Nitrogen retention in natural Mediterranean wetlands affected by agricultural runoff

Abstract

Abstract. Nitrogen retention efficiency in natural Mediterranean wetlands affected by agricultural runoff was quantified and the effect of season and hydrological/chemical loading was examined from March 2007 to June 2008 in two wetland-streams located in Southeast Spain. Nitrate-N (NO3−-N), ammonium-N (NH4+-N), total organic nitrogen-N (TON-N) and chloride (Cl−) concentrations were analyzed to calculate nitrogen retention efficiencies. These wetlands consistently reduced water nitrogen concentration throughout the year with higher values for NO3−-N (72.3%), even though the mean values of inflow NO3−-N concentrations were above 20 mg l−1. Additionally, they usually acted as sinks for TON-N (45.4%), but as sources for NH4+-N. Over the entire study period, the Taray and Parra wetlands were capable of removing a mean value of 1.6 and 0.8 kg NO3−-N a day−1, respectively. Retention efficiencies were not affected by temperature variation and did not follow a seasonal pattern. The temporal variability for NO3−-N retention efficiency was positively and negatively explained by the net hydrologic retention and the inflow NO3−-N concentration (R2adj=0.832, p<0.001), respectively. TON-N retention efficiency was only positively explained by the net hydrologic retention (R2adj=0.1997, p<0.05). No significant regression model was found for NH4+-N. Finally, the conservation of these Mediterranean wetland-streams may act as a tool to not only improves the surface water quality in agricultural catchments, but to also achieve a good ecological status for surface waters, this being the Water Framework Directive's ultimate purpose.