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Effects of artificial addition of acid precipitation on dissolved organic carbon (DOC) were studied at the Humic Lake Acidification Experiment (HUMEX) catchment on the western coast of Norway. Samples of soil water and lake water from a treatment and reference side were separated into different DOC fractions by the use of size fractionation and column chromatography with XAD-8 and ion-exchange resins. The main DOC components are hydrophobic and hydrophilic acids (HPO-A and HPI-A). While the amount and composition of DOC were stable down in a histosol profile, the amount decreased and composition changed down in a podzol profile. HPO-A decreased to a larger extent than HPI-A, and, therefore, the HPO-A/HPI-A ratio decreased from about 2 to 0.5. This ratio was high and stable in the dystric histosol profile. A lower percentage of the hydrophobic fractions was shown to be of lower molecular weight than found for the hydrophilic fractions. In plots most strongly affected by the acid treatment, the HPO-A/HPI-A ratio in soil water decreased. Preliminary results from DOC titration with aluminium indicate that HPO-A have less binding sites per mg C than HPI-A, but that the average complex constant is higher.
