Concentration-dependent NH 3 deposition processes for moorland plant species with and without stomata

Abstract

Currently, in operational modelling of NH 3 deposition a fixed value of canopy resistance ( R c) is generally applied, irrespective of the plant species and NH 3 concentration. This study determined the effect of NH 3 concentration on deposition processes to individual moorland species. An innovative flux chamber system was used to provide accurate continuous measurements of NH 3 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 NH 3 concentrations (1–140 μg m −3). NH 3 concentration directly affects the deposition processes to the vegetation canopy, with R c, and cuticular resistance ( R w) increasing with increasing NH 3 concentration, for all the species and vegetation communities tested. For example, the R c 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 NH 3 uptake were observed for higher plants, due to stomatal uptake; however, no diurnal variations were shown for non-stomatal plants. R c for C. vulgaris at 80 μg m −3 was 66 and 112 s m −1 during day and night, respectively. Differences were found in NH 3 deposition between plant species and vegetation communities: Sphagnum had the lowest R c (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).