The largest global reservoir of S is the lithosphere (Table 15.4). The atmospheric content of S represents a relatively small pool, but one that has increased significantly in recent times due to the burning of fossil fuels. Atmospheric emissions of S peaked at approximately 70 million metric tons (70 Tg) on an annual basis in the mid- to late-1980s. The result is acid rain, containing SO4-and NO-, which is acidifying surface waters and soils. These emissions have declined in recent decades where pollution abatement has been implemented. Areas with minimal atmospheric pollution receive approximately 1 kg S ha-1 year-1, while areas downwind from heavily industrialized areas can receive as much as 100 kg S ha-1 year-1. Many soils near cities receive more S from dry deposition through particulates than wet deposition from S dissolved in rainfall. Gases such as hydrogen sulfide (H2S), carbon disulfide (CS2), carbonyl sulfide (COS), methyl mer-captan (CH3SH), dimethyl sulfide ([CH3]2S), and dimethyl disulfide ([CH3]2S2) also enter the atmosphere through microbial transformations of both organic and inorganic S. Atmospheric S in aerosol form is most important to global climate change because of its impact on haze formation and changing the earth's reflectance to sunlight.
Most agricultural soils contain S in the range 20-2000 ^g S g-1 (Table 15.5). Many volcanic ash, organic, and tidal marsh soils contain 3000 or more ^g S g-1, and some desert soils have a total S content in excess of 10,000 ^g S g-1. In soils other than aridisols, the inorganic S fraction is typically small compared to the organic fraction (Table 15.5). Inorganic S exists in a number of oxidation states, ranging from +6 in SO4- to -2 in H2S and its derivatives (Table 15.6). More
TABLE 15.5 Amounts and Distribution of Sulfur in Some World Soils
Total sulfur (x106g)
Total sulfur Inorganic, Location Type of soil (p,g g-1) reducible C-bonded Ester sulfate
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