Micrometeorological measurements of net ammonia fluxes over oilseed rape during two vegetation periods

Ammonia fluxes were measured semi-continuously over oilseed rape (Brassica napus) as part of the EXAMINE North Berwick experiment during ripening of the growing plants and following cutting prior to harvest. The first period was investigated intensively, including flux measurements by continuous wet denuders at six heights, as well as by filter packs and passive flux samplers. The aerodynamic gradient method and eddy covariance were applied to estimate friction velocity (u*), sensible heat fluxes (H) and latent heat fluxes (λE), with best estimates provided by inter-comparisons for each. The measurements represent a major exercise in data acquisition and processing, and provide approximately 30 days of semi-continuous fluxes. The largest errors in estimating the NH3 fluxes arise from the concentration profile measurements. Fluxes from the ripening canopy were bi-directional, varying in the range −150 to 180 ng m−2 s−1, with the largest emission fluxes during the day, and both emission and deposition occurring at night. Larger net emissions were measured following cutting of the rape (up to 620 ng m−2 s−1), with an overall mean of 57 ng m−2 s−1 compared with 16 ng m−2 s−1 prior to cutting. The patterns may be explained in relation to the expected major driving forces controlling exchange (cuticular fluxes, foliar compensation point, leaf litter emissions), the link to temperature/wetness, and the potential for overlying leaves to recapture emissions from leaf litter. The comparison with the passive estimates indicates that continuous measurements of several months duration are required to make a reliable assessment of the passive approach