Mobility of different size fractions of organic carbon, Al, Fe, Mn and Si in podzols

The influence of size distribution on the translocation of organic carbon, Fe, Al, Mn and Si in soil waters, has been studied in Scandinavian podzol profiles from three different locations. Soil solutions from different horizons have been filtered through a 0.45 μm membrane filter prior to further size fractionation by ultramembrane techniques. Organic carbon, Fe, Al, Mn and Si were determined in: (I) a colloidal fraction–nominal molecular weight cutoff >3 kDa, (II) a medium size fraction–nominal molecular weight cutoff 1–3 kDa, and (III) a low molecular size fraction–nominal molecular weight cutoff <1 kDa. The colloidal fraction comprise large proportions of organic carbon in the O and the E horizons, while low molecular size organic carbon fractions increase with depth in the B horizon on a percentage basis. A major part of Fe and Al seems to be associated with the colloidal fraction of organic carbon in the O horizon, and results suggest that colloidal organic matter contribute to the vertical translocation of metals through the E horizon. Maximum concentrations of Si, Al and Fe are found in the E horizon. Concentration of colloidal Si is significantly higher in the E than the other horizons. Although the total concentration of Si is four times the concentration of Al, colloidal Si and Al are present in more or less similar concentrations. This suggest that Si and Al are associated with each other in colloids in the E horizon. Equilibrium calculations, however, indicate that proto-imogolite sols are not very likely, and correlation coefficients rather suggest interaction of Al with colloidal organic carbon. The major part of Fe is in the colloidal fraction in all horizons. In contrast, Mn decreases sharply with depth, and is generally in a low molecular weight form. This indicates that Mn is related to the litter turnover process and that Mn takes little part in the podzolization process.