Application of the N2/Ar technique to measuring soil-atmosphere N2 fluxes

RATIONALE: The emission of dinitrogen (N-2) gas from soil is the most poorly constrained flux in terrestrial nitrogen (N) budgets because the high background atmospheric N-2 concentration makes soil N-2 emissions difficult to measure. In this study, we tested the theoretical and analytical feasibility of using the N-2/Ar technique to measure soil-atmosphere N-2 fluxes. METHODS: Dual inlet isotope ratio mass spectrometry was used to measure dAr/N-2 values of gas sampled from surface flux chambers. In laboratory experiments using dry sand in a diffusion box, we induced a known steady-state flux of N-2, and then measured the change in the N-2/Ar ratio of chamber headspace air samples to test our ability to reconstruct this flux. We m\ odeled solubility, thermal, and water vapor flux fractionation effects on the N-2/Ar ratio to constrain physical effects on the measured N-2 flux. RESULTS: In dry sand, an actual N-2 flux of 108 mg N m(-2) day(-1) was measured as 111 +/- 19 mg N m(-2) day(-1) (+/- standard error (SE)). In wet sand, an actual N-2 flux of 160 mg N m(-2) day(-1) was measured as 146 +/- 20 mg N m(-2) day(-1) when solubility and water vapor flux fractionation were taken into account. Corrections for thermal fractionation did not improve estimates of N-2 fluxes. CONCLUSIONS: We conclude that our application of the N-2/Ar technique to soil surface fluxes is valid only above a detection limit of approximately 108 mg N m(-2) day(-1). The N-2/Ar method is currently best used as a validation tool for other methods in ecosystems with high soil N-2 fluxes, but, with future improvements, it holds promise to provide high-resolution measurements in systems with low soil N-2 fluxes. Copyright (C) 2012 John Wiley & Sons, Ltd.