Development of resistance models to describe measurements of bi-directional ammonia surface-atmosphere exchange

Ammonia fluxes over vegetated land are bi-directional, with differences between emission and deposition occurring in relation to environmental conditions (temperature and wetness), plant community (e.g. arable land, unfertilized ecosystems), as well as other factors such as plant phenology. In the present paper, measurements of net NH3 fluxes over arable cropland are reported in the range -50 to 40 ng m(-2)s(-1). These are used as examples to highlight the factors affecting the exchange process. Net fluxes are expected to depend on the competition between deposition to leaf surfaces and bi-directional exchange with a "stomatal compensation point" for leaf tissues (chi(s)). Current models describe this effect poorly, and two new resistance models are described here. These estimate a "canopy compensation point" (chi(c)), as the net potential for NH3 emission from the canopy. In the simpler of the two models, leaf surface uptake is parametrized using a resistance (R-w). This model is able to reproduce bi-directional fluxes, though there is also evidence that both cuticular NH3 adsorption and desorption may occur, dependent on previous fluxes. A more complex dynamic approach is therefore developed, treating cuticular uptake as a capacitance (C-d). The dynamic model is able to reproduce the pattern of desorption, but further development of both models is required to provide descriptions valid over longer periods and for a range of ecosystem types. (C) 1998 Elsevier Science Ltd. All rights reserved.