QUANTIFICATION OF AMMONIUM SORPTION IN ACID FOREST SOILS BY SORPTION ISOTHERMS

Quantity/Intensity relations of NH4+ were established to evaluate the role of NH4+ sorption as a N-sink as well as for the transport of NH4+ within the soil profile. Two haplic podzols (originating from Phyllite and Granite) from the German Fichtelgebirge were used. Exchange isotherms were obtained from batch experiments with NH4+ concentrations between 0.01 to 0.6 mmol NH4+ L(-1). Background solutions had a composition close to the natural soil solutions to identify the cations displaced by NH4+. Organic layers sorbed a maximum amount of 0.3-0.5 mmol(c) NH4+ 100 g(-1), A horizons 0.1-0.2 mmol(c) NH4+ 100 g(-1) and B horizons 0.09 mmol(c) 100 g(-1), corresponding to less than 1% of the cation exchange capacities. NH4+ displaced K+ and Ca2+ throughout the whole profile, and additionally Al in the subsoil horizons and H3O+ in the organic and A horizons. ''Initial mass isotherms'' (Quantity/Quantity isotherms) were established with distribution coefficients ranging from 0.17 to 0.35 for NH4+ with highest values in the O horizons. While exchangeable NH4+ in acid forest soils is considered to be only a minor long-term sink for deposited NH4+, it has its relevance in the seasonal dynamics and transport of NH4+.