Ways of soil development on stony substrate from hard coal mining spoil

Abstract

The effect of stones on soil development is presented with the example from a hard coal mining spoil. The investigation concerns the stone and fine earth content, total bulk density, bulk density of the stone and fine earth fractions, pore volume, total carbon, organic carbon and sulfur content, as well as pH. Samples were taken from two different mine spoil depositions, the Monopol and Achenbach spoils, of the vegetation trial at Waltrop in the Ruhr area, Germany. Sampling occurred at the start of the trial and at two times, 8 and 14 years later, respectively. The samples were taken in big rings which were pushed into the mine spoil soil. The depth intervals of samples were tight with 0-2, 2-5, 5-10 and 10-20 cm below surface. Total organic carbon (TOC) and sulfur content and the pH were measured using standard methods. TOC was determined by destruction with H2O2. This proved to be a reasonable method which is relatively easy to perform. Soil formation occurred very fast, within eight years, in substrate with stone content by weight of 60-80 %. However, the way of soil formation differed between the Monopol and Achenbach mining spoils. The results indicate that in Monopol spoil, accumulation of TOC occurred by the intrusion of fine earth into pores between stones. However, in Achenbach spoil, fine earth was formed by the weathering of stones. The TOC content of fine earth achieved 8 to 16 mg g-1. The TOC stock within 20 cm depth and 1 m2 of the Monopol and Achenbach spoils was 1.0 and 0.6 kg, respectively. This is a very low value compared to the up to 12 kg in 1 m depth of the farmland soils of the region. The mine spoils contain between 2 and 18 mg g-1 sulfur. Formation of sulfuric acid by the oxidation of sulfides in the spoil decreased the pH from 7.6 at the start of the trial to around pH 4.5 after 8 and 14 years in the Achenbach spoil. In the Monopol spoil, the pH increased with depth from pH 5.5 to 7.2. A negative relationship exists between pH and pore volume for the Monopol spoil. Due to the TOC accumulation of more than 6 mg g-1, the soils can be classified as Regosols from hard coal mining spoil substrate in the German classification system. Within US Soil Taxonomy, the soils would belong to the Entisols. In the WRB soil classification, the soils can be positioned to the Leptosols, with the qualifiers Technic, Skeletic and as an additional qualifier Sulfuric and Intrusic or alternative to the Technosols with the qualifiers Spolic, Skeletic, Sulfuric and Intrusic. The occurrence of stony spoil deposits from deep hard coal mining in the loess belt from Eastern France to Eastern Ukraine changed large areas of originally rich fertile soils to poor soils, but promoted the bio-diversity.