| The breed and commercial release of transgenic crops with high-yielding, good quality and resistance to adverse environments are important approaches to increase crop production. Along with more and more transgenic crops released to agroecosystems, the safety for both food and environment of transgenic crops are concerned recently. Genetic drift, non-target effect, biodiversity degradation and changes of nutrient (N, P, K and C) cycling were especially emphasized. Soil is an important component of the agroecosystem. Soil organisms, especially, microbes, are the foundation for the stability and healthy of soil system. Root exudation and decomposition of crop residues have a significant influence on the metabolic activity of soil microbes. Therefore, Determination of the effect of released transgenic plants on community structure of soil microbes and activity of functional groups of microorganism in soil is an important aspect of the ecological risk assessment of transgenic crops.In this study, transgenic rice with the character of resistantance to rice stem borers, and its parent cultivar(non-Bt rice) were used as material to study the influence of transgenic Bt rice on soil microbies and soil features. Field and controlled experiments of rice growing and straw decomposition were conducted in experimental farm and laboratory of Zhejiang University in 2001 and 2002. The population of bacteria, actinomyces, fungi, activity of denitrifying bacteria and phosphorus solubilizing microorganisms, Bt protein releasing rate, soil nutrient, straw decomposing rate were measured in the experiments. The results were showed as follows. (1) Bt protein and its fate in soilBt protein in aboveground part of Bt rice plants was higher than that in root system. Bt protein content in plants aboveground increased along with rice growth advancement but no significant changes of that were found in root system. The average content of Bt protein was 5.35ug/g in aboveground part, and 0.82ug/g in root system. In root exudation, the concentration of Bt protein was very low. At early heading stage, the content of Bt protein in root exudates was the highest (48.02ng.hill-1.day-1). The lowest content in the root secretion was detected at the riping stage (1.66ng.hill-1.day-1). No Bt protein was detected in rice rhizosphere soil without root residues. Results obtained by means of bio-assay indicated no fatality of boll worm fed with rhizospheric soil and soil extraction occurred.The content of Bt protein decreases sharply within the initial three weeks instraw-decomposing experiments conducted both in the field and in laboratory. The content kept at a low level within comparatively long time of the experiment. No Bt protein was dectected in soil after 120 days.(2) Changes of micro-ecological environment in rhizospheric soil of transgenic Bt riceSoil pH showed a constant decrease during rice growing period. No significant changes were observed in total nitrogen, available nitrogen, available phosphorus and available potassium in soil. No significant differences were found in soil organic matter, humus and nutrients (N, P, K) between Bt and non-Bt rice, indicating that there was no adverse impact of Bt rice on soil features.There was significant difference in the microbe community in rhizospheric soil between Bt rice and non-Bt rice. The clone formation unit (CPU) of bacteria in non-Bt rice was significantly higher than that of Bt rice (P<0.05), while CPU of fungi was lower than that of Bt rice. No significant differences were found between Bt rice and non-Bt rice in the number of aetinomyces, activity of phosphorus solubilizing microorganism (PSM) and denitrifying bacteria.(3) Soil microorganism community and nutrients during straw decompositionField and controlled lab experiments indicated that the straw decomposition of both Bt and non-Bt rice had significantly influences on the numbers of soil microbes. Numbers of microorganisms in both Bt rice and non-Bt rice were significantly higher than that in control without str... |
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