Abstract
Modern anthropogenic activities have significantly increased nitrate (NO3−) concentrations in surface waters. Stable isotopes (δ15N and δ18O) 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, δ15N- and δ18O-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 δ18O-NO3− values has been considered.
Highlights
Stable isotopes (δ15N and δ18O) in nitrate (NO3–) have been commonly measured for > 4 decades (see Heaton (1986) and papers therein)
We discuss the assumptions inherent in this figure and key improvements needed for improved understanding of NO3– isotopes in surface waters
The schematic biplot figure was originally designed for interpreting groundwater data where NO3– isotope values of different NO3– sources are preserved except bydenitrification (e.g., Böttcher et al, 1990; Aravena et al, 1993; Aravena and Robertson, 1998)
Summary
Stable isotopes (δ15N and δ18O) in nitrate (NO3–) have been commonly measured for > 4 decades (see Heaton (1986) and papers therein). Knowledge of isotope fractionation during NO3– production and consumption was summarised in Kendall (1998) yet, despite the many figures in this chapter, one figure described as “simplified” has become the ubiquitous interpretation scheme This figure visually summarises a compilation of NO3– isotope data with boxes by “dominant sources of nitrate” and encourages researchers to think only about one process, denitrification, this process may be uncommon in well oxygenated lake surfaces or streams and rivers. In this way, we need a better schematic figure that explicitly recognises the differences between NO3– sources and processes that produce and consume NO3–
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