The soils were HF-treated using the procedure of Skjemstad et al. (1994). Briefly, the HF-treatment consisted of nine successive treatments with 2% hydrofluoric acid solution. Each treatment involved shaking 3 g portions of soil in 50 mL of 2% hydrofluoric acid solution, end-over-end, for periods of 1 h (5 times), 16 h (3 times) and 64 h (once). Between treatments, samples were centrifuged, the supernatant discarded and replaced with fresh 2% hydrofluoric acid solution. Following the final treatment, the residue was rinsed 3 times with de-ionized water, then freeze-dried. Carbon contents of the HF-treated soils were measured on a LECO CR12 carbon analyzer.