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 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 ⁺.