Element cycles sulfur cycle

The harmful effects of air pollutants on human beings have been the major reason for efforts to understand and control their sources. During the past two decades, research on acidic deposition on water-based ecosystems has helped to reemphasize the importance of air pollutants in other receptors, such as soil-based ecosystems (1). When discussing the impact of air pollutants on ecosystems, the matter of scale becomes important. We will discuss three examples of elements which interact with air, water, and soil media on different geographic scales. These are the carbon cycle on a global scale, the sulfur cycle on a regional scale, and the fluoride cycle on a local scale. 

One of the things that environmental scientists do is to keep track of important elements in the biosphere—in what form do these elements normally occur, to what are they transformed, and how are they returned to their normal state Careful studies have given clear, although complicated, pictures of the "nitrogen cycle," the "sulfur cycle," and the "phosphorus cycle," for example. The "carbon cycle," begins and ends with atmospheric carbon dioxide. It can be represented in an abbreviated form as ... 

R. W. FairbriiXjE, Encyclopedia of Geochemistry and Environmental Sciences, Van Nostrand, New York, 1972.. See sections on Geochemical Classification of the Elements Sulfates Sulfate Reduction-Microbial Sulfides Sulfosalts Sulfur Sulfur Cycle Sulfur Isotope Fractionation in Biological Processes, etc., pp. 1123 - 58. 

The following sections summarize only the most prominent interactions between the elemental cycles and the links in the hydrologic cycle. Water also plays a role in many chemical and biological reactions that are beyond the scope of this discussion. The carbon, nitrogen, sulfur, and phosphorus cycles are discussed in detail in Chapters 11, 12, 13, and 14, respectively. 

This brief description leads to Fig. 7-13 which depicts the physical transformations of trace substances that occur in the atmosphere. These physical transformations can be compared to the respective chemical transformations within the context of the individual elemental cycles (e.g., sulfur). This comparison suggests that the overall lifetime of some species in the atmosphere can be governed by the chemical reaction rates, while others are governed by these physical processes. 

Abstract Inorganic polysulfide anions and the related radical anions S play an important role in the redox reactions of elemental sulfur and therefore also in the geobio chemical sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored and reactive radical anions S with n = 2, 3, and 6 are discussed, some of which exist in equilibrium with the corresponding diamagnetic dianions. 

Therefore, polysulfide ions play a major role in the global geological and biological sulfur cycles [1, 2]. In addition, they are reagents in important industrial processes, e.g., in desulfurization and paper production plants. It should be pointed out however that only sulfide, elemental sulfur and sulfate are thermodynamically stable under ambient conditions in the presence of water, their particular stabihty region depending on the redox potential and the pH value [3] ... 

Figure 4.8. The sulfur cycle where S° is elemental sulfur, H2S is hydrogen sulfide, S2032" is thiosulfate, SO32" is sulfite, SO/- is sulfate, R-OSO3H represents a sulfate ester, R-SO3H a sulfonic acid, R-S-R a thioether, and R-SH a thiol. (Adapted from Coyne MS. Soil Microbiology An Experimental Approach. Boston Delmar Publishers 1999.)...

The most noteworthy multistage element cycles in which bacteria play important roles are the nitrogen and sulfur redox cycles. The fixation of nitrogen is a reductive process that provides organisms with nitrogen in a form usable for the synthesis of amino acids, nucleic acids, and other cell constituents. In essence, the overall conversion to the key intermediate, ammonia, can be represented as ... 

Naturally occurring stable isotopes of C, N, and S have been used extensively for over a decade as direct tracers of element cycling in marine and terrestrial food webs (34-39). Carbon and sulfur isotopes fractionate very little between food and consumer thus their measurement indicates which primary producers or detrital pools are sources of C and S for consumers. For example, a study of plants and animals in Texas sand dunes showed that insect species had 813C values either like those of C3 plants or like those of C4 plants (-27 and -13%o, respectively). Rodent species had intermediate values near -20%o that indicated mixed diets of both C3 and C4 plants (40). The 13C measurements, used in simple linear mixing models, proved to be quick and reliable indicators of which plant sources provided the carbon assimilated by higher trophic levels. 

Sulfide. Sulfide appears in the water column of Lake Greifen only at the end of stagnation time and in the deepest water layers. This distribution indicated the biological reduction of sulfate in sediments. The reactions involved in the sulfur cycle are described in ref. 65. The occurrence of sulfide indicates a very low pe (p < 0) sulfide is an efficient reductant for many elements, including Fe(III), Mn(IV), As(V), and Cr(VI). The occurrence of sulfide also implies the possible precipitation of solid sulfide phases of various elements and the formation of dissolved complexes (21-23). 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

Sulfates interacting with the ecosystems and establishing the interaction of the sulfur cycle with other biogeochemical processes are one of the most important elements in the global cycle of sulfur. Numerous complicated transformations of sulfates in the environment are described by the set of fluxes H2, H%, 7/10, Hu,... 

Included in Table I are molybdenum enzymes that are as yet unclassified due to their partial characterization (46—49, 58). These enzymes includes polysulfide reductase that accomplish sulfur reduction to sulfide (46), underlining its role in the global sulfur cycling. Chlorate and selenate reductase are examples of relatively rare enzymes using simple oxyanions of third-row elements as substrates (47 19, 58). 

Living and dead organic matter compose a smaller, more rapidly cycled sulfur reservoir. Largely inert sulfur reservoirs include metal sulfides in rocks, elemental sulfur deposits, and fossil fuels. 

There are 5 major reservoirs in the Earth system atmosphere, biosphere (vegetation, animals), soils, hydrosphere (oceans, lakes, rivers, groundwater), and the lithosphere (Earth crust). Elemental cycles of carbon, oxygen, nitrogen, sulfur, phosphorus, and other elements interact with the different reservoirs of the Earth system. The carbon cycle has important aspects in tropical forests due to the large amount of carbon stored in the tropical forests and the high rate of tropical deforestation 0acob 1999)-... 

Then there are wider questions about the possible limiting roles of various trace elements in key biogeochemical processes Do trace elements other than iron limit phytoplankton growth and primary production Is the composition of phytoplankton assemblages controlled by trace elements Are various processes in the cycle of nitrogen limited by trace elements (e.g., N2 fixation by iron N2O reduction by copper) What are the links between trace elements and the reduced sulfur cycle in surface seawater ... 

Krouse H. R. and McCready R. G. L. (1979b) Reductive reactions in the sulfur cycle. In Biogeochemical Cycling of Mineral-forming Elements (eds. P. A. Trudinger and D. J. Swaine). Elsevier, Amsterdam, pp. 315—358. 

---