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ENVIRONMENTAL IMPACTS : SOIL IMPACTSnumerical abundance of their cells. Litter placement is known for its stronginfluence on the soil decomposer communities. The effects of the addition ofcrop residues on respiration and catabolic activities of the bacterial communitywere examined in microcosm experiments. Four cultivars of Zea mays L. of twodifferent isolines (each one including the conventional crop and its Bacillusthuringiensis cultivar) and one control of bulk soil were included in theexperimental design. The growth models suggest a dichotomy between soilsamended with either conventional or transgenic maize residues. The Cry1Abprotein appeared to influence the composition of the microbial community. Thehighly enhanced soil respiration observed during the first 72 h after the additionof Bt-maize residues can be interpreted as being related to the presence of thetransgenic crop residues. This result was confirmed by agar plate counting, asthe averages of the colony-forming units of soils in conventional treatments wereabout one-third of those treated with transgenic straw. Furthermore, the additionof Bt-maize appeared to induce increased microbial consumption of carbohydratesin BIOLOG EcoPlates. Three weeks after the addition of maize residues to thesoils, no differences between the consumption rate of specific chemical guildsby bacteria in soils amended with transgenic maize and bacteria in soils amendedwith conventional maize were detectable. Reaped crop residues, comparable topost-harvest maize straw (a common practice in current agriculture), rapidlyinfluence the soil bacterial cells at a functional level. Overall, these data supportthe existence of short Bt-induced ecological shifts in the microbial communitiesof croplands’ soils.http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.002012824. Castaldini M, Turrini A, Sbrana C, Benedetti A, Marchionni M, Mocali S,Fabiani A, Landi S, Santomassimo F, Pietrangeli B, Nuti MP, Miclaus N andGiovannetti M (2005) : Impact of Bt corn on rhizospheric and soileubacterial communities and on beneficial mycorrhizal symbiosis inexperimental microcosms. Appl Environ Microbiol. 71(11) : 6719-29.A polyphasic approach has been developed to gain knowledge of suitable keyindicators for the evaluation of environmental impact of genetically modified Bt 11and Bt 176 corn lines on soil ecosystems. We assessed the effects of Bt corn(which constitutively expresses the insecticidal toxin from Bacillus thuringiensis,encoded by the truncated Cry1Ab gene) and non-Bt corn plants and their residueson rhizospheric and bulk soil eubacterial communities by means of denaturinggradient gel electrophoresis analyses of 16S rRNA genes, on the nontargetmycorrhizal symbiont Glomus mosseae, and on soil respiration. Microcosmexperiments showed differences in rhizospheric eubacterial communitiesassociated with the three corn lines and a significantly lower level of mycorrhizalcolonization in Bt 176 corn roots. In greenhouse experiments, differences betweenBt and non-Bt corn plants were detected in rhizospheric eubacterial communities(both total and active), in culturable rhizospheric heterotrophic bacteria, and in(165)