Nitrate isotopes (δ15N,δ18O) in precipitation: best practices from an international coordinated research project

Stable isotope ratios of nitrogen and oxygen (15N/14N and 18O/16O)of nitrate (NO3 - ) are excellent tracers for developing systematicunderstanding of sources, conversions, and deposition of reactiveatmospheric nitrogen (Nr) in the environment. Despite recentanalytical advances, standardized sampling of NO3 - ) isotopes inprecipitation is still lacking. To advance atmospheric studies on Nrspecies, we propose best-practice guidelines for accurate andprecise sampling and analysis of NO3 - isotopes in precipitationbased on the experience obtained from an international researchproject coordinated by the International Atomic Energy Agency(IAEA). The precipitation sampling and preservation strategiesyielded a good agreement between the NO3 - concentrationsmeasured at the laboratories of 16 countries and at the IAEA.Compared to conventional methods (e.g., bacterialdenitrification), we confirmed the accurate performance of thelower cost Ti(III) reduction method for isotope analyses (15N and18O) of NO3 - in precipitation samples. These isotopic data depictdifferent origins and oxidation pathways of inorganic nitrogen.This work emphasized the capability of NO3 - isotopes to assessthe origin and atmospheric oxidation of Nr and outlined apathway to improve laboratory capability and expertise at aglobal scale. The incorporation of other isotopes like 17O in Nr isrecommended in future studies.