Sulfur Isotope Studies of the Pedosphere and Biosphere

Abstract

Recent developments in the use of sulfur isotopes for ecological research are recounted. It is shown that the S isotope composition of all components in an ecosystem may be consistent over a large area, yet in other situations, there are significant variations in one specimen over a few centimetres. Therefore it is not advisable to limit sampling of an ecosystem to one or two species for sulfur isotope investigations. The fact that different plants incorporate varied proportions of atmospheric and soil sulfur can be used to advantage. Epiphytic lichens and some mosses provide a cumulative record of the sulfur isotopic composition of atmospheric sulfur. Protected understory plants tend to sample almost solely soil sulfur. Depth trends in the soil record the penetration of pollutant sulfur. Data from insects, birds and animals should also reflect elevation trends similar to those displayed for vegetation. The extent to which the system is perturbed by sulfur additions is a more challenging and controversial problem. Some species may suffer while others benefit from pollutant sulfur. The use of sulfur isotope data with biological parameters such as coverage help in this assessment. Foliar delta 34S values more positive than those of available sulfur sources indicate that the plant is coping with excessive sulfur by emitting reduced sulfur compounds. Detection of this stress relief mechanism in the field by means other than sulfur isotopes would be difficult.