Changes in soil physical properties of forest floor horizons due to long-term deposition of lignite fly ash

Abstract

From the beginning of the twentieth century until the 1990s, energy in Upper Lusatia, Saxony in Eastern Germany was produced at power plants that burnt lignite coals. As a result, alkaline fly ash and aerosols from the combustion of brown coal have accumulated in adjacent areas that are partly under forestry. We ask the question, “how have these atmospheric depositions of fly ash influenced the soil physical properties (bulk density, particle density, saturated hydraulic conductivity, pore size distribution, and water repellency) of forest floor horizons?” The experimental sites represented typical soil types and stands of the sylviculturally used areas in the region of Upper Lusatia. Three forest sites were located close to the emission sources, where high amounts of fly ashes accumulated, and three control sites were without fly ash enrichment. Pore size distribution, saturated hydraulic conductivity, and bulk density were examined with undisturbed samples (metal cylinder 100 cm³). Disturbed samples were used for the characterization of particle density, texture, and water repellency (Wilhelmy plate method). Additionally, the carbon content was determined. Scanning electron microscopy was used to show fly ash enrichment. The enrichment of mineral fly ash particles could be proven for sites close to the emission source. Using scanning electron microscopy, spherical fly ash particles could be identified. Total quantities of persistent fly ash enrichment amounted to approximately 150-280 Mg ha–1. The enrichment of fly ash affected the soil-physical characteristics. Close to the emission source (sandy fly ashes), particle density, air capacity, and saturated hydraulic conductivity were significantly increased, whereas the plant available water was significantly reduced. With increasing distance from the emission source (silty fly ashes or no ash enrichment), air capacity and saturated hydraulic conductivity were reduced, while an increase of plant available water was observed. Furthermore, the forest floor horizons close to the emission source were characterized by significantly reduced water repellency due to the dominance of hydrophilic mineral fly ash particles. Fly ash deposition in Upper Lusatia must be considered as relevant for properties of forest soils. Mean particle density was significantly higher at sites with fly ash accumulation. This indicates the admixture of mineral particles. While bulk densities were not noticeably influenced, the increase of particle density and the dominance of sandy to coarse silty particles close to the emission sources cause an increase in total porosity, air capacity, and a relative reduction of plant available water. Hollows in spherical fly ash particles might contribute to the meso- and macropores. Due to the admixture of hydrophilic fly ash, the enriched forest floor horizons feature a distinct increase in potential wettability, which coincides with a higher pore and, hence, nutrient and contaminant accessibility. In combination with a higher saturated hydraulic conductivity, an increase in translocation of dissolved substances can be expected especially in the course of acidification, which causes an additional mobilization of nutrients and contaminants. With this study, we could prove the impact of fly ash enrichment on physical soil properties of forest floor horizons. Via SEM, we detected fly ash particles. The amounts of persistent fly ash accumulation could modify particle density, thickness, bulk density, and carbon content. To characterize hydraulic properties, we investigated the pore size distribution, the saturated hydraulic conductivity, and a water repellency parameter. Thereby, we detected a distinct increase of coarse pores and an accompanying extremely high saturated hydraulic conductivity. The water repellency parameter indicated a significant decrease of hydrophobicity of fly-ash-enriched forest floor horizons. Fly ash enrichment in forest floor horizons not only causes distinct chemical modifications but also alters soil physical properties, which must be considered in further hydrological investigations, as they may influence seepage of water and contaminant translocation within the soil and into groundwater.