Increased microbial activity and nitrogen mineralization coupled to changes in microbial community structure in the rhizosphere of Bt corn

• Total microbial activity was higher in the rhizosphere of the transgenic plants.
• CLPP was more influenced by sampling time than the nature of the plants.
• PCR-DGGE showed more differences between sampling times and sites than between plants.
• PCR-DGGE showed marked differences within the Betaproteobacteria.
• We describe for the first time the presence of Iamiaceae family in soil.

The interactions between plant roots and soil microorganisms are essential for the function and stability of ecosystems, primary agricultural production and plant health. Despite the importance of soil microbes the response of these microbes to large-scale cultivation of genetically modified (GM) crops is still poorly understood. This study evaluated the potential impact of two lines of transgenic Bt maize on rhizosphere microorganisms. A time-course field experiment was conducted over a period of two years in two fields in Guadalajara (Spain) with monthly sampling from April to September. Rhizosphere soil was collected from transgenic (TG) and unmodified (WT) maize plants from each field and sampling time for the analysis of several important functional and structural soil quality parameters. Total microbial activity, as determined by H3-Thymidine and C14-Leucine incorporation, was found to be higher in the rhizospheres of the transgenic plants. Similarly, differences in potential ammonification and nitrification were observed in the second year of the study. In contrast, bacterial and fungal microbial catabolic abilities, as determined by Biolog ECO and FF plate analyses, respectively, were more influenced by sampling time than the transgenic nature of the plants. Microbial community structure was also studied by bacterial and phylum-specific PCR-DGGE and PCR cloning approaches. In general, differences were again more pronounced between sampling times, as opposed to between TG versus WT plants, although marked differences were observed within the Betaproteobacteria between plant lines. For the first time it describes the presence of Iamiaceae family in soil, specifically to TG plant rhizosphere. To summarize, the study showed that some important properties of rhizopshere microbes may be impacted by Bt maize cultivation and highlighted the fact that such potential effects need to be viewed within the context of seasonal and spatial variability.