Quantification of diffuse nitrate inputs into a small river system using stable isotopes of oxygen and nitrogen in nitrate

To identify and quantify diffuse nitrate inputs into a river sub basin in Mecklenburg-Vorpommern (Germany) a dual-isotope approach with δ15N and δ18O in nitrate was carried out from October 2002 to March 2003. Three nitrate sources (water from artificially drained agricultural soils, groundwater, atmospheric deposition) and the river were sampled monthly to bimonthly. Nitrate of the drainage water had a concentration weighted mean (cwm) δ15N value of 10.4‰ and δ18O of 4.7‰, and was significantly different to the groundwater nitrate (cwm δ15N = 0.6‰; δ18O = 1.4‰). The low δ18O values indicated that most of the nitrate from these sources was formed during the nitrification process of soil organic N. Nitrate of atmospheric deposition had a cwm δ15N value of 0.1‰ and a δ18O value of 51.7‰. River nitrate showed cwm values of 9.0‰ in δ15N and 6.0‰ in δ18O close to the isotope values of the drainage water nitrate. The isotope values were used in a three source mixing-model, to determine the contribution of each sampled nitrate source to the total river nitrate. The mixing-model revealed that the nitrate from the drainage water contributed 86% of the river nitrate. Contribution of nitrate from groundwater and atmospheric deposition was 11% and 3%, respectively. These results agree with estimations of nitrate input data for this sub basin given by a nutrient emissions model.