New high precision approach for measuring ¹⁵N-N₂ gas fluxes from terrestrial ecosystems

Dinitrogen (N₂) production from denitrification and anaerobic ammonium oxidation represents a loss of reactive nitrogen (N) from terrestrial and aquatic ecosystems to the atmosphere. The large ¹⁵N additions required to detect ¹⁵N₂ production against the high atmospheric background precludes the use of the ¹⁵N tracer technique in natural terrestrial ecosystems. We present an isotope ratio mass spectrometry technique that dramatically improves the precision of ¹⁵N₂ measurements. The approach uses gas chromatography to remove oxygen and gas purification techniques to remove water vapor and trace gases that can interfere with ¹⁵N₂ analysis. The analytical precision for manual gas sample injection was 0.018‰ δ¹⁵N; this translates to a minimum detectable N₂ flux of 0.12ng-Ng^-1h^-1 over a 24h incubation using our experimental parameters. We measured denitrification-derived N₂ production rates of0.67±0.04ng Ng^-1drysoilh^-1 following the addition of 0.1μg 98 atom % ¹⁵N-NO₃^-g^-1drysoil toanaerobically-incubated soils; rates were significantly higher with the addition of 1.0μg 98 atom %¹⁵N-NO₃^-g^-1drysoil (p<0.001, n=5), averaging 1.3±0.03ngNg^-1drysoilh^-1. The N₂:N₂O ratio at 24h after ¹⁵N addition was 48±12 and 5.4±1.9 for the 0.1μg ¹⁵Ng^-1 and 1.0μg¹⁵Ng^-1 treatments, respectively. The decrease in the N₂:N₂O with the addition of only 1μg¹⁵Ng^-1 underscores the need to use very low rates of ¹⁵N addition to accurately characterize denitrification dynamics. This analytical advance will allow us to better estimate the N₂:N₂O ratio of denitrification, constrain ecosystem N budgets, and explore the mechanisms of and controls on N₂ production.