Ecological Responses Of Soil Nitrogen-fixing Bacteria Group To EPSPS Transgenic Herbicide-resistant Soybean

Since 1996, the first variety of genetically modified soybean cultivated commercially by Monsanto in the United States, genetically modified soybeans have brought huge economic benefits to human society and acreage increased year by year. The study of the impact of genetically modified crops including genetically modified soybeans on biological and environmental security has many reports, and some results showed that genetically modified crops possibly affected soil microbial communities. Soil microbial is an important part to maintain soil activity and function by involving in a variety of biochemical reactions, such as the decomposition of organic matter, humus formation, transformation and circulation of nutrients. Nitrogen-fixing bacteria group, a class of symbiotic nitrogen fixation with legume or autogenous nitrogen fixation microorganisms, plays a key role in the-natural nitrogen cycle. Herbicide-resistant genetically modified soybean is the first commercial cultivating and the widest cultivation range of genetically modified soybeans. Researching the impact of herbicide-resistant transgenic soybean after cultivation on soil microbial, especially on nitrogen-fixing bacteria group has a significant meaning to the ecological safety study of transgenic soybean.In this study, we chose a EPSPS transgenic herbicide-resistant soybean NZL06-698 and its receptor Mengdou 12 as well as a regional test variety Heihe 43 as materials. Rhizosphere soil samples were collected by field planting, through a variety of analysis,such as measurements of soil physicochemical properties, nitrogen cycle related activity and nitrogen content, soybean nodulation effect analysis, analysis of culturable soil nitrogen-fixing bacteria, analysis of overall abundance of nitrogen-fixing bacteria group and metagenome sequencing analysis of soil microbial, to illustrate the impact of EPSPS transgenic herbicide-resistant soybean after cultivation on soil microbial, especially on nitrogen-fixing bacteria group, then provide a data basis for the ecological safety study of transgenic soybean.The results showed that:after planting EPSPS transgenic herbicide-resistant soybean, there were no significant differences in soil moisture, pH and soil nitrogen cycling-related activity compared with its receptor Mengdou 12 and regional control Heihe 43.It illustrated that planting EPSPS transgenic herbicide-resistant soybean did not cause a significant impact on soil properties and N cycling-related activity. The measurement results of N content of plants, seeds and soil showed that in flowering and maturity periods, there were no significant differences among the three varieties of soybeans.The statistics of nodule number and size of the three kinds of flowering and maturity period soybean had consistent results:there was no significant difference in the nodule size among the three kinds of soybean, but as for the nodule number, it was significantly smaller in GM varieties than those in the receptor and regional varieties. It suggested that EPSPS transgenic herbicide-resistant soybean had an impact on soybean nodulation effect.Using the traditional method of microbial culture isolated cultivable soil nitrogen-fixing bacteria. The results showed that there were no significant differences among the three varieties of soybeans in the four periods. But the results of real-time quantitative PCR for soybean rhizosphere azotobacter group showed that transgenic varieties NZL06-698 was significantly lower than the receptor Mengdou 12 in rhizosphere azotobacter group abundance in flowering period. Due to limitations of media and culture conditions in traditional cultivation methods, it cannot completely reflect the actual situation of soil azotobacter group. But real-time quantitative PCR technology overcomes these limitations, so it can be more direct and in situ to study the changes in soil azotobacter group. Therefore, the results above showed that EPSPS transgenic herbicide-resistant soybean would have some impact on soil azotobacter group after cultivation.Metagenomic analysis results showed that there was a significant change in soil microbial community after transgenic soybean cultivation compared with the receptor. The performance was to reduce the overall richness of microorganisms and specifically enrich proteobacteria bacteria aboundance. Analysis of rhizobium bacteria found that most of rhizobia bacteria abundance had a significant difference between two soil samples, but it did not have the consistency. Any changes of the abundance of soil rhizobium bacteria or other class of bacterial groups involved in nodule formation will affect the formation of soybean nodules. Therefore, transgenic soybean significantly less than the receptor in nodule number is probably due to reduction of the abundance of rhizobium or other bacteria involved in nodule formation. The analysis results of gene abundance showed that after transgenic soybean cultivation, there was no significant difference in gene abundance involved in soil N cycling, but the abundance of nitrogen fixation genes had a significant difference compared with the receptor. This showed that the transgenic soybean had some impact on soil nitrogen fixation process after cultivation.Based on the analysis results above, it suggested that the cultivation of EPSPS transgenic herbicide-resistant soybean NZL06-698 would have an impact on soil microbial community, especially on nitrogen-fixing bacteria group, with reducing the abundance and diversity of microbial species, and the abundance of nitrogen-fixing bacteria group. Thereby it affected soybean nodulation effect, resulting in reducing the number of nodules, but had little impact on the amount of biological nitrogen fixation in soil and soybean nodules.