Kellogg Soil Survey Laboratory (KSSL) POX-C dataset

Forty two samples were selected from the Kellogg Soil Survey Laboratory (KSSL) archive. The soils (41) were taken from the A horizon except for one sample that came from an O horizon. The samples represented 9 of the 12 US soil Orders, including Mollisols (23), Alfisols (5), Ultisols (5), Andisols (2), Entisols (2), Inceptisols (2), Aridisols (1), Histosols (1) and Vertisols (1). The soils varied widely in SOC (3.0 – 288.4 g kg-1; mean 31 g kg-1), pH (4.3 – 8.5; mean 6.2) and clay content (3.6 – 47.0%; mean 21.5%) The geographic origin of the selected samples and the distribution of SOC concentrations, clay contents and pH values are in the sample selected materials.

All samples were analyzed using methods of the KSSL Manual (Soil Survey Staff 2014). Samples were dried in an open tray in a low temperature oven at 30-35 °C for 3-7 days and ground to <2 mm, using a roller on a flat, metal plate covered with paper to crush the peds. Clayey soils without coarse fragments were crushed using a laboratory jaw crusher (Retsch, Model BB200). Analyses were performed on air-dried <2 mm soil with resulting data reported on an oven-dry basis. Total soil C was determined by dry combustion and total SOC was estimated by deducting from total C the inorganic C content. Soil pH was determined in a 1:1 water solution. Particle-size analysis was determined by sieve and pipette, following pre-treatments for removal of organic matter and soluble salts and chemical dispersion with sodium hexametaphosphate. Horizon designation was assigned according to NRCS protocol (Schoeneberger et al. 2012) and taxonomic classification was assigned according to Soil Taxonomy (Soil Survey Staff, 2003).

Experimental design and treatments The experiment was a 2 x 5 factorial with 3 replications of the following treatments: 2 levels of mesh size (<2 mm or <0.18 mm) and 5 soil sample masses (0.25, 0.5, 1.0, 2.5, or 5.0 g). To prepare <0.18 mm material, ~15 g of the <2 mm soil was finely ground using a planetary ball mill with silicon nitride bowls and balls to pass a 0.18 mm mesh, except for the organic soil, where ~50 g soil was ground in a Wiley mill to pass a 0.18 mm mesh sieve.

POXC Determinations The POXC method involves reacting a dilute MnO4- solution with a soil sample of a given mass for a fixed time. The MnO4- solution has a deep violet colour that fades as SOC oxidation consumes MnO4-. The colour change is measured via spectrophotometry and related to the remaining MnO4-. A low absorbance value thus signifies high POXC. POXC was determined by the 6A2a1a1 method in the KSSL Manual (Soil Survey Staff, 2014) based on Weil, et al. (2003). Briefly, samples were mixed with 20 mL of 0.020 M KMnO4 in 50 mL centrifuge tubes. Each sample was thoroughly mixed by vortexing (~10 sec), allowed to stand for 10 min, and then centrifuged for 10 min at 2000 rpm or RCF = 1002 G. A 0.5 mL aliquot of the supernatant was combined with 49.5 mL of deionized water in a 50 mL tube and mixed by inversion. This extract was transferred by pipette to cuvettes, and absorbance (550 nm wavelength) determined on a 4-point calibrated UV-Visible spectrometer (Agilent Technologies, Cary 60 UV-Visible Spectrometer). Each analytical replicate was analysed within the same batch, on the same day, by the same analyst, using the same spectrophotometer. A new standard curve was made daily and applied to all POXC batches that were analyzed on that day. POXC was calculated from absorbance (Equation 1).

"POXC " ("mg " 〖"kg" 〗^"-1" )"= " ((0.020 mol L^(-1)-(a+(b×Abs_adj))×(9000 mg C mol^(-1) )×(0.02 L))/"Soil mass" (Eq. 1)

Where 0.020 mol L−1 = initial MnO4- concentration; a is the intercept of the standard curve, b is the slope of the standard curve, Absadj is the adjusted absorbance, 9000 = mg C (0.75 mol) assumed to be oxidized by 1 mol MnO4- changing from Mn7+ to Mn4+ (Weil et al., 2003); 0.02 L = volume of KMnO4- solution mixed with the soil sample; Soil mass = oven-dry mass of soil sample analysed (kg).

Data processing and analysis The results obtained for individual sample replicates were checked for their quantitative validity based on the following quantification limits for the final MnO4- concentrations:

Practical Quantification Limit (PQL) is defined as the minimum concentration that can be detected with a 99% confidence level that the analyte concentration (MnO4- in case of the POXC method) is greater than zero. This is based on the Method Detection Limit (MDL). The MDL was determined following the recommendations of the US Environmental Protection Agency (EPA, 2016), resulting in an MDL for the POXC method at KSSL of 0.004 mM MnO4-. To calculate the PQL, the MDL was multiplied by a factor of 3, resulting in a PQL of 0.012 mM. MnO4- concentrations below this value were considered invalid or outside the quantifiable range.

Upper Detection Limit (UDL). The UDL is the final MnO4- concentration that is statistically distinguishable from a blank reading. The UDL was estimated as the average blank MnO4- concentration from over 100 validated batches and subtracting three times the standard deviation of that mean, yielding a UDL of 0.175 mM. Thus, a final MnO4- concentration >0.175 mM was considered indistinguishable from a blank solution without soil, and the calculated POXC value was considered outside of the quantifiable range.

Values of POXC were reported for each sample, based on the average of three analytical replicates for a given sample, grind and mass class. Only samples for which all three replicates had POXC values within the quantifiable range (i.e. MnO4- concentrations between the UDL and PQL) were used in further analysis. No further data cleaning or removal of outliers was performed.

The analytical variability in POXC values was reported as the Coefficient of Variation (CV) of three analytical replicates that had POXC results within the quantifiable range.

• Resource Title: POX C Data Descriptors.

  File Name: POXC_DataDictionary.csv

  Resource Description: CSV




• Resource Title: KSSL POX C replicates.

  File Name: POX_Data_replicates.csv

  Resource Description: Replicates as measured by the Kellogg Soil Survey Lab




• Resource Title: KSSL POX Selected Sample Info.

  File Name: Table.S1.KSSL_SelectedSamples.csv

  Resource Description: Background information about POX C samples




• Resource Title: KSSL POX C Sample Data.

  File Name: Table.S2.SampleData.csv

  Resource Description: POX C sample data (aggregated from replicates)