Tracing sources of nitrate in agrucultural catchment by natural stable isotopes

The report summarises the results from isotope studies performed as part of the Strategic Institute Programme ‘Watershed-oriented management of large rivers’ and the research programme INTRA – ‘Integrated risk assessment for the transport of particles, nutrients and pesticides in agricultural catchments’. Abundances of stable isotopes of nitrogen and oxygen (δ15N and δ18O) in the nitrate molecule (NO3-) were analysed in runoff from 9 small agricultural catchments in SE Norway. The main focus of the studies was to investigate the potential of the dual isotope technique of NO3- to classify drainage pipe water or small streams with regard to their dominant sources of NO3-. The data generally indicate that the methodology should be used with care in catchment-oriented studies, where multiple NO3- sources, a mosaic of land-use types and complex hydrology easily result in overlapping isotopic signals. On the other hand, it may still add value to more traditional chemical assays and source apportionment methods, especially in combination with N transformation studies.