Bt Research 2025, Vol.16, No.5, 214-223 http://microbescipublisher.com/index.php/bt 221 Bt toxins are relatively few, but there are some representative cases. Study on the impact of Bt corn debris in streams - A study in the United States focuses on the effects of field residues (leafs, cobs, etc.) after corn harvest on aquatic organisms after rainwater is washed into adjacent streams. They detected the presence of Bt toxic protein in a stream in the Bt corn producing area and conducted indoor experiments to feed Bt corn debris to the larvae of the crusty-eating crusty-eating crusty-Hodoptera larvae. It was found that the survival and growth of certain aquatic insects were affected at extremely high doses and prolonged exposure (Liu et al., 2016). This suggests that Bt toxin may pose potential risks to stream ecology. However, further field investigations and subsequent experiments mostly failed to reproduce significant negative effects. The Bt protein content in stream water samples is usually very low, far from enough to have a toxic effect on most non-target aquatic insects (Pott et al., 2020). Moreover, Bt protein is quickly diluted and degraded in a running water environment, and the above effects are mostly transient and limited. 7.3 Dynamic cases of degradation of Bt toxin under plant straw return to the field Straw returns to the field is the current soil fertilizer and waste utilization measures advocated by agriculture. In Bt crop production, the return of straw (severed) will also involve the issue of returning to the field. Researchers tracked the dynamics of Bt toxins in the fields after Bt crop straw was returned to the field. In situ return of Bt cotton straw to the field fertility impact test - This test was continuously planted with Bt cotton for 1 and 2 years, and then crushed the cotton straw and returned it directly to the field. The Bt protein content and soil fertility index in the soil were analyzed. The results showed that after straw was returned to the field, the content of Bt protein in the soil dropped rapidly to an extremely low level, and the difference was not obvious compared with the control fields that did not return to the field; at the same time, fertility indicators such as soil organic matter and fast-acting nutrients have been improved, and the activity of enzymes such as soil urease and phosphatase also showed an upward trend (Zhang et al., 2017). This shows that the return of Bt cotton straw to the field not only did not cause the problem of Bt toxin residue, but instead improved the soil fertility and reflected the positive effect of returning the field. In a field experiment conducted in South China, Bt rice and control rice straw were placed in the fields to determine the growth and growth of Bt toxic proteins during straw decomposition. The results show that the content of Bt protein decreased by about 75% after 17 days of decomposition of Bt rice straw in the field, and the degradation rate slowed down. On day 53, the residue of Bt protein of about 5~10 ng·g⁻¹ was still detected in the straw (Liu et al., 2019). Despite a small amount of residue, considering that the straw quality has also been greatly reduced, the total amount of Bt protein entering the soil has been very low, and subsequent soil degradation can further remove it. This experiment also confirms that the release-degradation of Bt protein in straw occurs simultaneously. 8 Ecological and Environmental Risk Assessment and Prospects 8.1 Ecological safety evaluation of Bt toxin degradation products Bt toxin is eventually converted into products such as small peptides and amino acids through degradation. These degradation products are essentially common organic matter and have extremely low harm to the ecological environment. From the perspective of protein structure, the insecticidal activity of Bt toxin depends on its complete three-dimensional conformation and specific receptor binding domains, and once it is hydrolyzed into fragments, it no longer has its original function. From a macro perspective, Bt toxins eventually mineralize into inorganic nutrients such as ammonia nitrogen and nitrate nitrogen, which can even be reabsorbed and utilized by plants to complete a nutrient cycle process. Field studies have shown that after the decomposition of Bt crop straw, the content of ammonium nitrogen and nitrate nitrogen in the soil increased slightly, which may be due to the mineralization of organic nitrogen such as Bt protein. 8.2 Potential impact on non-targeted biology and ecosystems The assessment of the impact of non-target organisms is one of the important contents of the environmental safety assessment of Bt crops. A large number of studies and field observations have shown that the direct impact of Bt
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