CHANGES IN MINE SOIL PROPERTIES RESULTING FROM OVERBURDEN WEATHERING

Hard rock overburden is often used as a topsoil substitute for reclamation in the southern Appalachian surface mining region because of the limited availability of natural topsoil. In relatively unweathered overburden materials, soil-forming processes are accelerated and the resulting mine soils form rapidly. Morphological, physical, and chemical properties related to overburden weathering in mine soils formed from siltstone and sandstone overburden were observed for 6 to 8 yr. Distinct surface soil horizons, enriched with organic matter, occurred within two growing seasons after mine soil construction, and discernible subsurface horizonation developed within four growing seasons. The degree of development of surface and subsurface soil horizons became more pronounced after 8 yr. Coarse fragment content of sandstone-derived mine soils decreased, and silt content of siltstone-containing mine soils increased during six growing seasons. Although pH and extractable Ca and Mg levels decreased during the first three growing seasons, apparently due to rapid leaching of exposed carbonates, these properties rebounded to near initial levels after six growing seasons due to release of Ca and Mg from continued carbonate weathering and phytocycling. Levels of extractable Fe tripled between the first and second growing seasons and then remained relatively constant for the rest of the study. The changes in soil properties in these mine soils indicated that geochemical weathering and oxidation was occurring rapidly. This study demonstrated the complexity of mine soil weathering and development processes and the difficulties involved in predicting all eventual mine soil properties from premining overburden analysis.