1 [1]{Université Rennes 1CNRS, CAREN Research federation UMR 6118 Géosciences, 5 Campus de Beaulieu, Avenue Leclerc, Rennes, France} [2]{Université Rennes 1CNRS, CAREN Research federation UMR 6553 EcoBio, Campus de Beaulieu, Avenue Leclerc, Rennes, France} [3]{Université Européenne de Bretagne, UEB (European University of Brittany)} Correspondence to: L. Aquilina (!"#) 10 Keywords: hydrological flowpaths – microbial activity nitrate reduction – peat Factors influencing the dynamics of nitrate and sulphate concentration observed in a south Normandy peatland were determined experimentally. The effects of high or low nitrate input, 15 and oxic or anoxic conditions on microbial activity were investigated in bioreactors, using peat samples from field sites influenced by different hydrologic regimes. Site S, unlike site G, was characterized by the presence of hydrogeological gradients inducing water fluxes from stream to peat during most of the hydrological cycle. Peat samples from both sites were subjected to similar experimental conditions to distinguish between the chemical effects 20 (NO 3 ,O 2 concentrations) and the physical effects (hydrologic regimes and peat soil moisture). [Cl ], [SO 4 2 ] and [NO 3 ] were monitored for 240 hours. Nitrate was significantly reduced in most experiments: (1) Removal of 70% of the initial nitrate content after 51 hours under anoxic conditions; (2) Complete nitrate reduction after 240 hours in soil from the S site. This 25 reduction was interpreted as heterotrophic denitrification. Sulphate monitoring revealed that 400 mg.L 1 were produced in peat from site S under aerobic conditions. Sulphate changes under anaerobiosis were not significant or, for samples from G, under any conditions. Clear differences in chloride content (deviance analysis, P<0.05), sulphate concentration and nitrate consumption dynamics (deviance analysis, P<0.0001) were observed between the G and S 30 sites. Our results demonstrate that the rates of nitrate removal and sulphate production differ