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

Abstract

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. Dual inlet isotope ratio mass spectrometry was used to measure δAr/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. 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. 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.