Bt Research 2025, Vol.16, No.5, 214-223 http://microbescipublisher.com/index.php/bt 219 and returning to the field, is currently the recommended measure. For Bt crop fields, this means that a certain amount of Bt protein will return to the soil with the residue. The researchers conducted relevant dynamic monitoring for this scenario. In a 2-year field trial, Bt cotton plants planted for 1 and 2 years were harvested and then crushed and returned to the field respectively to determine the residual amount and changes of Bt protein in the soil. The results show that, whether it is continuous planting for one or two years, the initial content of Bt protein in the soil after straw is returned to the field is relatively low, and it quickly dropped to an undetectable level within 2 months after returning to the field (Zhang et al., 2017); at the same time, compared with conventional cotton fields, the nutrient content and enzyme activity of Bt cotton fields did not change adversely, but the organic matter and fast-acting nutrients were improved. This suggests that straw return to the field does not lead to the accumulation hazard of Bt protein in the soil (Liu et al., 2019). 6 Bt Toxin Degradation in Atmosphere and Special Environments 6.1 Effect of UV radiation on degradation of Bt toxins Bt toxins in the atmospheric environment mainly come from the spraying and drift of Bt preparations and the spread of pollen, epidermal debris, etc. of genetically modified Bt crops. Under open atmospheric conditions, the primary factor affecting the retention of Bt toxins is solar ultraviolet radiation. Strong UV radiation will quickly destroy the active center of Bt protein, causing its insecticidal function to be lost. Experiments show that exposing Bt crystal protein to simulated sunlight can reduce its virility by more than 90% within a few hours (Sanna and Moretti, 2008; Pan et al., 2017). Therefore, it is difficult for Bt particles floating in the air or Bt toxins attached to the plant surface to remain active in the sun for a long time (Pan et al., 2017). This is also why Bt biopesticides usually need to be sprayed in the evening or under low light conditions to reduce the destructive effects of UV light. In addition, oxidative components in the atmosphere (such as hydroxyl radicals) may also be involved in the degradation of Bt toxins under photochemical action. These strong oxidants are able to react with certain amino acid residues in protein molecules, resulting in structural modification and breakage of Bt proteins. 6.2 Study on the stability of Bt toxin under extreme environmental conditions Extreme environmental conditions may have a special effect on the degradation behavior of Bt toxins. In extreme low temperature environments, microbial activity almost stagnates, and the biodegradation of Bt toxins will be greatly slowed down. This means that the Bt protein may remain in its near-primitive state throughout the winter and does not begin to degrade until the temperature rises after the microorganisms become reactive. However, low temperature environments are often accompanied by freeze-thaw cycles and physical weathering. Bt toxin crystals may undergo mechanical breakage during freeze-thawing, which helps them to be more easily enzymatically dissolved when thawed. At the same time, the content of Bt protein in plants will also decrease under low temperature stress, which is mainly due to the reduction of protein synthesis and increased decomposition of the plant itself (Parimala and Muthuchelian, 2010). The cotton cryogenic test pointed out that the low temperature stress lasting 48 hours significantly reduced the content of insecticidal protein in Bt cotton fibers, and detected an increase in free amino acids and increased proteolytic enzyme activity in plants. This shows that although low-temperature conditions inhibit microorganisms, the enzymatic decomposition of the plants themselves may still be carried out to a certain extent. For extreme drought conditions, water deficiency inhibits microbial activity and causes the degradation of Bt toxins to almost pause. However, drought is often accompanied by higher daytime temperatures and strong light, which in turn accelerates the non-biodegradation of Bt protein. 6.3 Adsorption and degradation of Bt toxin by atmospheric particulate matter When the Bt preparation sprays or pollen released from the Bt crop drifts into the air, the Bt toxin may exist in the form of aerosol particles and attach to the atmospheric suspension particles. Atmospheric particulate matter provides a carrier, accelerating the dry and wet sedimentation processes of Bt toxin. Once the Bt toxin is attached to the dust particles, its migration behavior in the air depends on the physical properties of the particulate matter.
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