Effects of past fly ash deposition on the forest floor humus chemistry of pine stands in Northeastern Germany

Humus morphology and chemical characteristics of forest soils subjected to long-term deposition of alkaline and acid air pollutants were studied in pine forests on sandy substrates in Northeastern Germany. High emission rates of fly ash in the past caused an accumulation of mineral particles and ferromagnetic fly ash constituents in the organic layer. Mass and total ash content of organic horizons and magnetic susceptibility measurements suggested that fly ash was mainly accumulated in the F and H horizons of the forest soil. Total mass of organic layers at sites with heavy deposition loads were as high as 132 t ha−1. The F and H horizons of fly ash affected forest soils showed mineral contents of up to 59 and 79%, respectively. By measurements of magnetic susceptibility, it was shown that magnetic fly ash particles were found in the fine fractions (125–63 and <63 μm). Fly ash deposition significantly increased the pH values in the L, F and H horizon and mineral topsoil (0–10 cm). Lower pH values in the L layer compared to F and H horizons indicated a re-acidification of these forest soils due to reduced depositions of alkaline ash. Significantly higher concentrations of NH4Cl extractable cations (i.e. ‘effective cation exchange capacities’) and ‘base saturations (BS)’ of >90% were found in the humic horizons at sites where pH was increased due to high fly ash emissions. Stocks of basic cations were dominated by Ca2+ and decreased significantly along the fly ash deposition gradient from 46.33 to 6.91 kmol IE ha−1. Proportions of water extractable cations on NH4Cl extractable cations increased in the forest soil with decreasing deposition loads. Particularly monovalent cations (i.e. K and Na) are more mobile in fly ash influenced soils than bivalent cations (i.e. Ca, Mg). Stocks on organic C and total N decreased along the deposition gradient from 26.7 to 17.5 t C ha−1 and from 1.257 to 0.792 t N ha−1. In contrast, C/N ratios of the organic horizons increased from about 20 to 22 for F and H horizons. Measurements of hot water and cold water extractable organic C suggest that the availability of soil organic matter is reduced in soils with high historical fly ash loads.