Composition and radiocarbon age of HF-resistant soil organic matter in a Podzol and a Cambisol

Hydrofluoric acid (HF) is able to dissolve most minerals, so it is often used to enrich organic matter (OM) in soils and sediments, though significant OM losses sometimes occur. The objective of this study was to examine these carbon losses and the hypothesis that HF dissolves the mineral-associated OM along with its mineral carrier. We compared composition, radiocarbon activity and particle morphology of untreated and HF-treated soil samples, using bulk samples, density fractions > 2 g cm^-3 and fine particle size fractions < 6.3 μm from a Haplic Podzol and a Dystric Cambisol. A positive correlation between HF-soluble organic carbon and organic carbon in the dense fraction confirms a linkage between the dissolved fraction and the mineral-associated OM fraction. However, treatment of heavy soil fractions shows that removal of the mineral-associated OM fraction is not complete. In the topsoil carbon losses amount to only 20-30% of the mineral-associated C, compared to 40-55% for the sub-soil of the Dystric Cambisol and 70-85% for the sub-soil of the Haplic Podzol. Compositional OM change as a consequence of HF treatment was not detected in upper soil horizons, but was substantial for the lower horizons. With depth or age of soil horizon, the HF-soluble OM becomes enriched in O-alkyl C, aryl C, and carbonyl C compared to the HF-resistant OM. The HF-soluble material in all horizons of the Dystric Cambisol is dominated by high alkyl C content, whereas O-alkyl C dominates in deeper horizons of the Haplic Podzol. In A horizons, radiocarbon ages of the HF-resistant OM fraction were similar, but slightly younger than for bulk soil radiocarbon ages. In contrast, in the sub-soil horizons the HF-resistant fraction was up to 2000 yr older than the bulk soil age.