Soil organic carbon, permanganate fractions, and the chemical properties of acidic soils

Total organic carbon (TC) in 32 acidic surface (0-10 cm) soils was divided into 3 fractions (C1, C2, and C3) based on oxidisability by different strengths of KMnO4 (33 mM and 167 mM). With the methodology used, ease of oxidation decreased in the order C1>C2>C3. Several fundamental soil chemical properties were also determined, i.e. ECEC, CEC at pH 6.5 (CEC6.5), slope of the charge curve (Delta CEC), pH buffer capacity, (pHBC), P sorption capacity using a single addition index (PSI150), and content of organically complexed Al. All soils had pH(1:5 water) <6.5, and comprised a wide range of soil types and clay contents. Multiple step-up regression indicated that C fractions were significantly (P < 0.05) correlated with ECEC, Delta CEC, CEC6.5, and pHBC. These results reinforce the critical importance of soil organic matter to the fundamental soil chemical properties of predominantly variable charge soils. The intercorrelations between the various oxidisable C fractions made it difficult to elucidate if degree of oxidisability had any bearing on the reactivity of the organic matter. ECEC was primarily correlated with CZ, whereas all C fractions had highly significant (P < 0.01) effects on Delta CEC and pHBC. The fraction which was most difficult to oxidise, C3, made a significant (P < 0.01) contribution to CEC6.5 when combined with clay and ECEC in a multiple regression equation. Generally, one or other of the C fractions was better correlated with the fundamental soil chemical properties than TC. This simple empirical fractionation of soil organic C may therefore be a useful tool for assessing the effects of soil management on these properties.