Emissions of sulfur gases from acid sulfate soils

Many coastal lowland soils were deposited during and following the last sea-level rise and contain large amounts of sulfides, principally as pyrite. There are around 2.5x106 km2 of these soils in the world, with some 40,000 km2 in Australia and large areas in North America, many unmapped as yet. When these soils are drained for agriculture or development, the sulfides are oxidized producing a range of dissolved sulfur (S) species in highly acidic ground water. The soils are often referred to as acid sulfate soils (ASS). While soils generally are considered to be sinks for atmospheric sulfur dioxide (SO2), our studies have shown that ASS can be SO2 sources. Using static chambers and flux-gradient micrometeorological techniques, we have measured emissions of SO2 from Australian ASS developed for sugar cane production. Our early work suggested that SO2 evolution was coupled to evaporation of soil water containing sulfite. Our recent work, however, suggests strongly that SO2 emissions from ASS may also be linked to the formation of hydrogen sulfide (H2S) deep in the soil profile and its oxidation to SO2 at the surface under the influence of ultraviolet light. We have discovered a pattern of high H2S concentrations near the ground at night accompanying high H2S fluxes from the surface then, and low concentrations and fluxes by day. By contrast, SO2 concentrations and surface fluxes have the opposite time-course: lower at night; higher by day. The linkage between H2S and SO2 emissions is now being investigated in the laboratory. We estimate global emissions of S from ASS to be about 3TgS/y, which is of the same order as emissions from terrestrial biogenic sources, biomass burning and ship emissions (said to contribute to elevated concentrations in coastal areas), and is about 3% of known anthropogenic emissions of S.