Contamination of commercial 15N2: 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.