Effects of Bacillus thuringiensis transgenic cotton straw returning to field on soil fertility

With the rapid development of genetically modified organisms, how to use and deal with a large amount of Bacillus thuringiensis (Bt) transgenic cotton straw is an important issue. To study and clarify the feasibility and safety of straw incorporation of transgenic Bt cotton into the field, four kinds of Bt cotton with different levels of insect resistance and one non-transgenic conventional cotton (Simian no. 3) were analyzed. After one or two years of planting, all the cotton straws were mechanically crushed and returned to the field in situ. The content of residual Bt protein, activities of several soil enzymes, and nutrient contents in soil were detected and the changes were further analyzed. The effects of Bt cotton straw returning on nutrient characteristics and fertility of soil were analyzed. The Bt protein residues in soils were very few and almost undetectable after 40 days of straw recycling of transgenic Bt cotton with different insect resistance levels, and there was no significant differences between the transgenic field plots and non-transgenic ones. After cotton straws were returned into the field, the activities of urease, sucrase, protease, polyphenol oxidase, catalase, and alkaline phosphatase in soil obviously increased, while the activity of cellulose did not increase. The activity of soil cellulase was lower than that before cotton planting. The amounts of organic matter, available phosphorus, available nitrogen, available potassium, and total nitrogen in soil increased remarkably both for the first year and for the second year of cotton planting, and so did the pH value. Similarly, there were not any significant differences in variations of all nutrient content between Bt transgenic treatments and non-transgenic ones. The soil comprehensive fertility index resulted by straw recycling was calculated and the effect on the fertility suitability was evaluated. The fertility index rose from level Ⅲ to level Ⅱ in two years, which was not related to the insect resistance level of Bt transgenic cotton. The soil fertility was not adversely decreased but increased, in fact, by straw returning treatment of Bt cottons. Returning Bt cotton straw to the field could effectively improve the soil fertility index. In addition, this straw recycling scheme could effectively kill transgenic plants, and would meet the disposal requirements of transgenic plants. The collective data confirmed that returning treatment, to a certain extent, should be feasible and safe for dealing with mass residues of Bt transgenic cotton in practice.