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

Abstract

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.

Introduction

Although inputs of nutrients have been reduced in the recent years, the excessive load of nitrogen and phosphorous is still one of the major ecological problems of the Baltic Sea (Stålnacke et al., 1999). In 2000 an amount of 814 × 10³ t nitrogen entered the Baltic Sea, and more than 84% of the total N load derived from rivers (HELCOM, 2003). Worldwide, almost 70% of the riverine N load consists of dissolved organic nitrogen (DON; Meybeck, 1982), but experiments showed that its bioavailability may be as low as 2–16% (Stepanauskas and Leonardson, 1999), while nitrate is the dominant inorganic N-species lost to the aquatic environment (Addiscott et al., 1992) and is rapidly consumed. Diffuse nitrate inputs such as fertilizer runoff from farmland, atmospheric deposition and groundwater input are especially hard to identify, because they are emitted over large areas. Nowadays, a powerful tool to distinguish different nitrate sources is the determination of stable isotope ratios of nitrogen and oxygen (Wassenaar, 1995, Spoelstra et al., 2001, Chang et al., 2002, Mayer et al., 2002, Piatek et al., 2005). The δ¹⁵N values of nitrate from different sources often show overlapping ranges, but the additional measurement of the δ¹⁸O values allows a more precise classification (Mayer et al., 2002). Nitrate, derived from sewage and manure, is isotopically distinct from atmospheric nitrate in δ¹⁵N (7–20‰, −10‰ to 8‰, respectively), as well as in δ¹⁸O (<15‰ compared to 25–75‰; Wassenaar, 1995, Kendall, 1998). Nitrate originating from mineral fertilizers shows δ¹⁵N values of 0 ± 4‰ (Kendall, 1998), and δ¹⁸O values of 22 ± 3‰ (Amberger and Schmidt, 1987) because of their production from atmospheric nitrogen (δ¹⁵N = 0‰) and oxygen (δ¹⁸O = 23.5‰). However, the isotopic composition of nitrate collected in drainage tiles and runoff ditches does not reflect exactly the isotope values of the fertilizer applied, but is altered due to isotope fractionation processes (Kendall and Aravena, 1999). Flipse and Bonner (1985) demonstrated that groundwater nitrate produced under fertilized fields showed δ¹⁵N values up to 12.4‰ higher than the fertilizers applied, and explained this difference as the volatile loss of ammonia from the fertilizer, containing reduced nitrogen forms. Another fractionation process is denitrification, which increases the δ¹⁵N and δ¹⁸O values of the residual nitrate with an enrichment of δ¹⁸O:δ¹⁵N close to 1:2 (Böttcher et al., 1990). Nitrate uptake by plants (Högberg, 1997) and soil N-mineralization, including ammonification and nitrification may also modify the isotope signature of nitrate (Iqbal et al., 1997, Mayer et al., 2001).

The objective of this study was to test whether the isotopic composition of nitrate from three diffuse nitrate sources can be used to quantify the diffuse nitrate inputs into a sub basin of the Warnow River (444 km²), which also supplies the city of Rostock (200,000 inhabitants) with drinking water. The three nitrate sources, drainage water from fertilized fields, groundwater, and atmospheric deposition, as well as the river itself were sampled regularly from October 2002 to March 2003. The samples were analysed for nitrate concentration, δ¹⁵N and δ¹⁸O values. The data were used in a three source mixing-model (Phillips and Koch, 2002) to determine the percentage of every sampled nitrate source to the river nitrate. For the successful application of a conservative isotope mixing-model, it is necessary that the isotope values of the river nitrate are not altered due to fractionation processes. For this purpose, sampling was carried out during late fall and winter. At low water temperature microbial activity is reduced (Pfenning and McMahon, 1996) and therefore alteration of the river nitrate isotope values due to fractionation processes is minimal. The usefulness of this approach as an additional method for nitrate source quantification beside model estimations is discussed.