H2po4

Carbohydrates, proteins, phenolic molecules, alcohols, sulphydryls

Herbicides, fungicides, pesticides, PCB, solvents, detergents, petroleum products

HMoO-

Data compiled from various sources.

tration of these soil properties is to plot the main variables on a multi-axis graph (Fig. 2.10). Such graphs are useful visual aids when comparing several soils together (Alvim and Cabala, 1974; Alvim, 1978).

Cations (mM)

Cations (mM)

Anions (mM)

Organic matter (%)

Fig. 2.10. Graphic for representation of the chemical properties of the soil and soil solution (see Alvim and Cabala, 1974; Alvim, 1978).

Anions (mM)

Organic matter (%)

Fig. 2.10. Graphic for representation of the chemical properties of the soil and soil solution (see Alvim and Cabala, 1974; Alvim, 1978).

Charged molecules and ions released from litter are often held tightly in the SOM as chelates (i.e. a strong chemical bond forms between an organic 'ligand' molecule and the inorganic 'prosthetic' group). Unlike cations which can bind to clays irreversibly, those bound to organic molecules are still accessible as nutrient. That is because the organic molecules can be ingested and digested to release the nutrients. This is important because organic molecules hold far more chemical charges per mass than clays, to bind ions. Some ions are held reversibly by dipole interactions with organic molecules. One effect of charged organic molecules reversibly binding ions is to contribute to the soil buffering capacity. Soils with higher organic matter content are more resistant to small changes in pH. Adhesion or accumulation of molecules, atoms or ions on soil particle surfaces (to organic matter, clays and water), by chemical or physical forces, is called adsorption. Desorption into soil water depends on the solubility of molecules, as determined by their solubility constant Ks, or Ksp for sparingly soluble substances. The intake of soluble nutrients into cells is called absorption. Once nutrients are inside cells, they are no longer part of the SOM and are said to be immobilized. Thus, the ratio of immobilized nutrients in the biomass to that in the SOM and soil water is an indication of nutrient distribution in the soil. A more useful assay is the rate of biomineralization (transfer of SOM through cells) which reflects how active the biological community is.

The composition of soil solution can be obtained from commercial lysimeters. A discussion of differences in solution chemistry is provided by Lajtha et al. (1995, 1999), Marques et al. (1996) and Haines et al. (1982). These studies do not discriminate between minerals present in the soil moisture and those released from lysis of cells in the collected solution. Ion exchange resins and membranes have been used to estimate the amount of soil water available ions in situ. The buried resins absorb soil available anions or cations which are then extracted in the laboratory for quantitation. They are thought to mimic uptake of minerals by plant roots, and thus provide some quantitation of mineral availability over time. However, there is concern over interpretation of the quantities as they do not always correlate well with other measures of mineral availability (Lajtha, 1988; Giblin et al., 1994).

Other procedures involve analysis of the wash filtrate from a soil sample, or ultracentrifugation of soil to separate the heavy mineral soil from most of the water (see Lajtha et al., 1990). A modified technique involves centrifugation of soil in a liquid immiscible with water and denser than water, such as tetrachloroethylene (C2Cl4). During centrifu-gation, the mineral soil sediments and water is displaced to the top by the immiscible liquid. Losses of minerals and ions in the immiscible liquid need to be accounted for. In all these techniques, living cells are lysed into the soil solution. The contribution of lysed cells to the solution must be subtracted from the data. This can only be done in a general approximate way, by using mean values for cell composition.

The composition of the soil solution is important to plant nutrition, as it reflects the availability of nutrients and minerals to the roots (Smethurst, 2000) and regulates the rate of primary production. There is a triangular association between the role of the interstitial soil community in decomposing the litter, the influence of mineral soil particles and the overarching role of climate in controlling decomposition and the interstitial species composition. These interactions have been the subject of many reviews and discussions (Kalbitz et al., 2000; Qualls, 2000; Neff and Asner, 2001).

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