Effects of nitrate contamination and seasonal variation on the denitrification and greenhouse gas production in La Rocina Stream (Doñana National Park, SW Spain)

Climatic influence (global warming and decreased rainfall) could lead to an increase in the ecological and toxicological effects of the pollution in aquatic ecosystems, especially contamination from agricultural nitrate (NO3−) fertilizers. Physicochemical properties of the surface waters and sediments of four selected sites varying in NO3− concentration along La Rocina Stream, which feeds Marisma del Rocio in Doñana National Park (South West, Spain), were studied. Electrical conductivity, pH, content in macro and microelements, total organic carbon and nitrogen, and dissolved carbon and nitrogen were affected by each sampling site and sampling time. Contaminant NO3− in surface water at the site with the highest NO3− concentration (ranged in 61.6–106.6 mg L−1) was of inorganic origin, most probably from chemical fertilizers, as determined chemically (90% of the total dissolved nitrogen from NO3−) and by isotopic analysis of δ15N-NO3−. Changes in seasonal weather conditions and hydrological effects at the sampling sites were also responsible for variations in some biological activities (dehydrogenase, β-glucosidase, arylsulphatase, acid phosphatase and urease) in sediments, as well as in the production of the greenhouse gases CO2, CH4 and N2O. Both organic matter and NO3− contents influenced rates of gas production. Increased NO3− concentration also resulted in enhanced levels of potential denitrification measured as N2O production. The denitrification process was affected by NO3− contamination and the rainfall regimen, increasing the greenhouse gases emissions (CO2, CH4 and especially N2O) during the driest season in all sampling sites studied.