Indicators of pedogenesis of Technosols developed in an ash settling pond at the Bełchatów thermal power station (central Poland)

Technogenic soils (Technosols) developed in an ash settling pond at the Bełchatów thermal power station, central Poland, were studied in order to identify soil property transformations over 30 years of pedogenesis. Standard pedological methods were applied in order to determine the properties of the studied samples. All investigated soils were classified according to WRB as Spolic Technosols with various supplementary qualifiers (Alcalic/Hypereutric, Arenic/Loamic, Protocalcic, Hyperartefactic, Immisic, Laxic, Ochric, and Protosalic). The studied materials can be arranged into a chronosequence starting from fresh (unweathered) ashes, by young Technosol BE1 (age: several months), up to older Technosols BE2 (about 20 years) and BE3 (about 30 years). The studies showed that weathering and soil-forming processes changed properties of ash in soil environment. Fresh ash was characterized by high pH (11.0 – fly ash, 8.7 – bottom ash), low content of carbonates (1.5% in both samples), variable concentrations of TOC (1.2% – fly ash, 6.9% – bottom ash), and very low total nitrogen content (0.04%). Electrical conductivity (ECₑ) was 2.6 and 2.1 dS·m⁻¹ in fly ash and bottom ash respectively. Young Technosol BE1 had the pH 9.2–10.0, contents of carbonates were in the range 2.4–3.3%, TOC 1.3–1.7%, and total nitrogen less than 0.03%. ECₑ in young Technosol was in the range 2.7–4.0 dS·m⁻¹. There was no plant cover present on that soil and no well-developed genetic horizons were distinguished in the profile. Finally, old Technosols BE2 and BE3 had lower pH (from 7.9 up to 9.1), and, in general, higher contents of carbonates (from 1.5 to 7.9%) than fresh ash and young Technosol BE1. Old Technosols contained high concentrations of TOC (up to about 38% in Oi horizon) and total nitrogen (up to 0.9%) in the topsoil, where O and A horizons developed due to accumulation of soil organic matter. ECₑ in old Technosols was in the range 0.8–1.5 dS·m⁻¹. All studied ashes and soils were characterized by very low or even absence of total potential acidity. Base cations predominated in the sorption complex of the investigated ash and soils and can be arranged in the following order according to the abundance: Ca>Mg>K>Na. Base saturation (BS) of fresh ashes and Technosols was nearly 100%. The study shows that the first indicators of pedogenesis of the studied technogenic soils within the first 30 years of formation are: (1) changes of consistence of ash material from firm to friable/very friable due to root action, (2) accumulation of soil organic matter in the topsoil and formation of O and A horizons, (3) decrease of pH, (4) formation of pedogenic carbonates in soils and (5) decrease in soil salinity.