Effects Of Bt Gene Transformation In Rice On Rihzosphere And Residue Decomposed Microbial Community

By 2009, genetically modified (GM) crops were adopted in 25 countries covering an area of more than 134 million hectares with an annual growth rate of 7%. The Chinese government implemented a＄3.5 billion GM Crops Initiative that included Bt rice to secure its food supply and to address land degradation, chronic water shortages, and a growing population that already numbers 1.3 billion. Transgenic Bt rice is not approved in any country so far, but is likely to be approved in China in the near future. However, the random insertion of a foreign gene into the plant genome may cause changes in the amount and composition of crop residues and may also affect organic matter decomposition, soil microbial community, andnutrient cycling in soil. So far, little is known about the effects of Bt rice on rihzosphere microbial community, residues composition and its decomposition process. Hennce in this thesis the effects of cry1Ab gene transformation in rice on rihzosphere and its residue decomposed microbial community were conducted under field conditions. The results were summaried as follow:1. The effect of Bt rice on rihzosphere soil microbial community compositionThere were no significant differences of arbuscular mycorrhiza fungi (AMF) spores and glomalin protein density between Bt and non-Bt rice variety indicating that Bt gene transformation have little effect on soil AMF in paddy field. Furthermore, the bacterial and fungal mommunity composition were shifted during the rice growing stage and environment condition in paddy field. The effects of Bt gene transformation on rihzosphere soil microbial community were minor.2. The effect of Bt transformation on rice residues compositionLignin, cellulose, carbon and nitrogen contents in Bt and non-Bt parental rice residues were not significantly different. However, significantly higher lignin, cellulose and carbon content, and lower nitrogen content were detected in rice straw compared with rice roots. The FITR showed no significant differences of surface functional groups between Bt and non-Bt residues. The physical histology of rice straw was quit different compared with root using scanning electron microscope (SEM). However, no differencs of physical histology were also shown in Bt and non-Bt rice residues.3. The effect of Bt rice on rice residue decomposition dynamicsFor rice straw in the litterbags, the time-course of AFMR, TN, TC and the C/N ratio showed similar trends despite a transient increase in TN at the beginning of decomposition. A significantly higher decomposition level of straw as indicated by AFMR, TN and TC under incorporated placement was observed as compared to the surface placement. However, there were no further significant differences between Bt and non-Bt treatments. Percent AFMR, TN, TC and C/N ratio of rice straws were significantly affected by time and placement factors.For rice root, there were no significant differences of AFMR TN, TC and the C/N between Bt and non-Bt treatment in there rapeseed-rice cropping system. However, concerns have to be raised in rice-rice cropping sysytem Bt roots in buried litterbags displayed significantly lower AFMR, TN and TC contents at the initial decomposition stage compared to the conventional rice. 4. The effect of Bt rice on rice residue decomposed microbial community compositionFor rapeseed-rice cropping system, AMMI analysis of two bacterial T-RFLP data sets revealed a relationship between soil microbial community and decomposition time for rice straw and roots. The MANOVA on the first two dimensions of AMMI scores for soil bacterial and fungal communities showed significant effect of decomposition time in all treatments except the second interaction principal component (IPC 2) of bacterial T-RFLP data during the root decomposition. The fungal T-RFLP fingerprints from both straw and root samples were also clearly grouped by time. An effect of litterbag placement on the microbial community associated with the residues was also detected by AMMI analysis. However, no significant effect of rice genotype was observed at any time for either bacteria or fungi associated with straw and root residues.For rice-rice cropping system, total AMMI model analysis revealed that microbial community composition in the litterbags was affected by temporal and spatial factors. Compared with the non-Bt rice residue treatment, Bt rice straw had no significant effects on the soil bacterial and fungal community composition during the study period, regardless of the litterbags being placed on the surface or buried in the soil. There were no significant differences in the bacterial community composition profiles in root decomposition between Bt transgenic and non-Bt varieties. However, significant differences in soil fungal community composition between the buried Bt and non-Bt rice roots were observed in soils sampled on days 31,68 and 137, indicating that Bt roots incorporated into paddy soil may affect soil fungal community during the initial stage of their decomposition...