Evaluation of a closed tunnel for field-scale measurements of nitrous oxide fluxes from an unfertilized grassland soil.

Emissions of the major greenhouse gas NO from soils are characterized by huge spatial variability. An upscaling based on conventional small-scale chamber measurements is thus questionable and may involve a considerable amount of uncertainty. In this feasibility study, we evaluated the applicability of a large, closed tunnel for field-scale measurements of NO fluxes from an unfertilized grassland soil. The tunnel, coupled to an open-path Fourier transform infrared spectrometer, covered 500 m. During a 2-yr campaign, concurrent closed-chamber measurements (area of 0.045 m) were performed at the tunnel plot. The tunnel system enabled high-density and precise NO concentration measurements under dry, stable, nocturnal atmospheric conditions, but higher wind speeds and rain limited its application. To calculate an unbiased, predeployment NO flux from the increase of NO concentrations during tunnel deployment, we propose a novel approach based on inverse modeling (IMQ0). We show that IMQ0 is appropriate for the specific non-steady state tunnel setup. Compared with conventional models, which were developed for gas flux calculation from concentration gradients measured in vented closed chambers, IMQ0 is most accurate. Whereas NO fluxes obtained from the tunnel measurements were generally small and at a typical background level, the chamber measurements revealed high spatial and temporal variability of NO emissions, including slight NO uptake and precipitation-triggered emission peaks. The cumulative NO fluxes of both methods differed by one order of magnitude and were smaller for the tunnel measurements. We argue that the chambers were occasionally susceptible to detection of hotspots and hot moments of NO emission. However, these emissions were evidently not representative for the field scale. Compared with available greenhouse gas measurement techniques, we conclude that the tunnel may serve as a gap-filling method between small-scale chamber and ecosystem-level micrometeorological techniques, particularly during stable nocturnal conditions.