Abstract
Cultivated surface soil samples of an acid Gleysol (soil 1, pH 4.9) and a slightly acid Podzol (soil 2, pH 6.7) were equilibrated for 48 h with oto 144 meq H+ kg-1 by a batch technique designed to simulate reactions of acid load with soil constituents. The pH of the titration suspensions ranged in soil 1 from 5.6 to 3.3, in soil 2 from 7.2 to 4.7. The exchange reaction with base cations on variable charge sites was an important mechanism for H+ inactivation. The quantities of cation equivalents released were, however, lower than the proton equivalents added. Calcium dominated the supernatant solutions, but as related to exchangeable reserves. Mg seemed to be more susceptible to acidification at high soil pH. Protons were also consumed in the mobilization of divalent base cations from a non-exchangeable pool to an exchangeable one. The experimental soils differed in their response of acid cation fractions to proton loading. In the rather neutral soil 2, the quantities of soluble and exchangeable acid cations were very low and not affected by acidification. The Al dissolved by proton attack was immobilized by complexation reactions. This mechanism did not operate in the acid soil 1 where the proton loading markedly increased the exchangeable Al pool and, consequently, the soluble Al in the supernatant solution. This was associated with a simultaneous reduction in the complexed Al and a small increase in complexed Fe. Furthermore, acidification diminished the effective cation exchange capacity (ECEC) decisively less in soil 1 than in soil 2, because the increase in exchangeable Al markedly compensated the reduction in the exchangeable base cations. As compared to freelydrained systems, the batch titration overestimated the release of Al to solution phase.
Highlights
Soil acidification is characterized by intensity and capacity factors
The reciprocal of the slope of the titration graph stands for the buffer capacity (BC), defined as the number of mmols or meq of H + that must be added to 1 kg of soil to lower pH by one unit
In order to monitor acid-evoked changes in the soil and solution cations, the titration was performed without background electrolyte
Summary
Soil acidification is characterized by intensity and capacity factors. Intensity factors are determinedby chemical properties and are independent of the size of the system considered, whereas capacity factors are a function of the size of the system (Van Breemen et al 1983). Soil pH is an attribute indicating the intensity of acidity as well as the chemical and biological conditions of a soil. Its alteration in response to acid loading is determined by the buffering properties of the respective soil. The impact of acid precipitation in a special edaphic ecosystem is dependent on the type of buffering reactions in volved. Buffering by certain mechanisms can be ample but ecologically harmful Ulrich 1981, SCHWERTMANN et al 1987) Buffering by certain mechanisms can be ample but ecologically harmful (e.g. Ulrich 1981, SCHWERTMANN et al 1987)
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