Concentration-dependent NH3 deposition processes for mixed moorland semi-natural vegetation

Dry deposition modelling typically assumes that canopy resistance (Rc) is independent of ammonia (NH3) concentration. An innovative flux chamber system was used to provide accurate continuous measurements of NH3 deposition to a moorland composed of a mixture of Calluna vulgaris (L.) Hull, Eriophorum vaginatum L. and Sphagnum spp. Ammonia was applied at a wide range of concentrations (1–100 μg m−3). The physical and environmental properties and the testing of the chamber are described, as well as results for the moorland vegetation using the ‘canopy resistance’ and ‘canopy compensation point’ interpretations of the data. Results for moorland plant species demonstrate that NH3 concentration directly affects the rate of NH3 deposition to the vegetation canopy, with Rc and cuticular resistance (Rw) increasing with increasing NH3 concentrations. Differences in Rc were found between night and day: during the night Rc increases from 17 s m−1 at 10 μg m−3 to 95 s m−1 at 80 μg m−3, whereas during the day Rc increases from 17 s m−1 at 10 μg m−3 to 48 s m−1 at 80 μg m−3. The lower resistance during the day is caused by the stomata being open and available as a deposition route to the plant. Rw increased with increasing NH3 concentrations and was not significantly different between day and night (at 80 μg m−3 NH3 day Rw=88 s m−1 and night Rw=95 s m−1). The results demonstrate that assessments using fixed Rc will over-estimate NH3 deposition at high concentrations (over 15 μg m−3).