Long term response of water and nitrogen fluxes to Good Agricultural Practices at field and catchment scales

International audienceFacing the nitrate pollution problem, the European Union has encouraged a code of “Good Agricultural Practices” (GAP) in order to recover a good chemical and ecological status of waterbodies. In this study, we hypothesized that the systematic application of GAP in time and space would allow to meet the EU standard of nitrate concentration (50 mg NO3 L−1) under arable cropping systems. Water and nitrogen fluxes were determined in an agricultural catchment (187 ha) during 22 years after GAP implementation, at field and catchment scales. The aquifer outlet is a set of springs which were monitored for water flow and nitrate concentration. GAP management mainly consisted in adjusting N fertilization rates and establishing catch crops. Crop N uptake, soil water and mineral N were measured respectively two and three times per year on 36 sites representing soil variability. These data were used to initialize the STICS model which simulated the fluxes of infiltrated water and nitrate leached below the rooting zone at field scale. The elementary fluxes (calculated for each site-year) were then used as independent inputs of the hydrological model MODCOU which made the integration at catchment scale. Simulations of agricultural scenarios allowed to calculate the cost/efficiency ratio of GAP implementation per soil type.The mean amounts of infiltrated water and N leached calculated below rooting depth in agricultural fields were 179 mm yr−1 and 19 kg N ha−1 yr−1, respectively, yielding a mean weighted nitrate concentration of 41 mg NO3 L−1 during 22 years over the whole catchment. The mean residence time of water in the catchment was estimated at 17–22 years using tritium and CFC tracers. The observed nitrate concentration in the main spring declined 11 years after GAP implementation and levelled off to 49 mg NO3 L−1. The agro-hydrological model satisfactorily predicted water flow and nitrate concentration in springs but overestimated their response time. It predicted a positive impact of GAP application on water quality. GAP management appeared to be efficient on the long term with a low cost/efficiency ratio. A more flexible and motivating management could consist in including GAP as a first step in a loop progress towards agro-ecological systems