DEVELOPMENT OF RESISTANCE MODELS TO DESCRIBE MEASUREMENTS OF BIDIRECTIONAL AMMONIA SURFACE-ATMOSPHERE EXCHANGE

Ammonia fluxes over vegetated land are bi-directional, with difference s between emission and deposition occurring in relation to environment al 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). The se are used as examples to highlight the factors affecting the exchang e process. Net fluxes are expected to depend on the competition betwee n deposition to leaf surfaces and bi-directional exchange with a ''sto matal compensation point'' for leaf tissues (chi(s)). Current models d escribe this effect poorly, and two new resistance models are describe d here. These estimate a ''canopy compensation point'' (chi(c)), as th e net potential for NH3 emission from the canopy. In the simpler of th e two models, leaf surface uptake is parametrized using a resistance ( R-w). This model is able to reproduce bi-directional fluxes, though th ere is also evidence that both cuticular NH3 adsorption and desorption may occur, dependent on previous fluxes. A more complex dynamic appro ach is therefore developed, treating cuticular uptake as a capacitance (C-d). The dynamic model is able to reproduce the pattern of desorpti on, but further development of both models is required to provide desc riptions valid over longer periods and for a range of ecosystem types. (C) 1998 Elsevier Science Ltd. All rights reserved.