Source identification of nitrate by means of stable nitrogen isotopes in the river Daugava and loads of nitrogen to the Gulf of Riga

Abstract

ABSTRACTNitrogen isotope ratio of nitrate provides a powerful tool to investigate nitrate sources and cycling mechanisms. Although the use of an isotope ratio method for 15N/14N allows identifying the nitrate sources in rivers by estimating a seasonal variation of N-NO3 concentration, however, there are some restrictions. Nitrification, the conversion of NH4+ to NO3-, can proceed with significant nitrogen isotope fractionation, preferentially accumulating 14N in the produced NO3-, and can make it difficult to identify the nitrate source with a high proportion of the isotope δ15N. However, the uptake and assimilation of NH4+ and NO3- have the capability of affecting isotopic compositions of riverine nitrogen compounds, and this may hinder the determination of whether the impact of the nitrate source with a high proportion of the isotope δ15N reduces. In addition, this study demonstrates that nitrate nitrogen concentration may correlate with δ15NNO3 values both positively and negatively. Such correlations are the result of isotope effects during nitrogen transformation processes (e.g. nitrification and assimilation) and isotopic variability in the various nitrate sources. A comparison of NO3- concentration and δ15NNO3 can be used to further distinguish mixing from biological processing. However, in order to get a more precise answer regarding the nitrate sources, it would be useful to take both the data of nitrogen isotopes and data of oxygen isotopes present in nitrates.