The effect of an extended agricultural reclamation on changes in physical properties of technosols in post-lignite-mining areas: A case study from central Europe

Opencast mining destroys soils that have formed over many thousands of years. Their place is taken by post-mining areas such as external and internal dumps with materials whose physical and chemical properties are unfavorable from the point of view of plant growth and development. In order to minimize the negative effects of opencast mining, reclamation is necessary. In this work, we examined the impact of 42-year agricultural reclamation, in which three variants of mineral fertilization (0NPK, 1NPK, and 2NPK) were applied in the cultivation of alfalfa with the addition of orchard grass on changes in physical properties of Technosols. After forty-two years of agricultural reclamation, there has clearly been a differentiation of post-mining material into genetic horizons that differ in physical properties. The surface part of Technosols was converted to a humus-enriched surface horizon (Ap), which consisted of two subhorizons: Ap1 and Ap2, differing in structure, color, and consistency. In the 1978–2020 period in Ap1 of fertilized soils, there was a decrease in bulk density (BD, 9.0–10.9%) and particle density (PD, 1.5%), but an increase in structural stability index (SI, 243–337%), soil porosity (SP, 15.5–19.2%), air-filled porosity (AFP, 10.8–11.7%), field capacity (FC, 49–21.6%), plant available water capacity (PAWC, 46.4–48.0%) and S-index (46.9–50%). In the Ap2 subhorizon, there was a similar trend of changes; however, BD (for 2NPK), FC (for 1NPK), and PAWC (for 1NPK and 2NPK) were not significant. BD (5.4–14.3%) increased in most horizons of parent material (Cd) between 1978 and 2020, and SP (10.9–29%), AFP (16. 2–73.9%), and S-index values (9.4–34.4%) decreased. On the other hand, PD, SI, FC, and PAWC values did not change significantly.
Cultivation o alfalfa with orchard grass in the 1978–2020 period significantly affected the profile variation in physical properties of Technosols, which was not observed when the field experiment was set up (1978). In the Ap horizon, DB and PD had significantly lower values, while SI, SP, AFP, FC, PAWC, and S had significantly higher values compared to parent materials (Cd). This is related to the indirect effect of soil organic carbon (SOC), which was beneficial to improving the surface horizon properties of developing Technosols. The diversity of physical properties of the 42-years old post-mining Technosols in the Ap was dependent on fertilization, especially in the Ap1 subhorizon. In fertilized Technosols, the BD and PD values were lower, and SP, DP, AFP, FC, PAWC, and S values were higher compared to unfertilized Technosols. Parent material (Cd) horizons after 42 years of reclamation had similar physical properties regardless of fertilization.