Resistance modelling of ammonia exchange over oilseed rape

Ammonia (NH 3) surface/atmosphere exchange is bi-directional and as such resistance models must include canopy concentrations. An existing single layer model that describes the exchange in terms of adsorption to leaf cuticles and bi-directional transport through leaf stomata, which is governed by a stomatal compensation point ( χ s), is applied here to NH 3 exchange over oilseed rape and compared with measured fluxes. For the first time the model is tested using values of χ s based on the apoplastic ratio [NH 4 +]/pH ( Γ s) measured directly in the field. Strong NH 3 emission from decomposing leaf litter at the ground and the likelihood of high [NH 4 +] in the siliques complicate the exchange pattern with oilseed rape and limit the application of the original model. This is therefore extended by: (a) the inclusion of a litter layer (2-layer model), with an emission potential ( Γ l), (b) additionally dividing the plant canopy into a foliage- and a silique-layer (3-layer model) and (c) considering the relative humidity ( h) dependency of Γ l. The 2-layer model is able to predict night-time emission, but daytime emission is estimated to originate from the litter layer, which is in contradiction to the NH 3 sources and sinks derived for this canopy. The 3-layer model using a constant value of Γ l requires an emission potential for the siliques of about 1300, which is consistent with bioassay estimates. Together with a parameterization of Γ l that increases with h this model indicates that during daytime emission originates from the siliques, in agreement with the source/sink analysis. It is concluded that the leaf stomata were an effective NH 3 sink, whereas the leaf litter dominates night-time emissions and the silique-layer (probably) daytime emissions. Although the 2-layer model reproduces the net exchange, the 3-layer model appears to be the mechanistically more accurate description.