Impact of Bt corn on rhizospheric and soil eubacterial communities and on beneficial mycorrhizal symbiosis in experimental microcosms.

A polyphasic approach has been developed to gain knowledge on suitable key indicators for the evaluation of environmental impact of genetically modified Bt 11 and 176 corn lines on soil ecosystems. We assessed the effects of Bt and non-Bt corn plants and their residues on rhizospheric and bulk soil eubacterial communities by means of DGGE analyses of 16S rDNA and rRNA, on the non-target mycorrhizal symbiont Glomus mosseae and on soil respiration. Microcosm experiments showed differences in rhizospheric eubacterial communities associated with the three corn lines and a significantly lower mycorrhizal colonization in Bt 176 corn roots. In greenhouse experiments differences between Bt and non-Bt plants were detected in rhizospheric eubacterial communities, both structural and functional, in culturable rhizospheric heterotrophic bacteria and in mycorrhizal colonization. Plant residues of transgenic plants, ploughed under at harvest and kept mixed with soil for up to four months, affected soil respiration, bacterial communities and mycorrhizal establishment by indigenous endophytes. The multimodal approach utilized in our work may be applied in field long-term studies aimed at monitoring the real hazard of genetically modified crops and their residues on non-target soil microbial communities.