Dataset: Fenton oxidation of particulate terrestrial organic matter (TOM) samples and sequential POM/DOM fractionation

Hillsborough Soil: a riverbank soil of the Hillsborough River (Florida). The river arises from the Green Swamp, also known as the “Heart of the Floridan Aquifer”, and flows 60 miles to an outlet in the city of Tampa on Hillsborough Bay. Its watershed was formerly covered by a rich, old-growth forest of Bald Cypress, Longleaf Pine, and Sand Live Oak. After decades of logging, the watershed’s cover has shifted to recently grown Water Ash and Water Locust trees. The soil was collected on 02 April 2019 at coordinates 28° 05′ 16.8″ N and 82° 20′ 56.3″ W. After removal of large debris, the soil was homogenized, sieved, oven-dried at 105 °C, and stored in the dark to prevent degradation during storage.

Elliott Soil: a standardized fertile prairie soil purchased from the International Humic Substances Society (IHSS). This sample was obtained from an undisturbed area on the grounds of the Joliet Army Ammunition Plant near Joliet, Illinois, currently managed by the US Forest Service as the Midewin National Tallgrass Prairie Reserve. It is a poorly drained soil on moraines and till plains having loess or silty clay loam glacial till. 

Pahokee Peat: a standardized peat soil purchased from the IHSS. It is an agricultural soil of the Florida Everglades obtained from the University of Florida Belle Glade Research Station. This is a poorly drained soil formed by organic deposits from freshwater marshes accumulating over limestone bedrock. 

Great Dismal Swamp Peat: a peat from a blackwater wetland in Virginia that has high iron loadings. Its hydric soils are formed under repeated saturation, long enough to develop anaerobic conditions in the upper horizons. The peat is formed by fast accumulation of carbon and slow decomposition yielding an organic-rich peat (Sleeter et al., 2017).

Elemental and Isotopic Characterization: Samples were prepared for total organic carbon (TOC) and isotopic composition (δ13C) analysis by wrapping weighed amounts of sample in tin capsules. Analysis was performed using a Thermo-Scientific Flash EA instrument (combustion at 1010 °C) equipped with a thermal conductivity detector and a Delta V Plus isotope-ratio mass spectrometer. The instrument's performance is validated daily by analysis of standards (mentioned below). TOC and δ13C values were obtained in duplicate (technical replicates). TOC values were obtained after calibration with the isotopic standards below. Carbon peak integrations are done automatically by the instrument and peak areas are related to C amounts using calibration curves in Excel. δ13C values determined by the isotope-ratio mass spectrometer and computed by its software were peak-size-corrected and scale-corrected (in Excel using calibration curves) using laboratory and authentic reference standards (glutamic acid: δ13C = −13.90‰; USGS40: δ13C = −26.39‰; USGS41a: δ13C = +36.55‰; and tyrosine: δ13C = −24.90‰). δ13C results are reported in delta notation relative to that of Vienna Pee Dee Belemnite. 

Oxidation experiments. Oxidation of TOM samples was performed by suspending 1-4 g of dried material (105 °C for 12 h) in 10-20 mL of 1 M H₂O₂ (Fisher, 30%), with iron concentrations unperturbed from the native sample compositions. Control experiments were done similarly but with ultrapure water (18.2 MΩ·cm). Experiments were performed in acid-cleaned and pre-combusted 60 mL glass vials, which were kept open to the air to ensure constant aeration and steady gas release, avoiding pressure buildup. As the  soil samples were largely insoluble in water, once the aqueous H₂O₂ was added to the soils, vials were sonicated for 30 minutes to ensure proper mixing and breakage of any formed clumps. The formed suspensions were let react for 48 h at room temperature in a shaking incubator (vials were covered with a large KimWipe). Then, the vials were frozen at -80 °C (to quench the oxidation) and the liquid was removed by freeze-drying (with the freeze drier maintaining temperature of -60 to -80 °C). Then, new 10-20 mL of peroxide were added for another 48 hours. 

Oxidation was performed for up to six consecutive 48-hour cycles, for a total of 288 hours. Samples were analyzed at 10 time points: 0, 12, 24, 36, 48, 96, 144, 192, 240, and 288 hours. The 12-, 24-, and 36-hour measurements were collected during the first oxidation cycle, while subsequent measurements corresponded to the completion of each 48-hour cycle. Oxidation times represent cumulative exposure to the oxidation treatment.

POM/DOM fractionation. After the suspensions were freeze-dried, the obtained OM was fractionated into POM and DOM via water-extraction: the OM residue was suspended in 20 mL of ultrapure water. Suspensions were sonicated for 30 min to ensure proper mixing and then placed on a shaking incubator. After 48 hours the suspensions were quantitatively transferred into pre-cleaned plastic falcon tubes. Following centrifugation, the supernatant liquid (containing the DOM) was decanted into a filter manifold (0.45 µm pre-combusted GFF) and then the remaining POM was resuspended in 20 mL water to wash the POM of any remaining DOM. The new suspension was centrifuged again, and the water wash was decanted into the filter manifold. The falcon tubes, containing the POM fraction, as well as the decanted and filtered DOM fractions, were freeze-dried. 

The resultant DOM and POM powders were homogenized. Then, about 50 mg of each sample was digested with 1 mL of 20% HCl to remove refractory carbonates such as siderite (Larson et al., 2008). The acid was evaporated at 80 °C (O’Kelly, 2005) and samples were homogenized again prior to analysis.