Nicosulfuron application in agricultural soils drives the selection towards NS-tolerant microorganisms harboring various levels of sensitivity to nicosulfuron

The action mode of sulfonylurea herbicides is the inhibition of the acetohydroxyacid synthase (AHAS) required for the biosynthesis of amino acids valine and isoleucine in plants. However, this enzyme is also present in a range of non-targeted organisms, among which soil microorganisms are known for their pivotal role in ecosystem functioning. In order to assess microbial toxicity of sulfonylurea herbicide nicosulfuron (NS), a tiered microcosm (Tier I) to field (Tier II) experiment was designed. Soil bacteria harboring AHAS enzyme tolerant to the herbicide nicosulfuron were enumerated, isolated, taxonomically identified, and physiologically characterized. Results suggested that application of nicosulfuron drives the selection towards NS-tolerant bacteria, with increasing levels of exposure inducing an increase in their abundance and diversity in soil. Tolerance to nicosulfuron was shown to be widespread among the microbial community with various bacteria belonging to Firmicutes (Bacillus) and Actinobacteria (Arthrobacter) phyla representing most abundant and diverse clusters. While Arthrobacter bacterial population dominated community evolved under lower (Tier II) nicosulfuron selection pressure, it turns out that Bacillus dominated community evolved under higher (Tier I) nicosulfuron selection pressure. Different NS-tolerant bacteria likewise showed different levels of sensitivity to the nicosulfuron estimated by growth kinetics on nicosulfuron. As evident, Tier I exposure allowed selection of populations able to better cope with nicosulfuron. One could propose that sulfonylureas-tolerant bacterial community could constitute a useful bioindicator of exposure to these herbicides for assessing their ecotoxicity towards soil microorganisms. © 2015, Springer-Verlag Berlin Heidelberg

A tiered assessment approach based on standardized methods to estimate the impact of nicosulfuron on the abundance and function of the soil microbial community

Pesticides impact soil microorganisms in various ways. Despite the pivotal role of the latter in ecosystem functioning, the assessment of pesticides soil microbial toxicity is lagging behind the recent methodological advances in microbial ecology. We investigated the impact of nicosulfuron, a low dose sulfonylurea herbicide, on the structure, abundance and function of the soil microbial community using standardized methodologies (PLFAs, taxa-specific qPCR and enzyme activities). For this purpose a Tiered approach involving assessment i) at extreme, long term (five repeated application cycles) exposure schemes in a microcosm experiment conducted under greenhouse conditions (x0, x10, x100 and x1000 the recommended dose, Tier I) and ii) at realistic field exposure scenarios (x0, x1, x2 and x5, Tier II) was followed. Significant reductions in the abundance of Gram negative (beta-proteobacteria, planctomycetes) and Gram positive bacteria (actinobacteria) were indicated by both PLFA and qPCR analyses at low soil concentrations of nicosulfuron (0.25-1 mu g g(-1)), while a reduction of fungi at equally low levels of nicosulfuron in soil was found only by qPCR analysis. C- and P-cycling enzymes were particularly sensitive even at low soil concentration of the herbicide (0-1 mu g g(-1)). In contrast, no inhibitory effects of nic-osufluron at field conditions were found. The only exception was cellobiohydrolase which were impaired at herbicide rates higher than the recommended. We suggest that the use of a tiered microcosm-to-field experimentation combined with the application of standardized methodologies could provide a comprehensive assessment of the soil microbial toxicity of pesticides. (C) 2014 Elsevier Ltd. All rights reserved