Revisiting the permanganate oxidizable carbon (POXC) assay assumptions: POXC is lignin sensitive

Permanganate oxidizable carbon (POXC) is a popular soil health test developed to measure “labile” C via the reduction of permanganate, dependent on several stoichiometric reduction oxidation assumptions. As a proof-of-concept experiment to evaluate the interpretation of POXC as “labile” C, we tested 17 compounds ranging in biological lability under standard POXC assay conditions at a fixed C mass (25 mg) in a quartz (2–0.053 mm diameter) matrix. POXC was high for lignin, whereas carbohydrates did not differ from the quartz control. Functional group-based reactivity partly explained permanganate reduction. These findings indicate that (i) POXC is not a labile C fraction and (ii) corroborate previous concerns that the stoichiometric oxidation–reduction assumptions in the calculation of C oxidation from permanganate reduced are not sound. POXC interpretation should regard POXC as a chemically defined fraction, report in units of moles permanganate reduced per kg soil, and avoid terms such as “labile” and “active.”