| The influence of transgenic Bt crops on soil ecological system, which was taken as an important part of risk assessment by American EPA in2000, has attract much attention of researchers in recent years. The ecological risk assessment of genetically modified crops has great significance for promoting the stable development and reasonable application of biological technology, and protecting ecological environment and human health.In this experiment, transgenic Bt rice (HUA1517A/09HNZ010and HUA1517A/09HNZ011) and non-transgenic rice (JIN YOU207and HUA1517A/09HNZ009) were used as materials to investigate the effect of rice straws that were returned to field in flooded conditions on soil physico-chemical properties, denitrifying enzyme, denitrifying rate and microbial diversity, so as to provide a scientific basis for ecological risk assessment of transgenic Bt crops. The main results and conclusions were as follows:1. After straws were returned to field, the residual Bt protein in soil showed3dynamic stages:early releasing stage (1-4days), rapidly declining stage (4-30days) and last stable stage (30-60days). Both the first-order kinetic model and shift-log model fitted the dynamics of residual Bt protein in the highest content (4-60days). According to calculation, DT50and DT90in the first-order kinetic model were within the ranges of24.47-28.21d and80.13-94.07d respectively, while the corresponding days of DT50and DT90in shift-log model were18.88-25.88d and108.7-267.3d.2. Compared with non-transgenic Bt rice, transgenic Bt rice straws in paddy soil did not have much differences in the content of the organic matter, total nitrogen, C/N, NO3-and NH4+.3. Compared with non-transgenic Bt rice, transgenic Bt rice straws in paddy soil had no significant effect on the activities of nitrate reductase and nitrite reductase. After non-transgenic Bt rice straws were returned to field, the calculated activity of soil nitrate reductase was within the ranges of1.72-9.36NO3-mg/g; for transgenic Bt rice straws, the activity was2.02-11.3NO3-mg/g. The activities of nitrite reductase reached the maximum on the1st day, and then dropped dramatically. The calculated activities of nitrate reductase were within the ranges of0.33-1.10NO2-mg/g and 0.27-1.39NO2-mg/g in the corresponding treatments of transgenic Bt rice straws and non-transgenic Bt rice straws.4. The denitrifying rate can be decreased by returning straws, which shows a higher rate in the treatments of transgenic Bt rice straws than non-transgenic Bt rice straws. Bt protein was not the main influential factor on soil denitrifying rate since it had less effect on soil denitrifying rate. The affection degree of each index obtained by SPSS multiple linear regression is displayed as follows:Organic matter> Total nitrogen> C/N> NH4+> NO3-> Bt protein> Nitrate reductase activities> Nitrite reductase activities.5. The results from Biolog Eco micro flat method showed that the microbial metabolic activity and the ability of using carbohydrate and amine can be improved by returning transgenic Bt rice straws to field, while it had no effect on the using of carboxylic acids, amino acids, polymer and phenols. After rice straws returning, the Shannon index, Simpsons dominance index and McIntosh index of soil species all were greater in transgenic Bt paddy soil than those in contrast treatment, while the evenness index was just the opposite. The use pattern of carbon source by soil microorganism had significant differences between transgenic Bt rice and non-transgenic Bt rice after rice straws returning. The cluster analysis indicated that there were significant differences between the two tested rices in the use pattern of carbon source at exponential stage after straws returning, while it was no longer significant in the stable stage. |
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