Measurement of bi-directional ammonia fluxes over soybean using the modified Bowen-ratio technique

Measurements of bi-directional ammonia (NH3) exchange over a fertilized soybean canopy are presented for an 8-week period during the summer of 2002. The modified Bowen-ratio approach was used to determine fluxes from vertical NH3 and temperature gradients in combination with eddy covariance sensible heat fluxes. The measurement site is located in an area of high NH3 emissions from animal production and fertilizer use. Ambient NH3 concentrations ranged from 0.01 to 43.9 μg m−3 (μ = 9.4 μg m−3) during the experiment. The mean flux was −12.3 ng m−2 s−1, indicating that the canopy was a net sink for NH3; however, emission fluxes were consistently observed during the late morning and early afternoon. Deposition rates were highest when the canopy was wet (μ = −29.9 ng m−2 s−1). Modeling results suggest that uptake via the leaf cuticle was the dominant deposition process and stomatal uptake only occurred during the first few hours after sunrise when the stomatal resistance and compensation point were low. The average stomatal compensation point was high (χs = 11.5 μg NH3 m−3), primarily due to high daytime temperatures (μ = 29 °C). Measured cuticular resistances were large (median Rw = 208 s m−1), most likely due to very dry conditions. The average NH3 flux corresponds to a dry-to-wet deposition ratio of 0.44. Median flux error was 51%, which was dominated by uncertainty in the vertical NH3 gradient due to sequential sampling between measurement heights.