Impact of long-term cropping of glyphosate-resistant transgenic soybean [Glycine max (L.) Merr.] on soil microbiome.

The transgenic soybean [Glycine max (L.) Merrill] occupies about 80 % of the global area cropped with this legume, the majority comprising the glyphosate-resistant trait (Roundup Ready(®), GR or RR). However, concerns about possible impacts of transgenic crops on soil microbial communities are often raised. We investigated soil chemical, physical and microbiological properties, and grain yields in long-term field trials involving conventional and nearly isogenic RR transgenic genotypes. The trials were performed at two locations in Brazil, with different edaphoclimatic conditions. Large differences in physical, chemical and classic microbiological parameters (microbial biomass of C and N, basal respiration), as well as in grain production were observed between the sites. Some phyla (Proteobacteria, Actinobacteria, Acidobacteria), classes (Alphaproteobacteria, Actinomycetales, Solibacteres) and orders (Rhizobiales, Burkholderiales, Myxococcales, Pseudomonadales), as well as some functional subsystems (clustering-based subsystems, carbohydrates, amino acids and protein metabolism) were, in general, abundant in all treatments. However, bioindicators related to superior soil fertility and physical properties at Londrina were identified, among them a higher ratio of Proteobacteria:Acidobacteria. Regarding the transgene, the metagenomics showed differences in microbial taxonomic and functional abundances, but lower in magnitude than differences observed between the sites. Besides the site-specific differences, Proteobacteria, Firmicutes and Chlorophyta were higher in the transgenic treatment, as well as sequences related to protein metabolism, cell division and cycle. Although confirming effects of the transgenic trait on soil microbiome, no differences were recorded in grain yields, probably due to the buffering capacity associated with the high taxonomic and functional microbial diversity observed in all treatments.

Impact of the ahas transgene and of herbicides associated with the soybean crop on soil microbial communities.

Although Brazil has recently reached the position as the second largest producer of genetically modified soybean [Glycine max (L.) Merr.], there are few reports on the effects of transgenic crops and the associated use of specific herbicides on soil microbial communities, both under the edaphoclimatic conditions in Brazil, and in other producer regions in the southern hemisphere. The aim of this study was to evaluate the effects of transgenic soybean containing the ahas gene conferring resistance to herbicides of the imidazolinone group, and of the herbicides associated with transgenic soybeans on the soil microbial community. Twenty field experiments were carried out during three growing seasons (summer of 2006/2007, short-season of 2007 and summer of 2007/2008), in nine municipalities located in six Brazilian states and in the Federal District. The experiments were conducted using a completely randomized block design with four replicates and three treatments: (1) conventional (non-transgenic) soybean cultivar Conquista with conventional herbicides (bentazone + acifluorfen-sodium and other herbicides, depending on the level of infestation in each region); (2) near-isogenic transgenic Cultivance (CV127) containing the ahas gene, with conventional herbicides; (3) transgenic Cultivance with specific herbicide of the imidazolinone group (imazapyr). As the objective of the study was to verify impacts of the transgene and herbicides on the soil microbial community of the whole area and not only a punctual rhizospheric effects, samples were taken at the 0-10 cm layer prior to cropping and at R2 soybean growth stage, between plant rows. Quantitative (microbial biomass C and N, MB-C and MB-N) and qualitative (DGGE of the 16S rDNA region) parameters of soil microbial community were evaluated. No qualitative or quantitative differences were found that could be attributed to the transgene ahas. A comparison of Cultivance soybean with conventional and imidazolinone-group herbicides applications also failed to reveal differences that could be attributed to the specific use of imazapyr, even after three consecutive croppings at the same site. Finally, no differences were detected between conventional (Conquista and conventional herbicides) and transgenic soybean managements (Cultivance and imazapyr). However, marked differences were observed in MB-C and MB-N between the different sites and times of year and, for the 16S rDNA-DGGE profiles, between different sites. In conclusion, microbial community evaluations were found to be sensitive and viable for monitoring different technologies and agricultural management methods, but no differences could be attributed to the ahas transgene for three consecutive cropping seasons.