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 delta N-15 and delta O-18 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) delta N-15 value of 10.4%. and delta O-18 of 4.7 parts per thousand, and was significantly different to the groundwater nitrate (cwm delta N-15 = 0.6 parts per thousand; delta O-18 = 1.4 parts per thousand). The low delta O-18 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 delta N-15 value of 0.1 parts per thousand and a delta O-18 value of 51.7 parts per thousand. River nitrate showed cwm values of 9.0 parts per thousand in delta N-15 and 6.0 parts per thousand in delta O-18 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. (c) 2006 Elsevier Ltd. All rights reserved.