Use of Sulfur Isotope Ratios To Determine Anthropogenic Sulfur Signals in a Grassland Ecosystem

Archived soil and herbage samples from the control plot of the Rothamsted Park Grass Experiment, established in 1856, were used to investigate the effects of dramatically changing SO2 pollution inputs on the concentrations and stable isotope ratios of S in the samples. Both herbage S concentrations and δ34S showed clear trends over the last 135 years. Herbage S concentrations correlated positively with annual total SO2 emissions in the U.K. (R 2 = 0.61), whereas herbage δ34S correlated negatively with SO2 emissions (R 2 = 0.83). These results indicate that local variations of anthropogenic S inputs were not important at the site and verify the usefulness of this unique sample set for environmental monitoring. In contrast, the concentrations of total and extractable S in the topsoil were less sensitive to the changing pollution, although the δ34S values of soil S also decreased significantly, particularly during the period 1900−1970. Based on these S isotope ratios, we estimated that anthropogenic S contributed up to 50% of the herbage S uptake at the peak of SO2 emissions and still accounts for about 30% of the S presently stored in the topsoil.