Today's conventinal agriculture still relies on the general use of pesticides. Even thought this guarantees production capabilities it also causes environmental pollution having impact on non-targeted organisms. Agricultural practices have been recognized by "The European Soil Framework Directive" as a major threat for soil biodiversity. However, without taking into consideration. recent methodological advances in microbiology, pesticide regulation at EU level relying solely on simple C and N mineralization tè-sts•. ln order to cope with this ECOFUN­-MICROBIODIV project .aim to develop and evaluate tools to estimate the impact of herbicide nicosulfuron on the function and population dynamics of broad microbial groups. This interdisciplinary project relied on experiments conducted at two different scales:(a) one settled in field (Tier II toxicity) with maize treated with x1, x2 and x5 of the recomrnended nicosulfuron dose and (b) pot experiment (Tier I) in controlled greenhouse conditions applying to the crop x10, x100 and x1000 nicosulfuron dos es. Agronomical parameters (yield of crop and weeds) and chemical monitoring (HPLC-UV) were recorded throughout experiments. Impact of herbicide on the microbial community was assessed by determining changes in the a) structure and abundance of fungal and bacterial communities (A-RISA, PLFA), b) abundance and activity of the functional communities involved in C and N cycling (qPCR, enzyme activities), c) arbuscular mycorrhizal fungal community {colonization, DGGE), d) bacterial population harboring resistant AHAS gene (pyrosequencing) and d) by monitoring adaptation of soil microflcra. Nicosulfuron dissipation curves suggested field experiment as the one conducted under ,,low nicosulfuron selective pressure". while pot as scenarion in which microbes were under "high selective pressure". While results suggested that under Tier Il scenario the abundance, diversity and the activity of soil microflora was not signifïcantly affected by nicosulfuron in Tier I scenario both bacterial and fungal communities were clearly affected at high and repeated exposures to nicosulfuron.