Measurement of Trace Gas Fluxes over an Unfertilized Agricultural Field Using the Flux-gradient Technique

Trace gas fluxes exhibit extensive spatial and temporal variability that is dependent on a number of factors, including meteorology, ambient concentration, and emission source size. Previous studies have found that agricultural fertilization contributes to higher fluxes of certain gases. The magnitude of trace gas fluxes over unfertilized crops is still uncertain. In the present study, deposition of ammonia (NH3), nitric acid (HNO3), and sulfur dioxide (SO2) was measured over unfertilized soybean using the flux-gradient technique. The eddy diffusivity was estimated from eddy covariance measurements of temperature fluxes, resulting in K-H of 0.64 +/- 0.30 m(2) s(-1). Flux means and standard deviations were -0.14 +/- 0.13, -0.22 +/- 0.19, and -0.38 +/- 0.54 mu g m(-2) s(-1) for NH3, HNO3, and SO2, respectively. Low concentrations of NH3 and HNO3 increased the relative uncertainties in the deposition velocities estimated from measured fluxes. This contributed to dissimilarities between deposition velocities estimated from the resistance analogy and deposition velocities estimated from fluxes. However, wet canopy conditions during the study may have led to an underestimation of deposition by the resistance analogy because the resistance method does not accurately describe the enhanced deposition rates that occur after dew formation. Quantification of vegetation characteristics, such as leaf wetness and apoplast chemistry, would be beneficial in future studies to more accurately determine stomatal resistance and its influence on fluxes.