Technical note: Gas Chromatography vs. Mid-Infrared Laser Absorption Spectroscopy: A comparison of methods for measuring greenhouse gas fluxes from arable soils 

Abstract. For the study of soil-atmosphere exchange of green-house gases, a commonly adopted method is to monitor the change of gas concentrations in closed chambers. Accurate determination of CO₂, CH₄, and N₂O concentrations is therefore essential for reliable flux estimations. This study compares two techniques to determine these gas concentrations: Gas Chromatography (GC) and mid-infrared laser absorption spectroscopy (LAS). We compared both techniques by carrying out simultaneous chamber measurements under field conditions on two separate days covering a range of fluxes. The GC method involved syringe sampling into gas-tight vials and subsequent laboratory analysis. In contrast to that, a LAS analyzer was directly connected to the chambers (tubing system) and thus enabled real-time, high-temporal resolution data. We calculated gas fluxes based on GC- and LAS-derived concentration measurements, using seven distinct flux calculation setups, including systematic variations in chamber enclosure times (30, 20 and 10 min) for LAS data. Across both measurement days, the comparison resulted in a high level of agreement for determined CO₂ fluxes with a normalized Root Mean Square Error (nRMSE): 5.79 – 16.70 %. A high level of agreement between the methods was also observed for N₂O fluxes (nRMSE: 14.63 – 24.64 %). In contrast, there was a comparatively low agreement between methods for CH₄ fluxes (nRMSE: 88.42 – 94.54 %). N₂O and CH₄ fluxes highlighted the superior precision of LAS, as it detected significant fluxes (> minimum detectable flux) that were not significant with GC. For CH₄ this explains the low agreement between methods regarding arable soils that are dominated by (low) CH₄-consumption fluxes.