Sulfate reduction soils

The ground water from well 6 (site 1) yielded both the most depleted S13C and most enriched 834S. Isotopic compositions of this dual nature are consistent with methanogenesis and bacterial sulfate reduction, arising form interaction with organic-rich (bog) soils from below. 

Little or no fractionation accompanies the uptake of sulfate in soils by plants during ASR (60.611. Chukhrov et al. ( Q) showed that in cases where atmospheric sulfate is not subject to bacterial reduction in the soil, the value of the plant sulfur was identical to rainfall sulfur. In soils subject to dissimilatory sulfate reduction, the 6 S value of plant sulfur differed from that of local rainfall. Additionally, Chukhrov et al. (60) found that plants from oceanic islands had a sulfur content with higher values than those from continental areas, which they attribute to the relative influence of marine sulfate to these areas. 

King, GM. (1983) Sulfate reduction in Georgia salt marsh soils an evaluation of pyrite formation by use of 35 S and 55Fe tracers. Limnol. Oceanogr. 28, 987-995. 

In soils that become deficient in oxygen, usually as a result of flooding, the sulfide level will increase to relatively high concentrations. The formation of sulfide by sulfate reduction in nature is enhanced in warm, wet, or waterlogged soils with a pH above 6.0. Sulfide accumulation may be particularly pronounced in sulfate-rich saline areas in which plant excretions (release of carbon compounds) serve as the oxidizable carbon source, in addition to the hy-... 

Goldhaber M. B. and Kaplan 1. R. (1975) Controls and consequences of sulfate reduction rates in recent marine sediments. Soil Sci. 119(1), 42—55. 

Spatial variation in the abundance of electron donors and acceptors explains large-scale and small-scale patterns of anaerobic metabolism. Sulfate reduction dominates anaerobic carbon metabohsm on about two-thirds of the planet because of the high abundance of SO4 in seawater (Capone and Kiene, 1988). Fe(III) reduction is important in aU anaerobic ecosystems with mineral-dominated soils or sediments, regardless of whether they are marine or freshwater (Thamdrup, 2000). Methanogenesis is important in freshwater environments generally, and it dominates the anaerobic carbon metabolism of bogs, fens, and other wetlands that exist on organic (i.e., peat) soils. 

Nonenzymatic reduction by sulfides is widely considered to be the primary mechanism for Fe(III) and Mn(IV) reduction in systems where SO is abundant and sulfate reduction dominates anaerobic metabolism (Aller and Rude, 1988 Burdige and Nealson, 1986 Jacobson, 1994 Kostka and Luther, 1995 Postma, 1985 Pyzik and Sommer, 1981). Yet, Fe(III)-reducing bacteria are abundant in marine soils and sediments (Lowe et al., 2000 Kostka et al, 2002c), and several workers have noted that Fe(II) continues to accumulate when H2S production is blocked by molybdate, apparently because of enzymatic Fe(III) reduction (Canfield, 1989 Canfield et al., 1993b Hines et al., 1997 Jacobson, 1994 Joye et al., 1996 Kostka et al., 2002c Lovley and Phillips, 1987 Sprensen, 1982). 

Sulfate reduction can take place in waterlogged soils, especially environments such as paddy fields. Soils with high sulfate content, particularly as a result of input of seawater, can over time accumulate large concentrations of sulfide through sulfate reduction. When these soils are exposed to air, they can become acidic through the production... 

A major constraint is an absolute requirement for anoxic conditions and the organisms are particularly active in sediments, aqueous basins and waterlogged soils where restricted water movement and oxidation of organic matter by aerobic organisms has led to a depletion of oxygen. Nevertheless, bacterial sulfate reduction may occur in oxidized sediments due to the presence of reduced microniches (Jorgensen, 1977a). 

Since, at the present time, there is no general large-scale accumulation of reduced sulfur in sediments and soils, the combined rates of biological and chemical oxidation of sulfide can be assumed to be in the same order as those of sulfate reduction. Unfortunately, the few recorded rates of sulfide oxidation in the environment are not directly comparable with those of sulfate reduction. Aside from difficulties posed by the experimental... 

In oxidized surface waters and sediments, dissolved iron is mobile below about pH 3 to 4 as Fe and Fe(lII) inorganic complexes. Fe(III) is also mobile in many soils, and in surface and ground-waters as ferric-organic (humic-fulvic) complexes up to about pH 5 to 6 and as colloidal ferric oxyhydroxides between about pH 3 to 8. Under reducing conditions iron is soluble and mobile as Fe(II) below about pH 7 to 8, when it occurs, usually as uncomplexed Fe ion. However, where sulfur is present and conditions are sufficiently anaerobic to cause sulfate reduction, Fe(H) precipitates almost quantitatively as sulfides. Discussion and explanation of these observations is given below. Thermodynamic data for iron aqueous species and solids at 25°C considered in this chapter are given in Table A12.1. Stability constants and A//° values computed from these data are considered more reliable than their values in the MINTEQA2 data base for the same species and solids. 

Until recently it has been assumed that sulfate-reducing bacteria always required a strictly anaerobic environment. These environments are found in deep coastal-plain areas, oil-field brines, and in black (organic-rich), waterlogged soils and muds associated with rivers, lakes, and swamps. Sulfate reduction has also been observed in local microenvironments such as those created by the decay of a fish buried in otherwise oxidizing sediments (Berner 1971). Contrary to traditional belief, active sulfate reduction has also been observed in the presence of dissolved oxygen in the photosynthetic zone of microbial mats (Canfield and Des Marais 1991). 

The SLillide produced from sulfate reduction plays a major role in metal sul-lide immobilization in sediments but has also been applied to bioremediation of metals in waters and soil leachates. One process used sulfur- and iron-oxidizing bacteria to liberate metals from soils in the form of an acidic sulfate solution that enabled almost all the metals to be removed by bacterial sulfate reduction (White et al., 1998). Large-scale bioreactors have in fact been developed using bacterial sulfate reduction for treating metal-contaminated waters (Barnes et al., 1992 Gadd, 1992b). 

There is little evidence of a direct or indirect role or soil bacteria on the dynamics of Cs. Russell et al. (2004) recently reported that bacterial sulfate reduction decreased the adsorption of Cs on arid and tropical soils, but proposed no mechanism. It is unlikely that the accumulation of Cs makes soil microorganisms an important pool for immobilized Cs in mineral soils (because adsorption on soil clays would be much greater). However, this may not be the case in organic soils (Sanchez et al., 2000). 

Sulfide methylation reactions couple dissimilatory sulfate reduction to DMS production and determine the rates of DMS emission in freshwater wetlands. This process involves acetogenic bacteria, some of which degrade aromatic acids to acetone. In soils, freshwater, and marine ecosystems a wide diversity of other anaerobic and aerobic bacteria can contribute to sulfur gas production. In addition, diverse aerobes (e.g. methylotrophs and sulfate oxidizers) and anaerobes (e.g. methanogenes) consume S gas, thereby regulating fluxes in the atmosphere-biosphere system. 

In semiarid climates, sulfate deposits in the soil are sometimes reduced under conditions of poor drainage. The conditions necessary for reduction are created in natural basins where water accumulates due to the very low water permeability of sodic soils. The sulfate reduction reaction ... 

Soil redox potential is also critical in controlling elemental mobility. Some elements are much more soluble and mobile in one oxidation state than another (examples include Cr, Mn, Se, and others). The elements classified as chalcophiles (e.g., Hg, Cu, Pb, Cd, Zn, As, Se) form insoluble sulfide minerals in reducing environments where sulfide (S ) is generated from sulfate reduction (see Chapters 4 and 7). Mobility for chalcophiles is then extremely low unless oxidizing conditions are restored in the soil. Those elements that, in the sulfide form, have the very lowest solubility products (notably mercury, copper, lead, and cadmium) are the most Ukely to become highly immobile and unavailable in reduced soils. ... 

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