High N retention in Mediterranean catchments enhanced by water management practices

Estela Romero*1,2, Josette Garnier1,3, Gilles Billen1,3, Franz Peters2, Luis Lassaletta1,4

1 Université Pierre et Marie Curie (UPMC), UMR 7619 Metis, Paris, 75005, France. Email: [email protected]

2 Institut de Ciències del Mar (CSIC), Barcelona, 08003, Spain

3 Centre National de la Recherche Scientifique (CNRS), UMR 7619 Metis, Paris, 75005, France

4 PBL, Netherlands Environmental Assessment Agency, 3721 MA, Bilthoven, The Netherlands

Abstract

The share of nitrogen (N) that is exported to the sea or accumulated on land (N retention, sensu lato) involves different environmental processes; coastal eutrophication and anoxia in the first case; the acidification of soils, the emission of ammonia and greenhouse gases, and the pollution of aquifers in the latter. Nevertheless, the factors involved in N retention are still poorly constrained, particularly in arid and semi-arid systems. The present study evaluates the N fluxes of 38 catchments on the Iberian Peninsula with contrasting climatic characteristics (temperate and Mediterranean), land uses, and water management practices. The contribution of physical and socio-ecological factors in the retention of N was partitioned, and the link between N retention and water regulation was explored. We hypothesize that the extreme flow regulation performed in the Mediterranean enhances the high N retention values associated with arid and semi-arid regions. Our 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, and notably in arid and semi-arid areas. Increased water demands will likely lead to a higher flow regulation, and the situation may resemble that of Mediterranean Iberian Peninsula catchments. High N retention and the associated environmental risks must therefore be considered as an important consequence of water regulation practices, and must be adequately managed.