Upscaling nitrogen removal processes in fluvial wetlands and irrigation canals in a patchy agricultural watershed

The evaluation of nitrogen (N) removal in aquatic ecosystems within human exploited watersheds may allow the setting of upper limits of N use in agriculture. However, such an evaluation is complex owing to the small scale heterogeneity of aquatic ecosystems, including dominant vegetation and inherent N-related processes. In this work, microbial denitrification and primary producer assimilation were estimated in fluvial wetlands and irrigation canals of the Mincio River watershed (~ 850 km2, Italy), and scaled-up to the whole watershed using GIS and remote-sensing techniques. Denitrification in the fluvial wetland area which covered only ~ 3% of the watershed was estimated to permanently remove 80% of the excess N, defined as N soil surplus (~ 5500 t N y−1) minus N river export (~ 1300 t N y−1). Adding the estimated N uptake by primary producers in the wetland system, approximately all the N excess produced in adjacent agricultural land of the Mincio watershed can be retained or removed by internal processes. In contrast, the canal network had a linear extent of over 1770 km but its estimated denitrification capacity was comparatively much smaller (up to 7% of the excess N). Combinations of N-budgeting at the whole basin scale, experimental data, GIS and remote sensing techniques offer the possibility to analyze N retention capacity in heterogeneous aquatic environments. The application of such approach to the Mincio watershed stresses the functional relevance of even small wetland areas in agricultural settings.