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

RATIONALE The emission of dinitrogen (N₂) gas from soil is the most poorly constrained flux in terrestrial nitrogen (N) budgets because the high background atmospheric N₂ concentration makes soil N₂ emissions difficult to measure. In this study, we tested the theoretical and analytical feasibility of using the N₂/Ar technique to measure soil-atmosphere N₂ fluxes. METHODS Dual inlet isotope ratio mass spectrometry was used to measure δAr/N₂ 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 N₂, and then measured the change in the N₂/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 N₂/Ar ratio to constrain physical effects on the measured N₂ flux. RESULTS In dry sand, an actual N₂ 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 N₂ fluxes. CONCLUSIONS We conclude that our application of the N₂/Ar technique to soil surface fluxes is valid only above a detection limit of approximately 108 mg N m ^-2 day^-1. The N₂/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 N₂ fluxes.