Sulphur uptake from the atmosphere by forest and farmland

Abstract

MEASUREMENTS of sulphur uptake by plants are needed to aid our understanding of the long-range transport of sulphur dioxide and the role of sulphur uptake in acidification and the nutrient balance of forest ecosystems¹. The traditional profile method of analysing sulphur uptake has proved inadequate for forest measurements because the vertical concentration gradients above forests are small and difficult to measure with current techniques²; the best estimates available are based on extrapolation of measurements from individual shoots to the whole forest³. We report here the first direct measurements of the uptake of sulphur from the atmosphere by a forest in the absence of precipitation (that is, dry deposition). These measurements were possible because of the development of a fast response sulphur detector which allowed, for the first time, use of the eddy correlation method⁴ for sulphur uptake measurements. The sulphur uptake ranged from 0–0.1 µg m⁻² s⁻¹ (as SO₂) for atmospheric sulphur concentrations up to 35 µg m⁻³ (expressed as SO₂). On average, ∼75% of the atmospheric sulphur was in gaseous form. These uptake rates support those derived from use of the recent hypothesis that sulphur dioxide uptake by vegetation takes place through the stomatal opening for a dry canopy³. Some measurements of dry uptake of sulphur over farmland were also made and are in close agreement with published measurements made over similar surfaces using the profile and other methods.