Biosphere-atmosphere exchange of ammonia

Substantial progress has been made in the last eight years in the understanding and quantification of ammonia exchange between the atmosphere and biosphere. Much of the work has been linked to the joint EC/EUROTRAC subproject BIATEX (BIosphere ATmosphere EXchange), which has served as the main European forum for work in this area. In the mid-1980s there was still much confusion and uncertainty over the rate and direction of ammonia fluxes with different ecosystems; although the results of isolated studies were available, there was no clear overview of the key factors affecting ammonia fluxes. Work since that time has highlighted the dominant effects of ecosystem type and management, as well as humidity and wetness, on ammonia exchange. Ammonia is a key component of plant metabolism, so that ammonia emission may occur from plants in relation to nitrogen nutrition and plant growth stage. In contrast, ammonia is highly soluble and may be efficiently captured by leaf cuticles and surface w etness allowing large deposition velocities. The consequence is that ammonia exchange is bi-directional over agricultural ecosystems, though for most semi-natural ecosystems dry deposition dominates, being a significant component of the total atmospheric nitrogen input. The work within BIATEX has focused in more detail on the processes controlling these differences and, using the results of both micrometeorological and controlled environment measurements, has developed new models that are able to provide the synthesis necessary to predict ammonia fluxes. Long term and regional estimates of ammonia net exchange are still uncertain, though the models developed now provide the necessary framework to guide future measurements