Physical properties of forest soils along a fly-ash deposition gradient in Northeast Germany

This study investigates the effect of predominant historical atmospheric depositions of fly ashes from brown coal power plants on physical properties of forest soils along a deposition gradient (0 to 33.9 km). Studies on physical properties of a pure fly ash show a sand dominated texture, high particle density, (macro-)porosity and saturated hydraulic conductivity as well as a slight water repellency. The enrichment of fly ash particles is more obvious in the forest floor horizons closer to the main emission source, where an increase of air capacity (R2 = 0.67) and saturated hydraulic conductivity (R2 = 0.53), and a slightly higher particle density is observed, while plant available water (R2 = 0.57) and water repellency (contact angle R2 = 0.71) decreased. The organic C composition of the forest floor horizons, characterized by organic substances rich in aliphatic groups, which appear to be responsible for the partially extremely high water repellency is not influenced by fly ash accumulation. However, an input of fly ashes into the organic soil layers reduces the water repellency by adding non-hydrophobic mineral compounds. A hierarchical cluster analysis supports the gradual influence of fly ash deposition; sites close to the emission source differ distinctly from sites further away. No impact of fly ash was found for the top mineral horizons.