Iron cycle soils

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

We can see that the soluble and exchange forms of these metals are present in small amounts accounting merely for a few percent of the total metal content in soil. The content of organometal species is relatively high in the upper profile rich in humic species, whereas it drops sharply in the mineral horizons. Copper is extensively involved in the biogeochemical cycle in the Forest ecosystems and this is less profound for cobalt. It is noteworthy that a large part of metals (in particular, of copper) become bound to iron hydroxides. This is typical for various trace elements, including arsenic, zinc and other elements with variable valence. 

Contamination of urban soils by POPs such as PCBs is of concern for human and ecological health. Soil contamination by metals and persistent organics disrupts nutrient cycling and is associated with declines in density of sod microfauna and lower rates of decomposition of organic compounds (Carreiro et al., 1999 Pouyat et al., 1994), an important process for the removal of organic contaminants from the environment. Ironically, urban warming may counteract this effect in some locations (McDonnell et al., 1997). 

When the concentration of relatively insoluble elements in soil solutions reaches saturation with secondary minerals, they precipitate in soils, particularly tropical soils. Lucas (2001) points out that precipitation of silicon, aluminum, and iron in tropical soils occurs because of the low solubility of clay minerals and oxides and the high concentrations of these elements in the topsoil horizons produced by plant cycling of these elements. 

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