Quantification of ammonium sorption in acid forest soils by sorption isotherms

Abstract

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 mmolc NH4 + 100 g-1 A horizons 0.1–0.2 mmolc NH4 + 100 g-1 and B horizons 0.09 mmolc 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 +.