Soil metal-organic complexes

System 7, 9, 10. roots-rizosphere (VII) plants (VIII) their biological reactions— metabolism (VIII) soil-soil solution, air (IV) aerosols—atmospheric air (26, 28). In this system, the influence of metal-organic complexes on the plant development and their metabolism is considered. Under deficient or excessive contents of some chemical species, the metabolism may be destroyed (see Figure 2). 

For understanding these tendencies, we will consider the values of the biogeo-chemical coefficient of aqueous migration. This coefficient Cw is the ratio between the content of an element in the sum of water-soluble salts and in geological rocks. The values of Cw for certain chemical species are smaller in Arid ecosystems than those in Forest ecosystems. We can suggest two explanations. First, soils of Forest ecosystems are enriched in water-soluble metal-organic complexes (see Chapter 7). Second, most chemical species are trapped in the transpiration barrier of upper soil layers of Arid ecosystems. 

As a result of microbial formation of metal-organic complexes with fulvic acids in soils of Tropical Rain Forest ecosystems, the surface and sub-surface runoff waters are enriched in some heavy metals like manganese and copper. A similar tendency has been shown for boron, strontium and fluorine. 

Griffith, S.M. and Schnitzer, M., 1975. The isolation and characterization of stable metal-organic complexes from tropical volcanic soils. Soil Sci., 120 126—131. 

Research on the metal speciation of the soil solution has been encouraged by the free metal ion hypothesis in environmental toxicology (Lund, 1990). This hypothesis states that the toxicity or bioavailability of a metal is related to the activity of the free aquo ion. This hypothesis is gaining popularity in studies of soil-plant relations (Parker et al., 1995). However, some evidence is now emerging that free metal ion hypothesis may not be valid in all situations (Tessier and Turner, 1995). Plant uptake of metals varies with the types of chelators present in solution at the same free metal activity. Furthermore, given the same chelate, total metal concentration in solution affects metal uptake by plants. Either kinetic limitations to dissociation of the complex or uptake of the intact complex could explain these observations (Laurie et al., 1991). The possible reactions of complexed metals at the soil-root interface and the potential uptake by plants of metal-organic complexes are depicted in Figure 1.8. 

In general, TEs in exchangeable and acid-soluble forms are considered to be easily bioavailable (see Table 12.1). The metal-organic complex-bound form is also relatively mobile (Krishnamurti et al., 1995). The reducible and oxidizable forms are relatively stable under standard soil conditions, yet easily reducible and oxidizable fractions may be readily mobilized, as detailed in the ensuing sections. 

The transition and heavy metals, referred to hereafter as trace metals, are important to plants and animals as both micronutrients and toxic elements. Many of them occur in the soil environment in cation form. As naturally occurring elements, some of these cations are incorporated into primary and secondary mineral structures and may be very unavailable. Schemes for complete extraction of these metals from soils require extreme treatments, including dissolution of certain minerals. As pollutants, the metals may enter the soil in organically complexed form or as metal salts. In the latter case, the metal cations then adsorb on mineral and organic surfaces. 

Owing to the diversity of organic materials capable of binding metals in sediment-water systems, the fixation or release of metals in soil-water systems does not exhibit the rather precise, predictable Eh-pH boundary conditions shown by simple aqueous systems. However, Eh and pH have been shown to influence metal-organic complex formation and stability. The effects of soil/ sediment Eh and pH on trace metal transformations in natural systems must therefore be studied empirically. 

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