Measuring the biosphere-atmosphere exchange of total reactive nitrogen by eddy covariance

摘要

The (net) exchange of reactive nitrogen (N_r) with the atmosphere is animportant driver for ecosystem productivity and greenhouse gas exchange. Theexchange of airborne N_r includes various trace compounds that usuallyrequire different specific measurement techniques, and up to now fastresponse instruments suitable for eddy covariance measurements are onlyavailable for few of these compounds.Here we present eddy covariance flux measurements with a recently introducedconverter (TRANC) for the sum of all N_r compounds(∑N_r). Measurements were performed over a managed grasslandfield with phases of net emission and net deposition of ∑N_rand alternating dominance of oxidized (NO_X) and reduced species(NH₃). Spectral analysis of the eddy covariance data exhibited theexistence of covariance function peaks at a reasonable time lag related tothe sampling tube residence time under stationary conditions. Using ogiveanalysis, the high-frequency damping was quantified to 19%–26% fora low measurement height of 1.2 m and to about 10% for 4.8 mmeasurement height.∑N_r concentrations and fluxes were compared to parallel NO andNO₂ measurements by dynamic chambers and NH₃ measurements by theaerodynamic gradient technique. The average concentration results indicatethat the main compounds NO₂ and NH₃ were converted by the TRANCsystem with an efficiency of near 100%. With an optimised sample inletalso the fluxes of these compounds were recovered reasonably well includingnet deposition and net emission phases. The study shows that the TRANCsystem is suitable for fast response measurements of oxidized and reducednitrogen compounds and can be used for continuous eddy covariance fluxmeasurements of total reactive nitrogen.