Degradation Of BT Cotton And BT Rice Straw And The Impact On Soil And Aquatic Ecosystem

The potential impact of transgenic crops on soil and aquatic ecosystem is a focus of environmental assessment in recent years. Genetically modified cotton has been released commercially for many years in China, but only few study on the degradation characteristics and the effect on soil protozoa of cotton stalk has been done. Most previous studies on environmental safety of transgenic rice only examined the impact on arthropod animal and soil microorganisms, but little attention was paid on aquatic ecosystems. For further evaluating safety of the Bt cotton and Bt rice on the natural ecological environment, the degradation characteristics were studied by field bury experiments in this paper. The differences of Bt cotton and non-Bt cotton impact on abundance and diversity of collembola and unicellular protozoan were investigated in3consecutive years. Under conventional field management practices, survey on diversity and richness of plankton in Bt rice and non-Bt rice layer has been conducted. Results were obtained as follows:1. Degradation characteristics of Bt cotton and Bt rice strawCotton root and stem degraded in a trend of fast in the early stage and slow in the later stage; the weight of cotton root and stem residues remained stable after6months of the experiment, which meant the residues had been mineralized fully.There was a significant difference in weight loss between Bt and non-Bt cotton root, while stem, leaves and shell had no significant differences.There were no significant differences in carbon content of cotton residues between Bt and non-Bt cotton.The content of nitrogen had no significant difference between Bt and non-Bt cotton straw, but a significant difference appeared in the later stage, which meant the activities of microorganisms maybe differed between Bt and non-Bt straw. The degradation rate of rice straw was faster than that of non-Bt cotton straw; the degradation rate were almost equal between Bt and non-Be rice straw; the content of N and C had no significant difference between Bt and non-Bt rice.The content of Bt protein in Bt rice and Bt cotton straw showed a fast degradation rate at the early stage, the degradation rate reached80%and70%respectively in the end of first month, and we could not test Bt protein in the end of fourth month. 2. Effects of Bt cotton on collembola and ciliates population densityThe results in2011showed that there were no significant difference in population density of collembola between Bt and non-Bt cotton except for one date in July24th. The results in2012showed that the difference of population density of collembola was very obvious between different investigation location.In the period of balling, collembola population density in experimental field of Hubei Academy of Agricultural Sciences (HAAS) was higher than that of Huazhong Agricultural University (HZAU) significantly, with densities of4000and1500individuals per trapping point respectively. Entomobryidae as the dominant species showed the same trend that the population density in HAAS was higher than that of HZAU significantly, with a density of1200and400individuals per trapping point respectively. But in the same survey, population density of collembola had no significant difference in Bt cotton and non Bt-cotton.The results in2013showed that the dominant species were Isotomidae, and the population density of these species and total density had no significant difference between Bt cotton and non Bt-cotton.The result of population density of ciliates in different cotton field showed that population density increased first and then decreased with the seasons. Ciliates maximum population density appeared in August15th both in HAAS and HZAU, with2200per g soil and1400per g soil, meaning that the population density of ciliates under the influence of soil physico-chemical properties and fertility greatly; ciliates population density ranged from2000to3500per g soil in environmental testing cotton field (Tianjin) and from2500to6000per g soil in HZAU cotton field, the average density of population had a significant differences between two places, which meant that the population density of ciliates was influenced by location. The twice survey showed that the population density of ciliates had no significant difference between Bt cotton and non-Bt cotton.3. The safety assessment of Bt rice on zooplanktonRice lines genetically modified with the crystal genes from Bacillus thuringiensis (Bt) have experienced rapid development, with biosafety certificates for two Bt rice lines issued in2009. There has still been no commercial release yet due to public concerns about environmental risks. Many studies have confirmed that Bt rice was as safe as conventional rice to non-target organisms when pesticides were not applied. Pesticides are still required in Bt rice, however, to control non-lepidopteran pests. We assessed the environmental effects of two Bt rice lines (Bt rice with crylAb/Ac and cry2a) using zooplanktons as indicator species under normal field management practices using pesticides when required. Field experiments showed that zooplankton abundance and diversity were up to95%and80%lower in non-Bt rice fields than Bt rice fields. Laboratory rearing showed that water from non-Bt rice fields was significantly less suitable for the survival and reproduction of Daphnia magna and Paramecium caudatum in comparison with water from Bt rice fields. High pesticide residues were detected in the water from non-Bt rice fields, accounting for the bad performance of zooplankton in this water. Our results demonstrate that Bt rice is safer to aquatic ecosystems than non-Bt rice, and its commercialization will be beneficial for biodiversity restoration in rice-based ecosystems.Conclusions:Bt cotton and Bt rice had no significant differences in degradation rate than their conventional control respectively, which indicated Bt rice and Bt cotton had no influence on material and energy circulation in the soil.Bt cotton had no negative effects on abundance of collembola and ciliates in the soil;Under the conventional field management practices, Bt rice was safer than non-Bt rice to zooplankton which indicated that the commercialization of Bt rice will be beneficial to the restore of the aquatic biodiversity in the rice-based ecosystems.