Contamination of commercial ¹⁵ N ₂ : potential impacts on nitrogen fixation assays and a simple method for quantification

Quantifying rates of nitrogen (N) fixation in complex biological systems is of specific and acute interest to a wide array of ecological, agricultural, and soil scientists. N-fixation assays are commonly achieved through incubation of the target system with N₂ gas that is greatly enriched in the less abundant heavy isotope, ¹⁵N. However, contamination of ¹⁵N₂ gas supplies with more biologically available N species, such as ammonia and nitrate/nitrite can profoundly interfere with such assays and lead to false positives and/or inflated estimates of N-fixation rates. Here, we identify the ongoing contamination of commercial ¹⁵N₂ stocks and quantify the potential impact to organismal and soil N-fixation studies. Further, we provide recommendations for mitigating and correcting for the effects of contamination in gas stocks, and present a simple, rapid procedure for quantification utilizing the same instrumentation that is typically employed to analyse N isotope ratios in N-fixation studies.