Environmental Context. Nitrogen is an essential element for all living organisms, and its biogeochemical cycle is connected to the cycling of carbon, sulfur, phosphorous, oxygen, and trace metals. Measurement of the isotopic composition of ammonium (NH₄⁺) and nitrate (NO₃^-) containing samples provides a better understanding of the nitrogen cycle. While the established ‘ammonium diffusion’ measurement has many advantages, it is not easy for inexperienced people to prepare samples. This paper shows how the method can be simplified, ideally for samples freshly collected from the field.

Abstract. Several methods have been developed for nitrogen isotope measurements on ammonium (NH₄⁺) and nitrate (NO₃^-) in solid or aqueous samples. We have tested the accuracy and reproducibility of the ammonium diffusion method for δ¹⁵N measurements on NH₄⁺ and NO₃^- and compared this technique to other established methods. Our results show that the ammonium diffusion technique is capable of generating accurate and reproducible δ¹⁵N values for minute quantities of NH₄⁺-N and NO₃^--N in aqueous samples, if sufficient care is taken to minimize nitrogen blanks and to optimize the extraction procedure. Hence, the ammonium diffusion method offers an attractive alternative to more labour-intensive and costly methods for determining nitrogen isotope ratios of NH₄⁺ and NO₃^- in aqueous samples.