CATIONS ADSORBED TO SOIL ORGANIC-MATTER - A REGULATORY FACTOR FOR THERELEASE OF ORGANIC-CARBON AND HYDROGEN-IONS FROM SOILS TO WATERS

Surface waters in northern forest ecosystems receive a substantial amo unt of drainage water from superficial soil horizons enriched in organ ic matter (SOM). Chemical reactions in the interface between the soil solution andf organic colloides will therefore affect the surface wate r chemistry. The mobilization of total organic carbon (TOC) and pH was studied as a function of amounts of organically adsorbed Na, Ca and A l in two O and one A horizon, which differed in the likelihood of cont ributing to the chemistry in runoff, in a forested watershed in northe rn Sweden. The samples were hydrogen ion saturated, washed and titrate d with NaOH, Ca(OH)(2) and Al(OH)(3) in a constant ionic medium of 0.0 1 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 me tal cations), which resulted in a low release of TOC. These horizons s howed a high potential of organic carbon solubility when Al was change d for di- or monovalent cations. Calculations suggested that the relea se of TOC would increase more than ten times if Al was exchanged for C a upon liming to pH 6.0. The pH values of all horizons were shown to b e determined mainly by the composition of adsorbed mono-, di- and triv alent cations.