Concentration-dependent NH3 deposition processes for moorland plant species with and without stomata

Currently, in operational modelling of NH3 deposition a fixed value of canopy resistance (Rc) is generally applied, irrespective of the plant species and NH3 concentration. This study determined the effect of NH3 concentration on deposition processes to individual moorland species. An innovative flux chamber system was used to provide accurate continuous measurements of NH3 deposition to Deschampsia cespitosa (L.) Beauv., Calluna vulgaris (L.) Hull, Eriophorum vaginatum L., Cladonia spp., Sphagnum spp., and Pleurozium schreberi (Brid.) Mitt. Measurements were conducted across a wide range of NH3 concentrations (1–140 μg m−3). NH3 concentration directly affects the deposition processes to the vegetation canopy, with Rc, and cuticular resistance (Rw) increasing with increasing NH3 concentration, for all the species and vegetation communities tested. For example, the Rc for C. vulgaris increased from 14 s m−1 at 2 μg m−3 to 112 s m−1 at 80 μg m−3. Diurnal variations in NH3 uptake were observed for higher plants, due to stomatal uptake; however, no diurnal variations were shown for non-stomatal plants. Rc for C. vulgaris at 80 μg m−3 was 66 and 112 s m−1 during day and night, respectively. Differences were found in NH3 deposition between plant species and vegetation communities: Sphagnum had the lowest Rc (3 s m−1 at 2 μg m−3 to 23 at 80 μg m−3), and D. cespitosa had the highest nighttime value (18 s m−1 at 2 μg m−3 to 197 s m−1 at 80 μg m−3).