title: Water management practices exacerbate nitrogen retention in Mediterranean catchments.
abstract: Nitrogen (N) retention sensu lato refers to all processes preventing new reactive nitrogen brought into watersheds through agricultural or industrial activities to be exported by river systems to the sea. Although such processes protect marine systems from the threat of eutrophication and anoxia, they raise other environmental issues, including the acidification of soils, the emission of ammonia and greenhouse gases, and the pollution of aquifers. Despite these implications, the factors involved in N retention are still poorly controlled, particularly in arid and semi-arid systems. The present study evaluates the N fluxes of 38 catchments in the Iberian Peninsula with contrasting climatic characteristics (temperate and Mediterranean), land uses, and water management practices. This diversity allows addressing the contribution of physical and socioecological factors in N retention, and more specifically, exploring the relation between N retention and water regulation. We hypothesise that the extreme flow regulation implemented in the Mediterranean enhances the high N retention values associated with arid and semi-arid regions. The results show that reservoirs and irrigation channels account for >50% of the variability in N retention values, and above a certain regulation threshold, N retention peaks to values >85-90%. Future climate projections forecast a decrease in rainfall and an increase in agricultural intensification and irrigation practices in many world regions, most notably in arid and semi-arid areas. Increased water demand will likely lead to greater flow regulation, and the situation in many areas may resemble that of Iberian Mediterranean catchments. High N retention and the associated environmental risks must therefore be considered and adequately addressed.
authors: Université Pierre et Marie Curie (UPMC), UMR 7619 Metis, Paris 75005, France; Institut de Ciències del Mar (CSIC), Barcelona 08003, Spain. Electronic address: estela.romero@upmc.fr.
authors: Estela
authors: E
authors: Romero
authors: Université Pierre et Marie Curie (UPMC), UMR 7619 Metis, Paris 75005, France; Centre National de la Recherche Scientifique (CNRS), UMR 7619 Metis, Paris 75005, France.
authors: Josette
authors: J
authors: Garnier
authors: Université Pierre et Marie Curie (UPMC), UMR 7619 Metis, Paris 75005, France; Centre National de la Recherche Scientifique (CNRS), UMR 7619 Metis, Paris 75005, France.
authors: Gilles
authors: G
authors: Billen
authors: Institut de Ciències del Mar (CSIC), Barcelona 08003, Spain.
authors: Franz
authors: F
authors: Peters
authors: Université Pierre et Marie Curie (UPMC), UMR 7619 Metis, Paris 75005, France; PBL, Netherlands Environmental Assessment Agency, 3721 MA Bilthoven, The Netherlands.
authors: Luis
authors: L
authors: Lassaletta
keywords: Irrigation; Mediterranean climate; N retention; Reservoirs; River basin; Water regulation