The exchange of atmospheric ammonia with vegetated surfaces. I: Unfertilized vegetation

Abstract Micrometeorological techinques have been used to measure the surface/atmosphere exchange of gaseous ammonia (NH3) over semi‐natural and unfertilized plant communities. A range of temperate ecosystems have been examined, including moorland, cut meadows and coniferous forest. Apart from a recently harrowed calcareous grassland, where deposition velocities, V d (1 m), were in the range 1–11 mm s −1 , ammonia was found to deposit rapidly (V d typically 15–20 mm s −1 for short vegetation) with surface resistances, R c , not significantly different from zero. Limited measurements over forest were consistent with a larger V d (mean 66 mm s‐1) as a consequence of increased turbulence over the aerodynamically rough surface. These observations show that, except for the harrowed calcareous site, ammonia is efficiently deposited to leaf surfaces, and that the mean air concentration at the surfaces approaches zero. The results are used as a basis for estimating annual dry deposition of ammonia to semi‐natural and unfertilized vegetation. For example sites in the British Isles, subject to typical background air concentrations, the estimated annual dry deposition of ammonia is in range 2‐41 kg fixed nitrogen per hectare, showing that this is a major source of fixed atmospheric nitrogen.