Effect And Mechanis Of Root Exudates From Transgenic Bt Cotton On The Soil N Cycling

With the massive planting of genetically modified crops all over the world,increasingly concern has been paid to the environment safety. Studying the environmental effects of transgenic Bt crops contributes to the healthy development of transgenic Bt crops. We collected the root exudates of the transgenic Bt cotton and analyzed it’s components. Firstly, we added the root exudates of two different cottons in bud stage, Bt protein and artificial root exudates into the red soil respectively. Then the physical and chemical properties, nitrogen forms, urease, hydroxylamine reductase,denitrification enzyme, nitrification-denitrification rate and Bt protein of the red soil were determined. In the end, the 16 S rRNA gene and nirK functional gene were used to study the influence of the transgenic crops on microbial diversity and further explore the effect and mechanism of transgenic Bt cotton on soil N cycle. The main results were as follows:（1） From the seedling stage, the seedling to the bud stage, the root exudates pH of both common and transgenic Bt cotton were change from the weak alkaline to the weak acid and the water-soluble carbon, water-soluble nitrogen, Bt proteins, organic acids,amino acids and total soluble sugar were increased significantly with the extension of growth period. At the same growth stages, there were no difference in main components of the two cotton root exudates, but the most root exudates component of common cotton（such as the water-soluble carbon, water-soluble nitrogen, total soluble sugars, amino acids, organic acids, etc.） were higher than those in Bt cotton, especially in the bud stage.As the main organic acid in the root exudates, formic acid in transgenic Bt cotton root exudates accounted for 54%, 75% and 96% at the seding, seedling and bud stages,respectively, and that in common cotton root exudates accounted for 74%, 39% and 93%,respectively. Therefore, the content of the formic acids is the main difference in the organic acids between the common and transgenic Bt cottons. As one of main amino acids in root exudates, threonine in the transgenic Bt cotton and common cotton at the bud stage accounted for 42% and 36%, respectively. Compared to the transgenic Btcotton, the common cotton root exudates can provide more carbon and nitrogen resources for plant growth.（2） When the Bt protein and the transgenic Bt cotton root exudates were added into the red soil, the degradation rate was fast at the first 7 days, then become more and more slowly, and reached to the equilibrium after another 7 days. To study the characteristic of the Bt protein degradation, the double-constant model was used for fitting and it fitted well. Then, we concluded that the DT50 Values of transgenic Bt cotton and Bt protein treatment were 170 and 14.5 days, respectively.（3） The bud stage root exudates of Bt and common cotton, pure Bt protein and artificial root exudates in the soil had no significant effect on the soil pH. The decomposition of organic matter was accelerated by both root exudates and the common cotton played a better role. The transgenic Bt cotton root exudates, common cotton root exudates and Bt protein in soil made NH₄⁺-N, NO₃^--N and NO₂^--N content higher than the CK treatment, although there were no significant differences among the treatments,and the NO₃^--N, NO₂^--N contents were increased; Compared to the CK, soil NO₃^--N,NO₂^--N contents showed a decreasing trend in the artificial treatment. Soil urease and hydroxylamine reductase activity were increased by the transgenic Bt cotton root exudate,common cotton root exudate, Bt protein and artificial root exudate; and the transgenic Bt cotton had a strongest effect. Compared to the CK, the soil nitrate reductase and nitrite reductase activity were inhibited by the other treatments. The sequence of the inhibition was: artificial root exudates > common cotton root exudates > Transgenic Bt cotton root exudates ≈ Bt protein > CK. Compared with the denitrification rate of each treatment, the transgenic Bt cotton root exudates and Bt proteins promoted the denitrification process,and the other treatments had no significant difference. These results suggest that the root exudates of transgenic Bt cotton and Bt protein can speed up the nitrogen transformation process.（4） MiSeq sequencing analysis showed that, after 7 days, compared to the CK,quantity of the 16 S rRNA gene dominant species increased, Acidobacteria and Chloroflexi which participate in decomposition of organic matter increased in some degree; meanwhile, the relative abundance of the Proteobacteria and Actinobacteriawhich involved in the carbon deposition decomposition conversion process in the crop rhizosphere decreased. Between the two treatments, the common cotton played a much more significant role. nirK genes are mainly distributed in denitrifying bacteria, and its Shannon diversity index were as follows: there were no significant different between the CK and the transgenic Bt cotton root exudates treatment, and the CK was much higher than the common root exudates treatment. The results showed that the described common cotton root exudates reduced the nirK denitrifying bacteria diversity. Proteobacteria abundance was decreased significantly by Bt cotton root exudate treatment, while the abundance of bacteria which were not identified were increased, and it confirmed that the root exudate of transgenic Bt cotton can increase the soil nirK denitrifying bacteria diversity.