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

Abstract

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 (CEC_6·5), slope of the charge curve (ΔCEC), 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, ΔCEC, CEC_6·5, 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 C1, whereas all C fractions had highly significant (P < 0·01) effects on ΔCEC and pHBC. The fraction which was most difficult to oxidise, C3, made a significant (P < 0·01) contribution to CEC_6·5 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.