Characterization of metal-organic complexes in aspen and birch forest soils in interior Alaska

Abstract

Complexation of iron, copper, and manganese by organic substances contained in aqueous soil extracts obtained from the mineral B horizon of contiguous aspen (Populus tremuloides) and birch (Betula papyrifera) forests was characterized by gel-permeation chromatography (GPC), thin-layer chromatography (TLC), ion-exchange chromatography, atomic absorption spectrometry, and infrared spectroscopy. Fractionation of these extracts on Sephadex G-25 (medium) yielded 6 fractions for birch extracts and 5 fractions for aspen extracts. This result indicated that the chemical composition of the birch extracts differed from the chemical composition of the aspen extracts. The largest quantity of metals was found in fractions that represented substances with apparent molecular weights less than 5000 daltons. Metal distribution patterns indicated different metal-organic associations within fractions of a forest-type extract as well as between fractions of the two forest-type extracts. Fractionation of each GPC fraction on thin-layer plates yielded a total of 27 TLC bands for the birch GPC fractions and 19 TLC bands for the aspen GPC fractions. This result indicated that fewer components were separated in the aspen GPC fractions than in the birch GPC fractions. Solutions derived from the TLC bands, when passed through a Chelex 100 column, provided qualitative information regarding the strength of copper, iron, and manganese complexation by organic substances in those solutions. Manganese was not strongly bound by organic substances derived from birch soil or aspen soil. Copper and iron were usually strongly complexed by organic materials regardless of the source. Significant differences in copper and iron complexation were observed within a forest-type soil extract and between forest-type soil extracts. TLC bands that exhibited little evidence of undissociated carboxylic acid character, as revealed by infrared spectroscopy, contained most of the iron and suggested that carboxylate anion was the principal complexing moiety for iron. Copper generally appeared to be concentrated in TLC bands that fluoresced which was in contrast to those containing iron. This result suggested separations of copper complexes from iron complexes.