Elsevier

Geoderma

Volume 404, 15 December 2021, 115392
Geoderma

To standardize by mass of soil or organic carbon? A comparison of permanganate oxidizable carbon (POXC) assay methods

https://doi.org/10.1016/j.geoderma.2021.115392Get rights and content

Highlights

  • Current POXC methods subject a varying amount of soil C to oxidation across samples.

  • Total MnO4 reduction was more consistent when standardizing by soil C in the assay.

  • Distribution of results (on a soil mass basis) did not differ by standardization method.

  • The standard method of using 2.5 g of soil had the most repeatable results.

  • Interpretation of POXC should reflect uncertainty about oxidant and functionality.

Abstract

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.

Introduction

In recent years, there has been an increasing interest in soil health indicators. Many of these indicators center on measuring properties directly related to soil organic matter content or dynamics. Soil organic matter is linked to many desirable outcomes—including ecosystem services—and is the main energy source for the soil biotic community (Schmidt et al., 2011). While measures of total organic matter are the most commonly used and clearly interpretable indicator of soil health (Bünemann et al., 2018), these often respond slowly to management, are labor intensive, have a high cost of analysis, and/or have insufficiently rapid turnaround of results for timely decision-making. Ideally, a soil health indicator should be sensitive to management, amenable to a high-throughput analysis, repeatable, and interpretable (Stott, 2019).

Permanganate oxidizable carbon (POXC) has recently gained attention as a potential measure of soil health. Although permanganate (MnO4)—a moderately strong chemical oxidant—has long been used to oxidatively fractionate soil organic matter (Warington and Peake, 1880, Syme, 1909, Willard et al., 1956, Matsuda and Schnitzer, 1972), several of the more recent implementations have leveraged its oxidizing capabilities to measure a pool of presumably labile organic matter (Lefroy et al., 1993, Blair et al., 1995, Weil et al., 2003). Across applications, the amount of organic matter oxidized—often presumed to be soil C—is inferred by the disappearance of MnO4 in solution after its reduction (Mn7+ to either Mn4+ and/or Mn2+). Thus, measurement of soil organic matter pools by MnO4 oxidation have been interpreted as indirect measures of organic matter lability or quality and microbial decomposition (Moebius-Clune et al., 2017, Stott, 2019). The variation of this method initially proposed by Weil et al. (2003) and slightly modified by Culman et al. (2012)—now generally referred to as the POXC method—is increasingly used in soil health assessment frameworks (Hurisso et al., 2016, Moebius-Clune et al., 2017, Stott, 2019). In recent years, the methods have been further questioned and analyzed. Several studies have shown that POXC is amenable to similar processing and handling as traditional agronomic soil tests: POXC is unaffected by drying temperature prior to analysis (Gasch et al., 2020), does not require a greater spatiotemporal sampling density (Hurisso et al., 2018a), and can be reliably quantified using a standardized <2 mm ground soil (Hurisso et al., 2018b, Pulleman et al., 2020, Wade et al., 2020). An additional methodological consideration that remains unresolved is how or if the mass of soil to be subjected to oxidation by MnO4 influences POXC results (Pulleman et al., 2020).

Initial studies using MnO4 (as KMnO4) to evaluate labile organic matter sought to standardize the ratio of oxidant to reductant (i.e. MnO4 : soil organic carbon [SOC]) in the assay of a given soil sample (Lefroy et al., 1993, Blair et al., 1995). Thus, the mass of soil used in the assay varied, but both the amount of MnO4 and the mass of SOC subjected to oxidation by MnO4 were constant. Though maintaining a constant ratio of oxidant (MnO4) and reductant (SOC) is needed to ensure strict comparability of oxidant reduction, this approach requires a priori knowledge of SOC content and requires the additional labor to calculate and weigh out sample-specific masses of soil. In order to increase the throughput rate for widespread soil health testing and commercialization, Weil et al. (2003) standardized the mass of soil at 5.0 g, an amount which was later decreased to 2.5 g by Culman et al. (2012). Although the use of 2.5 g soil mass has become the standard protocol for POXC analysis (Stott, 2019, Culman et al., 2020), the finite amount of MnO4 in solution can result in ‘bleaching’ from reduction of all MnO4, generally observed at SOC contents above 8–10% (Pulleman et al., 2020, Wade et al., 2020). Although most of the agricultural soils for which the POXC method was designed fall well below this threshold, bleaching limits the applicability of the metric.

Moreover, the decision to standardize the analysis by soil mass rather than SOC content conflicts with fundamental principles of redox chemistry and contradicts the assumptions underlying the POXC method. Specifically, standardizing by soil mass will result in a variable amount of SOC being subjected to oxidation, thereby altering the ratio of oxidant (MnO4) to substrate (presumably SOC). In numerous evaluations of MnO4 oxidation of individual organic compounds (e.g. amino acids, aromatics, etc.), altering the ratio of MnO4 to substrate can substantially alter the rate of oxidation (Verma et al., 1976, Perez-Benito et al., 1987, Brillas et al., 1988). This means that altering the ratio of MnO4 to SOC could impact how much MnO4 is reduced in a given soil sample. Therefore, when comparing across soils with varying SOC contents, a fixed soil mass would produce a range of reaction rates that could confound comparisons of soil C oxidation amongst different samples. When using a fixed soil mass, samples with lower SOC will have a higher MnO4 : SOC ratio than samples with higher SOC, favoring greater oxidation rates and thus a greater proportion of total SOC oxidized. With these considerations in mind, Pulleman et al (2020) suggested that standardizing by SOC mass could improve comparability across samples, relative to the current method of standardizing by soil mass.

We evaluated this suggestion for a set of 69 diverse soils that have been carefully selected to represent all 12 USDA soil orders and a broad range of edaphic conditions that are common in agricultural settings of the US. In order to capture a wide range of potential amounts of substrate for oxidation by MnO4, SOC content was the primary characteristic of interest. Our objectives were to evaluate the effect of standardizing POXC analyses by SOC mass on: 1) the amount (μmol) of MnO4 reduced, 2) the absolute values of POXC on both a soil and SOC mass basis, and 3) the repeatability of the metric in terms of MnO4 reduced and the resulting POXC values. We hypothesized that standardizing by SOC mass will (1) result in a more consistent amount of MnO4 reduced and (2) produce substantially higher POXC values, both on an absolute basis (mg POXC kg−1 soil) and as a proportion of SOC (POXC/SOC). Additionally, we hypothesized that (3) the more consistent MnO4 to SOC ratio will also result in more repeatable values of MnO4 reduced and POXC.

Section snippets

Soil selection and characterization

A total of 69 soils were selected to reflect a range of edaphic characteristics. A minimum of 3 soils from each of the twelve USDA Orders were included in this dataset, which were obtained from a combination of collections and new field sampling. We used surface A or A/B horizons for mineral soils and O horizons were for organic soils (Histosols, Gelisols). Soils were air-dried and sieved to <2 mm before physical and chemical characterization. We determined texture by shaking 40.0 g of soil in

Detection rates

Detection rates of MnO4 across treatments were high (>90%) and did not differ between the treatments with a fixed SOC mass (i.e. 15 or 25 mg SOC) and the standard treatment of fixed soil mass (i.e. 2.5 g soil). In the 15 mg SOC treatment, 92.3% of the samples had detectable POXC values and in the 25 mg SOC treatment, 91.0% of the samples had detectable POXC values. Although the proportion of detectable values obtained using 15 or 25 mg SOC was slightly greater than values obtained by the

Implications and conclusion

Measuring POXC using a fixed amount of SOC in the sample appears to offer few advantages relative to standardization using a fixed soil mass. As hypothesized, using a fixed amount of SOC resulted in generally more consistent amount of MnO4 reduced (Fig. 2a) than using a fixed soil mass, but this did not greatly alter the proportion of detectable results or change the distribution of MnO4 reduced per unit of soil mass (Table 2). Similar distribution of POXC values among sample mass treatments

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to thank several anonymous reviewers for their comments and attention to detail, which greatly improved the quality of this manuscript. We would also like to acknowledge Rich Ferguson for his input and the many conversations which catalyzed this study. Data and reproducible code is available for this study online at https://github.com/jordon-wade/POXC-fixed-SOC-mass.

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