Chemical properties of forest soils along a fly-ash deposition gradient in eastern Germany

Chemical characteristics of forest soils subjected to long-term deposition of alkaline and acid air pollutants were analysed in spruce (Picea abies (L.) Karst.) stands in eastern Germany. Three forest sites along an emission gradient of 3, 6, and 15 km downwind of a coal-fired power plant were selected, representing high, intermediate, and low fly-ash input rates. Past emissions caused an accumulation of mineral fly-ash constituents in the organic layer, resulting in an atypically high mass of organic horizons of forest soils, especial v in the F and H horizons. Total mass of organic layers at the site with heavy deposition loads was as high as 128 t ha(-1), compared to 58 t ha(-1) at the low input site. Fly-ash deposition significantly increased the pH values in the L, F and H horizons and mineral topsoil (0-10 cm). Significantly higher concentrations of NH4Cl-extractable cations (i.e. effective cation exchange capacities) and base saturations of > 66% were found in the humic horizons at sites where the pH was increased due to the direct and indirect (i.e. higher proportions of deciduous trees) effects of fly-ash emissions. Stocks of basic cations were dominated by Ca2+ and decreased significantly along the fly-ash deposition gradient from 33.6 to 5.3 kmol(c) ha(-1). Proportions of water-soluble basic cations out of the total potentially exchangeable (i.e. NH4Cl-extractable) basic cations generally increased in the forest soil with decreasing deposition loads following, the cation exchange capacity and base saturation along the fly-ash gradient. Higher proportions of monovalent cations, such as K+ and Na+, were observed in the water extracts from fly-ash-affected forest soils, while the NH4Cl-extracts were dominated by bivalent cations, such as Ca2+ and Mg2+. These results suggest a greater leaching tendency for monovalent cations in these soils. Stocks of organic C and total N in the humus layer decreased from sites with high fly-ash deposition levels to sites with low levels, from 57.4 to 46.4 t C ha(-1) and from 2.43 to 1.99 t N ha(-1). The C/N ratios of the organic horizons varied from 22 to, 25, revealing no distinct pattern along the fly-ash gradient. Measurements of hot-water-extractable and water-soluble organic C suggested a reduced availability or a faster decomposition of soil organic matter in soils with historically high fly-ash loads.