Biogeosciences (Nov 2012)
Measuring the biosphere-atmosphere exchange of total reactive nitrogen by eddy covariance
Abstract
The (net) exchange of reactive nitrogen (N<sub>r</sub>) with the atmosphere is an important driver for ecosystem productivity and greenhouse gas exchange. The exchange of airborne N<sub>r</sub> includes various trace compounds that usually require different specific measurement techniques, and up to now fast response instruments suitable for eddy covariance measurements are only available for few of these compounds. <br><br> Here we present eddy covariance flux measurements with a recently introduced converter (TRANC) for the sum of all N<sub>r</sub> compounds (∑N<sub>r</sub>). Measurements were performed over a managed grassland field with phases of net emission and net deposition of ∑N<sub>r</sub> and alternating dominance of oxidized (NO<sub>X</sub>) and reduced species (NH<sub>3</sub>). Spectral analysis of the eddy covariance data exhibited the existence of covariance function peaks at a reasonable time lag related to the sampling tube residence time under stationary conditions. Using ogive analysis, the high-frequency damping was quantified to 19%–26% for a low measurement height of 1.2 m and to about 10% for 4.8 m measurement height. <br><br> ∑N<sub>r</sub> concentrations and fluxes were compared to parallel NO and NO<sub>2</sub> measurements by dynamic chambers and NH<sub>3</sub> measurements by the aerodynamic gradient technique. The average concentration results indicate that the main compounds NO<sub>2</sub> and NH<sub>3</sub> were converted by the TRANC system with an efficiency of near 100%. With an optimised sample inlet also the fluxes of these compounds were recovered reasonably well including net deposition and net emission phases. The study shows that the TRANC system is suitable for fast response measurements of oxidized and reduced nitrogen compounds and can be used for continuous eddy covariance flux measurements of total reactive nitrogen.
