Soil exchange complexes

Once precipitation begins, a quasi-steady state will eventually be attained in which the soil pe and pH are poised by the redox and precipitation equilibria operating. In the transition to the steady state, protons will be provided by dissociation of acids in the soil solution—e.g. H2CO3 derived from C02-and by reactions with the soil exchange complex. The course of reduction and the eventual steady state will depend on these reactions and it is therefore necessary to allow for them in predicting what the steady state conditions will be. 

Blum et al. (2002) and Probst et al. (2000) have found, in areas similarly cation-depleted by acid deposition, that a considerable proportion of calcium released by weathering came from apatite dissolution. This apatite was utilized directly by ectomycorrhizal tree species (spruce and fir) bypassing the soil exchange complex. In the Blum et al. (2002) study, ectomycorrhizal fungi associated with the roots of conifers provided 95% of the calcium found in the foliage of the trees and 35% of the Ca leaving the mixed conifer hardwood watershed in stream water. 

Stage 5 starts when the excess acid loading from the atmosphere stops. With continued weathering, part of the supply of base cations is adsorbed onto the impoverished soil exchange complex, lowering the runoff cation concentrations to values below pre-acidification values. The concentration of base cations decreases to a minimum value. 

If the soil exchange complex is saturated with both H+ and Ca + ions at equilibrium Schoefield s ratio law states ... 

Figure 3.7. Coricepmai diagram iilasnacmg calcium cycle in forest watersheds. Inputs of calcium include weathering and atmospheric deposition of these weatijering is usually the greatest. U)sses of calciitm include tree accuinitJation and stream runoff. Under conditions of elevated acidic de x>sitfon stream losses increase, potentially depleting available calcium from the ecosystem, particularly ifom the soil exchange complex...

Large amounts of carbon are found in the terrestrial ecosystems and there is a rapid exchange of carbon between the atmosphere, terrestrial biota, and soils. The complexity of the terrestrial ecosystems makes any description of their role in the carbon cycle a crude simplification and we shall only review some of the most important aspects of organic carbon on land. Inventories of the total biomass of terrestrial ecosystems have been made by several researchers, a survey of these is given by Ajtay etal.(1979). 

At high electrolyte concentrations of the soil solution, the double layer is compressed so that clay remains flocculated. A decrease in ion concentration, e.g. as a result of dilution by percolating rain water, can result in dispersion of clay and collapse of aggregates. If the exchange complex is dominated by polyvalent ions, the double layer may remain narrow even at low electrolyte concentrations and consequently aggregates remain intact (FAO, 2001). 

Becanse of the complexity of soils, there are no general relations between the proportions of two cations on the exchange complex and their reduced activity ratio in solntion. Bnt two equations are commonly used the Gaines and Thomas... 

Metal cations may be soluble, readily exchangeable, complexed with organic matter, or hydrous oxides, substituted in stoichiometric compounds, or occluded in mineral structures (see reviews by Brummer et al., 1986 Beckett, 1989 Forstner, 1991). The chemical factors that affect the retention of a specific chemical form of a trace metal (e.g. effects of pH and I on specific adsorption ) are well documented (Jones and Jarvis, 1981 Tiller, 1983 McBride, 1989 1991 Alloway, 1990 Forstner, 1991). When several components co-exist in a soil, the distribution of a trace metal among them will also depend on the type and relative quantities of the soil components how they change with pH, I, etc. and the extent of saturation of adsorption sites on soil adsorbents. 

The sodium adsorption potentials of two humid soils are presented in some detail and the SAR-ESP data are shown in Figures 11.1-11.4. For comparison purposes, these figures also include the SAR-ESP relationship of salt-affected soils found in arid-region soils (western United States). Figures 11.1-11.4 show that for any given SAR, the ESP for either one of the two humid soils is greater than the ESP of the western U.S. soils. This indicates that the two humid soils adsorb sodium on their exchange complex more effectively than the western U.S. soils. 

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