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

A new resistance model is described to interpret the biosphere-atmosphere exchange fluxes of ammonia (NH 3 ) with vegetation, and compared with previous modelling approaches for NH 3 . 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 NH 3 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 NH 3 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 (X c ) may be derived. The net flux is determined by the relative magnitude of X c and the NH 3 air concentration above the vegetation. The two-layer canopy compensation point is applied to: (i) an oilseed rape canopy, in which NH 3 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 X c 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 NH 3 exchange in atmospheric transport models over a very wide range of vegetation types.

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