LAUSR

Abstract The relationships between microbial communities biodiversity and soil functions are difficult to identify. In most cases, the establishment of these links requires alteration of microbial communities. In this work, we deliberately chose to subject soils to an unrealistic dose of biocide (fungicide 1000 times the recommended dose) in order to modify soil microbial communities. Two different soils were chosen from a luvisol, with contrasting land-uses: a long-term arable cropping plot (CC) and a permanent grassland (PG). Microbial communities were assessed by the measurement of the total microbial biomass via the quantification of total soil DNA. Total bacterial and fungal abundances and different bacterial taxa were measured by qPCR of rRNA genes. Results showed that fungicide stress significantly reduced total microbial and fungal biomass in PG and CC soils and decreased bacterial biomass only in CC soil. In both soils, fungicide stress decreased the abundance of Acidobacteria and Bacteroidetes. The Proteobacteria groups were insensitive or increased after fungicide addition. Enzyme activities and soil carbon mineralization were evaluated. In both soils, fungicide stress reduced cellulase, β-glucosidase and dehydrogenase activities, while N-acetyl-glucosaminidase was not affected. Xylanase activity was not affected in PG soil and it was inhibited in CC soil. Changes in structural and functional profiles of microbial communities were not correlated in the PG soil and were negatively correlated in the CC soil. To conclude, this work comfirms that the responses of microbial communities to stress depend on the nature of applied stress, measured functions and initial state of microbial communities.