Impact of multi-resistant transgenic deBacillus thuringiensisBt maize on straw decomposition and the involved microbial communities

A general concern associated with the use of transgenic Bt maize is its possible negative influence on non-target organisms and ecosystem functions such as organic matter transformation. Our study was carriedout to assess the impact of the multi-resistant Bt maize hybrid MON 89034 × MON 88017 on straw decom-position and the residue-colonizing microbial communities in comparison to the near-isogenic controland two conventionally bred varieties. Straw decomposition was analyzed by a substrate-induced res-piration method and monitoring of the complete decomposition after14C pulse labelling of the maizeplants. Both approaches indicated significant differences in the decomposition rate between the conven-tional varieties, but the transgenic Bt hybrid and its near-isogenic control could not be distinguished.Potential effects on the residue-colonizing bacterial and fungal communities were analyzed by the quan-tification of metabolically active microbial groups using taxon-specific primers and T-RFLP analyses ofthe small-subunit ribosomal RNA gene. Results obtained over three years and three sampling dates didnot reveal significant differences between the transgenic hybrid and the control. The abundance of themetabolically active microbial groups varied between the varieties only in some cases and temporarily restricted. Similarly, T-RFLP analysis did not show an impact of the plant genotype on the bacterial and fungal community structure. In contrast, seasonal effects given by different sampling dates as well asvarying soil properties within the field trial were identified as drivers of the microbial communities colonizing the rotting maize straw. Conclusively, the multi-resistant Bt hybrid MON 89034 × MON 88017 did not indicate an adverse impact on straw decomposition and the involved microbial communities.