Application of the N 2 /Ar technique to measuring soil‐atmosphere N 2 fluxes

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Abstract
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 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. 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 © 2012 John Wiley & Sons, Ltd.

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