact on soil microbial communities. For instance, a glasshouse experiment comparing Bt maize and pyrethroid insecticide (deltamethrin) found that the insecticide had a more significant effect on altering nematode community structure and increasing plant Bt concentration than the Bt trait itself (Griffiths et al., 2006). Similarly, a study on the impact of various pesticides on soil microbial functions related to carbon cycling found that insecticides like fipronil had a more substantial effect on stimulating cellulolytic and chitinolytic microorganisms compared to Bt applications (Sim et al., 2022). These findings suggest that while Bt crops do influence soil microbial communities, their impact is generally less severe than that of chemical pesticides, making them a potentially more sustainable option for pest management in agriculture. 4 Microbial Functions Affected by Bt Applications 4.1 Impact on nitrogen-fixing bacteria Bt applications can significantly influence nitrogen-fixing bacteria in the soil. Studies have shown that the presence of nitrogen-fixing bacteria, such as Azospirillumsp. and Nostoc edaphicum, is crucial for plant growth and soil health. However, the impact of Bt applications on these bacteria can vary. For instance, single bacterial inoculation without mycorrhiza does not positively affect plant growth, while co-inoculation with arbuscular mycorrhizal fungi (AMF) improves plant growth and vitality (Ogar et al., 2015). Additionally, nitrogen fertilization, which can be influenced by Bt applications, has been shown to reduce the abundance of nif genes, indicating a decrease in nitrogen fixation potential (Liao et al., 2021). 4.2 Effects on decomposers and nutrient cycling Bt applications can also affect decomposers and nutrient cycling processes in the soil. Decomposers, such as saprotrophic fungi, play a critical role in breaking down organic matter and recycling nutrients. Nitrogen enrichment, which can be a consequence of Bt applications, has been shown to decrease microbial biomass carbon (MBC) and alter the composition of decomposer communities (Jia et al., 2020). Furthermore, the presence of AMF can modify the soil bacterial community and nitrogen cycling during litter decomposition, indicating that Bt applications that affect AMF can subsequently influence decomposer activity and nutrient cycling (Nuccio et al., 2013). The addition of fungicides and bactericides, often used alongside Bt applications, can also have mixed effects on soil respiration and nitrogen cycling, further complicating the impact on decomposers (Ullah and Dijkstra, 2019). 4.3 Changes in microbial symbiosis with plants (e.g., mycorrhizal fungi) Bt applications can lead to significant changes in microbial symbiosis with plants, particularly with mycorrhizal fungi. Mycorrhizal fungi, such as AMF, form symbiotic relationships with plant roots, enhancing nutrient uptake and plant growth. However, nitrogen enrichment and other factors associated with Bt applications can reduce the abundance of AMF and alter their community composition (Zhang et al., 2018; Jia et al., 2020). Studies have shown that AMF can shift the community composition of nitrogen-cycling microbes and suppress soil N2O emissions, indicating that changes in AMF populations due to Bt applications can have broader ecological impacts (Zhang et al., 2022). Additionally, the presence of AMF has been shown to buffer the impact of varying nutrient inputs on the decomposer community, highlighting the importance of maintaining healthy mycorrhizal symbioses in the face of Bt applications (Schmidt et al., 2017). 5 Soil Health and Ecological Balance 5.1 Bt applications and soil fertility Bt crops, which are genetically modified to express Bacillus thuringiensis (Bt) toxins, have been widely adopted in agriculture. These crops can influence soil fertility through various mechanisms. For instance, Bt proteins
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