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

Abstract Dinitrogen (N 2 ) production from denitrification and anaerobic ammonium oxidation represents a loss of reactive nitrogen (N) from terrestrial and aquatic ecosystems to the atmosphere. The large 15 N additions required to detect 15 N 2 production against the high atmospheric background precludes the use of the 15 N tracer technique in natural terrestrial ecosystems. We present an isotope ratio mass spectrometry technique that dramatically improves the precision of 15 N 2 measurements. The approach uses gas chromatography to remove oxygen and gas purification techniques to remove water vapor and trace gases that can interfere with 15 N 2 analysis. The analytical precision for manual gas sample injection was 0.018‰ δ 15 N; this translates to a minimum detectable N 2 flux of 0.12 ng-N g −1  h −1 over a 24 h incubation using our experimental parameters. We measured denitrification-derived N 2 production rates of 0.67 ± 0.04 ng N g −1  dry soil h −1 following the addition of 0.1 μg 98 atom % 15 N–NO 3 −  g −1  dry soil to anaerobically-incubated soils; rates were significantly higher with the addition of 1.0 μg 98 atom %  15 N–NO 3 −  g −1  dry soil ( p n  = 5), averaging 1.3 ± 0.03 ng N g −1  dry soil h −1 . The N 2 :N 2 O ratio at 24 h after 15 N addition was 48 ± 12 and 5.4 ± 1.9 for the 0.1 μg 15 N g −1 and 1.0 μg  15 N g −1 treatments, respectively. The decrease in the N 2 :N 2 O with the addition of only 1 μg  15 N g −1 underscores the need to use very low rates of 15 N addition to accurately characterize denitrification dynamics. This analytical advance will allow us to better estimate the N 2 :N 2 O ratio of denitrification, constrain ecosystem N budgets, and explore the mechanisms of and controls on N 2 production.