Nitrogen retention in natural Mediterranean wetland-streams affected by agricultural runoff
Abstract. Nitrogen retention efficiency in natural Mediterranean wetland-streams affected by agricultural runoff was quantified and the effect of the temporal variability and hydrological/chemical loading was examined from March 2007 to June 2008 in two wetland-streams located in Southeast Spain. Nitrate-N (NO−3-N), ammonium-N (NH+4-N), total nitrogen-N (TN-N), total organic nitrogen-N (TON-N) and chloride (Cl−) concentrations were analyzed to calculate nitrogen retention efficiencies. These wetland-streams consistently reduced water nitrogen concentration throughout the year with higher values for NO−3-N (72.3%), even though the mean value of inflow NO−3-N concentrations was above 20 mg l−1. Additionally, they usually acted as sinks for TON-N (8.4%), but as sources for NH+4-N. Over the entire study period, the Taray and Parra wetland-streams were capable of removing on average 1.6 and 0.8 kg NO−3-N a day−1, respectively. Retention efficiencies were not affected by temperature variation. NO−3-N retention efficiency followed a seasonal pattern with the highest retention values in summer (June–September). The temporal variability for NO−3-N retention efficiency was positively and negatively explained by the hydrologic retention and the inflow NO−3-N concentration (R2adj=0.815, p<0.01), respectively. No significant regression model was found for TON-N and NH+4-N. Finally, the conservation of these Mediterranean wetland-streams may help to not only improve the surface water quality in agricultural catchments, but to also achieve good ecological status for surface waters, this being the Water Framework Directive's ultimate purpose.
