Sulfur cycle

In the global phosphorus cycle we have seen that the lithospheric phase is predominant (Figure 18.21a), whereas the nitrogen cycle has an atmospheric phase of overwhelming importance (Figure 18.21b). Sulfur, by contrast, has atmospheric and lithospheric phases of similar magnitude (Figure 18.21c).

Three natural biogeochemical processes release sulfur to the atmosphere: (i) the formation of the volatile compound dimethylsulfide (DMS) (by enzymatic breakdown of an abundant compound in phytoplankton -dimethylsulfonioproprionate); (ii) anaerobic respiration by sulfate-reducing bacteria; and (iii) volcanic activity. Total biological release of sulfur to the atmosphere is estimated to be 22 Tg S year-1, and of this more than 90% is in the form of DMS. Most of the remainder is produced by sulfur bacteria that release reduced sulfur compounds, particularly H2S, from waterlogged bog and marsh communities and from marine communities associated with tidal flats. Volcanic production provides a further 7 Tg S year-1 to the atmosphere (Simo, 2001). A reverse flow from the atmosphere involves oxidation of sulfur compounds to sulfate, which returns to earth as both wetfall and dryfall.

The weathering of rocks provides about half the sulfur draining off the land into rivers and lakes, the remainder deriving from atmospheric sources. On its way to the ocean, a proportion of the available sulfur (mainly dissolved sulfate) is taken up by plants, passed along food chains and, via decomposition processes, becomes available again to plants. However, in comparison to phosphorus and nitrogen, a much smaller fraction of the flux of sulfur is involved in internal recycling in terrestrial and aquatic communities. Finally, there is a continuous loss of sulfur to ocean sediments, mainly through abiotic processes such as the conversion of H2S, by reaction with iron, to ferrous sulfide (which gives marine sediments their black color).

The combustion of fossil fuels is the major human perturbation to the sulfur and acid rain global sulfur cycle (coal contains 1-5%

sulfur and oil contains 2-3%). The SO2 released to the atmosphere is oxidized and converted to sulfuric acid in aerosol droplets, mostly less than 1 |lm in size. Natural and human releases of sulfur to the atmosphere are of similar magnitude and together account for 70 Tg S year-1 (Simo, 2001). Whereas natural inputs are spread fairly evenly over the globe, most human inputs are concentrated in and around industrial zones in northern Europe and eastern North America, where they can contribute up to 90% of the total (Fry & Cooke, 1984). Concentrations decline progressively downwind from sites of production, but they can still be high at distances of several hundred kilometers. Thus, one nation can export its SO2 to other countries; concerted international political action is required to alleviate the problems that arise.

Water in equilibrium with CO2 in the atmosphere forms dilute carbonic acid with a pH of about 5.6. However, the pH of acid precipitation (rain or snow) can average well below 5.0, and values as low as 2.4 have been recorded in Britain, 2.8 in nitrogen and acid rain the sulfur cycle has atmospheric and lithospheric phases of similar magnitude

Scandinavia and 2.1 in the USA. The emission of SO2 often contributes most to the acid rain problem, though together NO, and NH3 account for 30-50% of the problem (Mooney et al., 1987; Sutton et al., 1993).

We saw earlier how a low pH can drastically affect the biotas of streams and lakes (see Chapter 2). Acid rain (see Section 2.8) has been responsible for the extinction of fish in thousands of lakes, particularly in Scandinavia. In addition, a low pH can have far-reaching consequences for forests and other terrestrial communities. It can affect plants directly, by breaking down lipids in foliage and damaging membranes, or indirectly, by increasing leaching of some nutrients from the soil and by rendering other nutrients unavailable for uptake by plants. It is important to note that some perturbations to biogeochemical cycles arise through indirect, 'knock-on' effects on other biogeochemical components. For example, alterations in the sulfur flux in themselves are not always damaging to terrestrial and aquatic communities, but the effect of sulfate's ability to mobilize metals such as aluminum, to which many organisms are sensitive, may indirectly lead to changes in community composition. (In another context, sulfate in lakes can reduce the ability of iron to bind phosphorus, releasing the phosphorus and increasing phytoplankton productivity (Caraco, 1993).)

Provided that governments show the political will to reduce emissions of SO2 and NO, (for example, by making use of techniques already available to remove sulfur from coal and oil), the acid rain problem should be controllable. Indeed reductions in sulfur emissions have occurred in various parts of the world.

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