Increases in extractable sulphate following soil submergence with water, dilute sulphuric acid or acid rain

Abstract

A marked increase in the concentration of LiCl-extractable sulphate occurred when samples of two acid brown earth soils (one from a site exposed to heavy atmospheric pollution and the other from a relatively unpolluted site) were waterlogged in the laboratory with either deionized water, dilute sulphuric acid (pH 4·0) or acid rain (pH 3·8). The greatest increases in sulphate concentration occurred in the heavily polluted soils. However, no similar increases were found when two neutral loam soils were waterlogged. The pH of the brown earths increased on waterlogging, while soil Eh fell, but only slowly. Sulphate reduction eventually occurred in the brown earths following most treatments, resulting in a decrease in sulphate concentration. No free H₂S was evolved under these conditions, however. The acid rain treatment had the same effect as dilute sulphuric acid and deionized water on sulphate transformations in these woodland soils. The increases in the concentration of LiCl-extractable sulphate which occur when brown earths are waterlogged appear to result from a combination of organic sulphur mineralisation and increased desorption of adsorbed sulphate.