Effect of long-term herbicide applications on the bacterial community structure and function in an agricultural soil.

Little is known about the chronic effect of herbicides on the soil microbial community, with most studies focusing on acute impacts. In this study, we investigated the effect of 20 years of atrazine and metolachlor application on the community structure, abundance and function of bacterial groups in the bulk soil of a maize monoculture. Group-specific PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) of 16S rRNA genes was used to characterize the composition of the microbial community. Primers specific for the entire bacterial group, as well as for the Acidobacterium group, the actinomycetes, the ammonium oxidizers, and the Type I and II methanotrophs were used in the PCR-DGGE analysis. Cluster analysis of the DGGE patterns obtained with the different primer sets differentiated between the herbicide-treated and the non-treated control soil. In particular the methanotrophic community differed, with three bands (phylotypes) being absent (or showing a lower intensity) in the DGGE patterns of the herbicide-treated soils compared to the patterns of the control soil. The differences in the methanotrophic community structure were not reflected in the methane oxidation capacity, which was similar for the two treatments. The prevalence of methanotrophs as evaluated with real-time PCR analysis also did not differ between the herbicide-treated and non-treated soil. Our results indicate that the long-term use of the herbicides atrazine and metolachlor resulted in an altered soil community structure, in particular for the methanotrophic bacteria. These observed changes did not cause a decreased community function (methane oxidation), probably because the total abundance of the methanotrophs in the soil system was preserved.