A two‐layer canopy compensation point model for describing bi‐directional biosphere‐atmosphere exchange of ammonia

Abstract

AbstractA new resistance model is described to interpret the biosphere‐atmosphere exchange fluxes of ammonia (NH3) with vegetation, and compared with previous modelling approaches for NH3. The new model constitutes an extension of an existing one‐layer canopy compensation point model: in addition to bi‐directional foliar stomatal exchange and deposition to leaf cuticles, the model treats NH3 emission from a ground layer. This may originate from fertilizer evaporation, the soil or decomposing plant parts. The emission potentials of the foliage and ground surface are given by the NH3 gas concentrations at equilibrium with the ammonium (NH+4) concentration in the apoplastic fluid or soil solution. From these concentrations, as well as the transfer resistances of the different exchange pathways, the net compensation point of the canopy (Xc) may be derived. The net flux is determined by the relative magnitude of Xc and the NH3 air concentration above the vegetation. The two‐layer canopy compensation point is applied to: (i) an oilseed rape canopy, in which NH3 emission from decomposing leaf litter at the ground surface presents a second major source; and (ii) a wheat stubble field, in which emission from the soil contributes significantly to the net exchange. For both canopies, the model performance is contrasted with the single‐layer Xc model, which is not able to reproduce the temporal patterns of exchange. The two‐layer model is proposed as the optimum compromise between simplicity and accuracy, capable of describing bi‐directional NH3 exchange in atmospheric transport models over a very wide range of vegetation types.