Chemical effects of pH 3 sulphuric acid on a soil profile

Abstract

Cores of podzolic soil (monolith lysimeters) were treated for 4.8 yr with 1500 mm yr−1 of either 0.5 mM H2SO4 at pH 3, equivalent to 24 g S m−2 yr−1 (acid treated) or distilled water (controls). The acid treatment was about 37 times greater than the average annual input of H3O+ from rain at the site from which the monoliths were taken. Acid treatment acidified the litter (from pH(CaCl2)3.4 to pH(CaCl2)2.6) and the mineral soil to a depth of 80 cm (mean pH(CaCl2) decrease of 0.2 unit). In the litter and upper A horizon, ion-exchange reactions provided the main neutralizing mechanism, resulting in a decrease in the reserves of extractable (in 2.5 % acetic acid) Ca, Mg, and Mn of about 70 to 80 %. Dissolution of solid phase Al from hydrous oxides provided most neutralization below this depth. Al3+ was the principal soluble Al species throughout the profile. In the litter and upper A horizon, some of the mobilized Al3+ was retained on cation exchange sites resulting in an increase in exchangeable Al. Deeper in the profile, where the exchange sites were effectively saturated with Al3+, no increase in exchangeable Al occurred, and Al3+ was, therefore, available for leaching. Some reversible adsorption of SO4 2−, associated with hydrous Al oxides, occurred in the Bs and C horizons. The results are discussed in relation to possible effects of acid deposition over regions of Europe and N. America.