Dry deposition of reduced nitrogen

The paper reviews the results of measurements of ammonia (NH₃) exchange and considers the application of this information in approaches to modelling atmospheric NH₃ inputs to ecosystems. Ammonia dry deposition is seen to be highly dependent on land management and the magnitude of agricultural nitrogen inputs. Efficient dry deposition is observed to many semi-natural and forest ecosystems, with typical deposition velocities (V _d) in the range 5–50 mm s^-1. By contrast, intensively grazed pastures and fertilized arable croplands frequently show net NH₃ emission, with fluxes dependent on the form of N input, environmental conditions (temperature, wetness) and plant growth stage. Other factors affecting the exchange of NH₃ with ecosystems include the existence of an NH₃“compensation point” within plant tissues and interactions with acid gases such as SO₂. Available information on NH₃ exchange over the sea and aquatic water bodies suggests that both emission and deposition can occur depending on NH₃ air concentrations and water compensation points. Approaches to mapping NH₃ dry deposition include inferring deposition from monitored air concentrations and values of V _d (“inferential technique”), and atmospheric transport models. The former may be more site specific, but is limited by lack of NH₃ concentration monitoring, while the latter provides good regional coverage, but poor estimates of actual inputs to specific ecosystems (grids often > 50 km X 50 km). A possible advance is to apply the air concentration field outputs of large scale transport models, with locally derived V _d, though this fails to treat the significant local variability in NH₃ emissions and air concentrations. An improvement would be the further development of local scale atmospheric transport models (e.g., 5 km X 5 km grids) incorporating land use information and ecosystem specific V _d on a fine spatial scale.