The misuse of permanganate as a quantitative measure of soil organic carbon

Abstract

In the last two decades, permanganate has been used to define what is assumed to be a labile or “active” soil carbon (C) pool, commonly referred to as “permanganate-oxidizable carbon” (POXC). However, uncertainties in the reduction reaction (Mn⁷⁺ → Mn⁴⁺/Mn²⁺) and even greater uncertainties in the oxidation reaction (C^? → C^?) as well as the reaction of non-C reductants in the soil sample preclude the calculation of milligram C per kilogram of soil oxidized. Combined variation in the reduction–oxidation reactions can entail up to fivefold variation in how much soil organic C is oxidized per unit permanganate reduced. Without determining final reduction state of Mn and the initial and final oxidation states of C, the amount of C oxidized cannot be calculated. Unless a concrete understanding of the reduction and oxidation half-reactions is achieved, an alternative expression of permanganate reactivity of a soil sample (i.e., not mg C kg⁻¹ soil) is needed.