In the Seine Basin, characterised by intensive arable crops, most of the surface and groundwater is contaminated by nitrate (NO3-). The goal of this study is to investigate nitrogen leaching on commercial arable crop farms in five organic and three conventional systems. In 2012-2013, a total of 37 fields are studied on eight arable crop rotations, for three different soil and climate conditions. Our results show a gradient of soil solution concentrations in function of crops, lower for alfalfa (mean 2.8 mg NO3-N l(-1)) and higher for crops fertilised after legumes (15 mg NO3-N l(-1)). Catch crops decrease nitrate soil solution concentrations, below 10 mg NO3-N l(-1). For a full rotation, the estimated mean concentrations is lower for organic farming, 12 +/- 5 mg NO3-N l(-1) than for conventional farming 24 +/- 11 mg NO3-N l(-1), with however a large range of variability. Overall, organic farming shows lower leaching rates (14-50 kg NO3-N ha(-1)) than conventional farms (32-77 kg NO3-N ha(-1)). Taking into account the slightly lower productivity of organic systems, we show that yield-scaled leaching values are also lower for organic (0.2 +/- 0.1 kg N kg(-1) N year(-1)) than for conventional systems (0.3 +/- 0.1 kg N kg(-1) N year(-1)). Overall, we show that organic farming systems have lower impact than conventional farming on N leaching, although there is still room for progress in both systems in commercial farms.
