The use of permanganate oxidizable carbon (POXC) as a soil health indicator has gained traction due to its low analysis cost and potential for high-throughput analysis. Permanganate (MnO₄⁻) has long been proposed to fractionate soil organic matter. A methodological alteration was proposed to allow for its use as a rapid soil health indicator (POXC) is to allow the MnO₄⁻ to reach with a fixed soil mass, rather than a fixed mass of soil organic carbon (SOC). However, this modification may compromise the robustness of the results by altering the consistency of the stoichiometry in the reduction–oxidation reaction (MnO₄⁻ : SOC). Here we use a diverse set of 69 U.S. soils to evaluate whether using a fixed soil mass (2.5 g) substantively undermines the theoretical requirement of a fixed amount of SOC (15 or 25 mg SOC per sample) per unit of oxidant (MnO₄⁻). We found that the use of a fixed SOC mass entailed a more consistent reduction of MnO₄⁻ than a fixed soil mass and also resulted in a greater range of absolute (mg kg⁻¹) and relative POXC values (as a % of SOC) across soils. This broader range of values was not driven by large differences in the amount of MnO₄⁻ reduced per unit of SOC analyzed, but rather resulted from normalizing the amount of MnO₄⁻ reduced on a soil mass basis. The underlying distribution of MnO₄⁻ reduction did not substantively change, suggesting that the interpretation when comparing relative differences in POXC would similarly be unchanged. Unexpectedly, the use of a fixed SOC mass decreased the repeatability of the metric relative to the use of a fixed soil mass. Given the current interpretation of POXC, we see few upsides of using fixed SOC mass and several downsides (i.e. lower throughput and decreased reliability), relative to the current use of a fixed soil mass. To minimize unfounded assumptions, we further propose that POXC values be reported strictly as MnO₄⁻ reduced, specifically as μmol MnO₄⁻ reduced kg⁻¹ soil. Our results further underscore that the results of POXC assays should be cautiously interpreted. Specifically, interpretations should be qualified by the operationally-defined nature of the POXC assay as an indirectly measured, chemically defined fraction.

使用高锰酸盐可氧化碳 (POXC) 作为土壤健康指标因其低分析成本和高通量分析潜力而受到关注。长期以来，高锰酸盐 (MnO ₄ ^- ) 被提议用于分离土壤有机质。建议将其用作快速土壤健康指标 (POXC) 的方法改变是允许 MnO ₄ ^-达到固定的土壤质量，而不是固定质量的土壤有机碳 (SOC)。然而，这种修改可能会通过改变还原氧化反应 (MnO ₄ ^-: SOC)。在这里，我们使用一组不同的 69 种美国土壤来评估使用固定的土壤质量 (2.5 g) 是否实质上破坏了每单位氧化剂 (MnO ₄ ^-）。我们发现，与固定土壤质量相比，使用固定 SOC 质量需要更一致地减少 MnO ₄ ^{- ，}并且还导致更大范围的绝对（mg kg ^-1）和相对 POXC 值（作为 SOC 的百分比）跨越土壤。这种更广泛的值范围不是由分析的每单位 SOC 所减少的 MnO ₄ ^-量的巨大差异驱动的，而是由于对 MnO ₄ ^-的量进行归一化所致^。在土壤质量的基础上减少。MnO ₄ ^-还原的潜在分布没有实质性改变，这表明比较POXC 的相对差异时的解释同样不会改变。出乎意料的是，相对于使用固定土壤质量，使用固定 SOC 质量降低了度量的可重复性。鉴于当前对 POXC 的解释，相对于当前使用固定土壤质量，我们认为使用固定 SOC 质量几乎没有好处和几个缺点（即较低的吞吐量和降低的可靠性）。为了尽量减少毫无根据的假设，我们进一步建议将 POXC 值严格报告为 MnO ₄ ^-还原，特别是 μmol MnO ₄ ^-还原 kg ^-1土壤。我们的结果进一步强调，应谨慎解释 POXC 检测的结果。具体而言，解释应通过 POXC 检测的操作定义性质作为间接测量的化学定义部分进行限定。