Molecular Soil Biology 2024, Vol.15, No.4, 183-192 http://bioscipublisher.com/index.php/msb 187 are often complex and context-dependent. For instance, research indicates that Bt cotton does not adversely affect microbial populations or enzymatic activities in the soil, suggesting a neutral or potentially synergistic relationship with beneficial bacteria (Yasin et al., 2016). Additionally, soil microbial diversity and evenness indices did not significantly shift with the addition of Cry1Ac protein, indicating that Bt proteins might not disrupt beneficial bacterial communities (Zhaolei et al., 2018). 6.2 Impacts on soil pathogens and microbial disease suppression The presence of Bt toxins in the soil can influence soil pathogens and the microbial suppression of diseases. For example, Bt crops have been shown to increase the activities of certain soil enzymes, such as dehydrogenase and urease, which are involved in nutrient cycling and can indirectly affect soil pathogen dynamics (Li et al., 2019). However, the overall impact on soil pathogens remains inconclusive, as some studies report no significant changes in microbial community structures, including those of pathogenic fungi, due to Bt crop residues (Lu et al., 2010). This suggests that while Bt toxins may alter some aspects of microbial activity, they do not necessarily enhance or suppress soil-borne diseases in a consistent manner. 6.3 Effects on microbial-driven nutrient availability and uptake by plants Bt crops can also affect nutrient availability and uptake by plants through their interactions with soil microorganisms. The cultivation of Bt cotton, for instance, has been associated with higher cation exchange capacity and increased levels of essential nutrients such as nitrogen, phosphorus, and potassium in the soil, compared to non-Bt cotton. This enhancement in nutrient dynamics is likely mediated by microbial processes, as Bt crops have been shown to increase the population of beneficial bacteria involved in nutrient cycling (Yasin et al., 2016). Moreover, the decomposition of Bt crop residues does not significantly alter the microbial community composition, suggesting that nutrient availability driven by microbial activity remains stable (Lu et al., 2010). 7 Case Studies and Comparative Research 7.1 Field studies on Bt crops: key findings Field studies have provided significant insights into the impact of Bt crops on soil microbial communities. For instance, a global meta-analysis revealed that Bt crops can alter soil enzymatic activities, with specific enzymes like dehydrogenase and urease showing significant increases, while neutral phosphatase decreased under Bt crop cultivation without Bt residues incorporation (Li et al., 2019). Another study conducted in tropical western India found that Bt cotton significantly altered microbial community composition and functional diversity in rhizospheric soil compared to non-Bt cotton (Saha et al., 2021). Additionally, a systematic review and meta-analysis highlighted that while there is considerable variation among soil invertebrate orders, there was no significant overall effect of Cry proteins on soil invertebrates (Krogh et al., 2020). These findings underscore the complexity and variability of Bt crop impacts on soil ecosystems. 7.2 Laboratory vs. field conditions: variability in outcomes The outcomes of Bt crop studies can vary significantly between laboratory and field conditions. Laboratory studies, such as those involving soil incubation with Cry1Ac protein, have shown no significant changes in soil microbial diversities and population sizes over a 100-day period (Zhaolei et al., 2018). In contrast, field studies often reveal more nuanced effects. For example, field research on Bt rice indicated that while bacterial communities in the rhizosphere did not show significant changes, the root and leaf endospheres exhibited mild and transient differences compared to non-Bt rice lines over three consecutive years (Wu et al., 2021) (Figure 2). This variability highlights the importance of considering environmental factors and the complexity of natural ecosystems when evaluating the impacts of Bt crops. 7.3 Global perspectives: regional studies and differing environmental conditions Regional studies have shown that the impact of Bt crops on soil microbial communities can vary widely depending on environmental conditions. For instance, a study in tropical western India found that Bt cotton significantly altered microbial community composition and functional diversity in rhizospheric soil (Saha et al., 2021). In contrast, a global meta-analysis of soil enzymatic activities under Bt crops found that the response ratios
RkJQdWJsaXNoZXIy MjQ4ODYzMg==