Micronutrient And Trace Metal Cycling In Soil

A generic cycle diagram for micronutrient and trace metals in soils is illustrated in Fig. 15.8. Concentrations of elements as free ions or soluble complexes are strongly influenced by abiotic reactions such as changes in oxidation state, fixation to mineral surfaces, complexation with organic matter, and formation of insoluble minerals. Microorganisms are able to solubilize minerals and change the soil redox potential and pH. Plant availability of micronutrients is therefore highly dependent on microbial activity. Recycling of micronutrients occurs when plant litter is returned to the soil and mineralized by the microbial biomass, thereby releasing the trace metals. The production and secretion of various chelating agents by plant roots and microorganisms promote the dissolution and weathering of minerals and facilitate the movement of micronutrients to roots. Inputs of most heavy metal elements are from anthropogenic sources. Human activities since the late 1800s have released at least 106 metric tons of Hg into the environment. Coal contains 1200 to 21,000 ng Hg g-1, and the burning of oil and coal releases 2 to

FIGURE 15.8 A generic cycle diagram for micronutrient and metal elements in soil (with permission from Stevenson and Cole, 1999).

7 X 104 metric tons Hg year-1. Most of this Hg eventually finds its way into the oceans. An additional transformation that many potentially toxic metals undergo, resulting in their loss from the soil system, and that some micronutrient metals do not, is volatilization. This transformation is a detoxification strategy for organisms and releases the toxic metal to the atmosphere in a volatile form.

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