HR: 0800h
AN: H41E-0337 INVITED     [Abstracts]
TI: Isotopic Tracing of Atmospheric Sulfate in Forest Soils
AU: * Bernhard, M
EM: bmayer@ucalgary.ca
AF: University of Calgary, Department of Geology and Geophysics, 2500 University Drive NW, Calgary, AB T2N 1N4 Canada
AB: Stable isotope ratios of sulfur and oxygen are a useful tool for tracing sulfate in terrestrial and aquatic ecosystems. Several studies throughout the last two decades have shown that the isotopic composition of atmospherically deposited sulfate is usually not preserved in aerobic forest soils. Whereas stable sulfur isotope ratios of sulfate in soil solutions tend to remain close to those of atmospheric sulfate, a marked shift to lower oxygen isotope ratios of soil sulfate has been observed between the top of the forest floor and the mineral horizons. This suggests that atmospherically deposited sulfate does not behave conservatively in the unsaturated soil zone, but undergoes redox reactions e.g. via immobilization into organic sulfur compounds and re-mineralization to inorganic sulfate. This implies a significant retardation of atmospherically deposited sulfate in the pedosphere. To identify the sulfur retardation processes in forest soils and to asses their rates, a long-term isotopic tracer study has been conducted. In July 1990, an equivalent of 70 kg S/ha was applied as K2SO4 solution to a forest ecosystem stocked with Norway spruce near Munich (Germany). The sulfur isotope ratio of the deposited sulfate was approximately 25 per mil higher than that of S in soil and seepage water before tracer application. Sulfur isotope ratios for seepage water sulfate collected at 5 cm depth indicated that the tracer passed through the forest floor within four months after application. Mass and isotope balances showed that circa 70 percent of the applied K2SO4 had passed the mineral soil horizons in 20 cm depth 2.5 years after tracer application, but no labeled sulfate was detected in seepage water in 100 cm soil depth during the observation period. Isotope ratio measurements on various sulfur compounds from soil samples obtained in 1991, 1995, 1999 and 2003 confirmed that the majority of the labeled sulfate was retained in the mineral soil for several years. Circa one decade after tracer application some of the labeled sulfur re-appeared in the forest floor. This suggests that substantial amounts of tracer sulfate were taken up by the forest stand and were returned as organic sulfur to the forest floor via needle fall. The study provides evidence that some of the atmospherically deposited sulfate undergoes the immobilization re-mineralization cycle in forested ecosystems, providing a feasible explanation for the observed depletion of oxygen-18 in seepage water sulfate.
DE: 1860 Runoff and streamflow
DE: 1875 Unsaturated zone
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting