Characteristics of pipestems from acid sulfate soils of Finland and Australia

Abstract

This study describes iron (Fe) pipestems formed around and within root channels during the development of a homogeneous unripe sulfidic sediment into a ripe soil where redox- or pH-active elements segregate. Pipestems and adjacent soil material samples were characterised by chemical and mineralogical analyses and scanning electron microscopy (SEM) associated with energy dispersive spectroscopy (EDS). Pipestems occurred at 60–190 cm and at 30–130 cm in a young Finnish soil and a more mature Australian soil, respectively, while sulfidic materials were found below 170–180 cm. The pipestems consisted of mineral grains cemented together by Fe precipitate. There was an up to 24-fold enrichment of Fe and an up to 13-fold enrichment of sulfur (S) in the Finnish pipestems compared to the adjacent soil material, whereas the corresponding enrichment in the Australian soil was up to 27-fold for Fe and up to 8-fold for S. In the Australian pipestem matrices, the Fe concentration was as high as 40% compared to 14% in the Finnish ones. It was estimated that about 70 ton ha⁻¹ S had been mobilised from the sulfidic material in the Finnish soil at the depth of 50–150 cm. Part of S has leached out but a substantial amount remained in the soil constituting the stock of retained acidity. Almost all pipestems contained a new solid phase precipitated within the former cortex cells of the plant roots. In the Finnish samples, this precipitate consisted of jarosite and schwertmannite. After oxidative exhaustion of sulfidic material in the surrounding soil, these metastable minerals are gradually hydrolysed, associated with leaching of S. In the mature Australian soil, most of these minerals had already been transformed to goethite. Pipestems formed after roots in unripe soil are sites for synthesis and hydrolysis of minerals and serve as routes for atmospheric oxygen into the reduced subsoil and for soluble reaction products to exit the soil. Pipestems have an important role in the ripening of the soil profile.