A mechanistic inquiry into the applicability of permanganate oxidizable carbon as a soil health indicator

We determined the characteristics that render organic matter responsive to dilute permanganate oxidation to find a mechanistic explanation for the frequently observed responsiveness of permanganate oxidizable carbon (PoxC) to changes in soil health. We conclude that PoxC is primarily a measurement of phenolic and polyphenolic structures rather than of compounds traditionally viewed as preferred microbial food. Upon subjecting pure compounds to the PoxC method, we found that the susceptibility of organic molecules to permanganate oxidation was determined by the presence of specific structural features abundant in phenolic compounds, which were highly reactive (3–98 mg PoxC·g⁻¹ C), and in a subset of low-molecular-weight organic acids (0–180 mg PoxC·g⁻¹ C). In comparison, proteins were minimally reactive (0–13 mg PoxC·g⁻¹ C) and carbohydrates were unreactive when subjected to the PoxC method. Permanganate reactivity was not found to be a predictor of decomposability in biosolids through the mirroring of C:N ratio. However, permanganate reactivity increased up to 27% throughout the decomposition process. We suggest a reinterpretation of the mechanisms that render PoxC useful as a soil health indicator. Rather than representing a preferred microbial food source, PoxC measures a carbon fraction with unique molecular properties that may be an indicator of plant-mediated soil processes. Our research corroborates the usefulness of PoxC as a soil health indicator, albeit for entirely different reasons than previously assumed. This opens exciting avenues of further inquiry and sets the stage for intensified investigations into the general role of polyphenolic compounds in soil health.