Towards a global interpretation of dual nitrate isotopes in surface waters

Modern anthropogenic activities have significantly increased nitrate (NO3-) concentrations in surface waters. Stable isotopes (delta N-15 and delta O-18) in NO3- offer a tool to deconvolute some of the human-made changes in the nitrogen cycle. They are often graphically illustrated on a template designed to identify different sources of NO3- and denitrification. In the two decades since this template was developed, delta N-1(5)- and delta O-1(8)-NO3- have been measured in a variety of ecosystems and through the nitrogen cycle. However, its interpretation is often fuzzy or complex. This default is no longer helpful because it does not describe surface water ecosystems well and biases researchers towards denitrification as the NO3- removal pathway, even in well oxygenated systems where denitrification is likely to have little to no influence on the nitrogen cycle. We propose a different scheme to encourage a better understanding of the nitrogen cycle and interpretation of NO3- isotopes. We use a mechanistic understanding of NO3- formation to place bounds on the oxygen isotope axis and provide a means to adjust for different environmental water isotope values, so data from multiple sites and times of year can be appropriately compared. We demonstrate that any interpretation of our example datasets (Canada, Kenya, United Kingdom) show clear evidence of denitrification or a mixture of NO3- sources simply because many data points fall outside of arbitrary boxes which cannot be supported once the range of potential delta O-1(8)-NO(3)(- )values has been considered.