Cations adsorbed to soil organic matter — A regulatory factor for the release of organic carbon and hydrogen ions from soils to waters

Surface waters in northern forest ecosystems receive a substantial amount of drainage water from superficial soil horizons enriched in organic matter (SOM). Chemical reactions in the interface between the soil solution andf organic colloides will therefore affect the surface water chemistry. The mobilization of total organic carbon (TOC) and pH was studied as a function of amounts of organically adsorbed Na, Ca and Al in two O and one A horizon, which differed in the likelihood of contributing to the chemistry in runoff, in a forested watershed in northern Sweden. The samples were hydrogen ion saturated, washed and titrated with NaOH, Ca(OH)₂ and Al(OH)₃ in a constant ionic medium of 0.01 M NaCl in order to give rise to a population of manipulated samples differing in the composition of adsorbed cations. The highly humified SOM accumulated in the O_h and A_h horizons of a Gleysol close to the draining stream was stabilized by flocculating Al (95% of adsorbed metal cations), which resulted in a low release of TOC. These horizons showed a high potential of organic carbon solubility when Al was changed for di- or monovalent cations. Calculations suggested that the release of TOC would increase more than ten times if Al was exchanged for Ca upon liming to pH 6.0. The pH values of all horizons were shown to be determined mainly by the composition of adsorbed mono-,di- and trivalent cations.