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 t he 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 sampl ers. The aerodynamic gradient method and eddy covariance were applied to estimat e friction velocity (u*), sensible heat fluxes (H) and latent heat fluxes ( lambdaE), with best estimates provided by inter-comparisons for each. The m easurements represent a major exercise in data acquisition and processing, and provide approximately 30 days of semi-continuous fluxes. The largest er rors in estimating the NH3 fluxes arise from the concentration profile meas urements. Fluxes from the ripening canopy were bi-directional, varying in t he range -150 to 180 ng m(-2) s(-1), with the largest emission fluxes durin g the day, and both emission and deposition occurring at night. Larger net emissions were measured following cubing 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 exp ected major driving forces controlling exchange (cuticular fluxes, foliar c ompensation point, leaf Litter emissions), the link to temperature/wetness, and the potential for overlying leaves to recapture emissions from leaf li tter. The comparison with the passive estimates indicates that continuous m easurements of several months duration are required to make a reliable asse ssment of the passive approach. (C) 2000 Elsevier Science B.V. All rights r eserved.