Dataset: Oxidation data of whole terrestrial organic matter samples (soils) collected ....

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.

Santa Fe Soil: a riverbank soil of the Santa Fe River (Florida). The river arises from Lake Santa Fe in northern Florida and flows westward for 75 miles until it empties into the Suwannee River. The Santa Fe River is a slow-flowing river in a heavily forested watershed (mainly with Bald Cypress). The soil was collected on 03 April 2019 at coordinates 29° 55′ 22.8″ N and 82° 25′ 37.8″ W. After removal of large debris, the peat 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. 

Caitlin Soil: a deep, moderately well drained silt loam soil on a 2% till plain in a cultivated field in Ogle County, Illinois. It is a fine-silty, mixed, superactive, mesic Oxyaquic Argiudoll. The soil was sampled and standardized by the U.S. Department of Agriculture. The soil is formed in loess or other silty material and in the underlying loamy calcareous till (slope ranging from 0 to 15%).

Sharpsburg Soil: a deep, moderately well-drained silty clay loam soil formed in loess in Lancaster County, Nebraska. It is a fine, smectitic, mesic Typic Argiudoll. The soil was sampled and standardized by the U.S. Department of Agriculture. This soil is formed on interfluve and hill slopes on uplands (slope ranging from 0 to 18%) and on treads and risers on stream terraces in river valleys.

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. 

Okefenokee Peat: a peat from the largest blackwater swamp in North America. This is a highly acidic (pH 3 to 4) low-nutrient environment (Murray & Hodson, 1984). Its Spodosol soils are formed by the deposits of muck and peat mixed with fine salt and silt.

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).

Oxidation experiments. Oxidation of TOM samples was performed by suspending 2 g of dried material (105 °C for 12 h) in 10 mL of 1 M H2O2 (Fisher, 30%), with iron concentrations unperturbed from the native sample compositions (Table 1 of the associated publication, DOI: 10.1021/acs.est.4c12913). Control experiments were done similarly but with ultrapure water (18.2 MΩ·cm). Experiments were performed in acid-cleaned and pre-combusted 20 mL glass vials, which were kept open to the air to ensure constant aeration and steady gas release, avoiding pressure buildup. As the eight soil samples were largely insoluble in water, once the aqueous H2O2 was added to the soils, acid-cleaned and pre-combusted rods were used to break any formed clumps and homogeneously distribute the peroxide solution. The formed suspensions were let react for 48 h at room temperature in a dark fume hood (vials were covered with a large KimWipe). Then, the peroxide solution was evaporated for 12 h at 105 °C to retain the particulate and dissolved components and also to quench the oxidation reactions for obtaining samples at exact time points.

Considering that the oxidation had continued to occur during sample drying, the total oxidation is estimated to be 60 h (2.5 days) per time point. After drying, a sample was sacrificed, and the remaining samples were used for furtheroxidation by adding new 10 mL of 1 M H2O2 for another 2.5 days of oxidation. This is graphically explained in Figure S1 in the Supporting Information (SI) of the associated publication (DOI: 10.1021/acs.est.4c12913)  Thus, oxidation was subsequently performed in six cycles for a total oxidation time of 15 days. Section 1 of the SI of the associated publication at DOI: 10.1021/acs.est.4c12913 includes more details on the oxidation protocol, justifications for the choices of experimental conditions, and results from ancillary control experiments (Figure S2 of the associated publication at DOI: 10.1021/acs.est.4c12913) and milder oxidation studies with 0.1 M H2O2 (Figure S3 of the associated publication at DOI: 10.1021/acs.est.4c12913). In summary, the in vitro experimental conditions were chosen to ensure that the TOM samples experience a strong oxidation gradient that will yield significant carbon losses (minimum of 66% and above 90%). This would allow for testing if the carbon isotopic composition, C/N ratios, and bulk organic structure become altered at significant carbon losses comparative to carbon losses characteristic for TOM export from land to estuaries and the open ocean (∼66%) and to deep sediments (above 95%).

Elemental and Isotopic Characterization. Oxidized samples were homogenized and dried at 105 OC prior to analysis. Elemental analysis as described below was performed to acquire total nitrogen (TN) content on unacidified samples. Then, samples were digested with 20% HCl to remove refractory carbonates such as siderite (Larson et al., 2008). The acid was removed by freeze-drying and the samples were ground into fine powders. They were then used for total organic carbon (TOC) and isotopic composition (δ13C) analyses.  Sample preparation prior to TOC, TN, and δ13C was done 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 TN values were obtained in triplicate (technical replicates), while δ13C values were measured three times on the same sample (instrumental replicates) and averaged into a single measurement. TOC values were obtained after calibration with the isotopic standards below while TN values were obtained after calibration with nicotinamide and aspartic acid. Carbon and nitrogen peak integrations are done automatically by the instrument and peak areas are related to C or N 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. C/N ratios were determined by the formula (TOC/1.00784)/(TN/12.011). Residual carbon amounts after the oxidation were determined by dividing the the mass of carbon in the oxidized sample divided by the mass of carbon in the control sample. Mass of carbon is determined by multiplying the mass of the sample by the corresponding TOC value. 

This method yielded the "Residual carbon, %", "Stable carbon isotopic signature (δ13C)", and "Carbon-to-nitrogen ratio" reported in the dataset. 

Structural Characterization by 13C Solid-State NMR. xidized samples were homogenized and dried at 105 OC prior to analysis. Samples were packed in a 4 mm zirconia (ZrO2) rotor with a polychlorotrifluoroethylene (Kel-F) cap. Analysis was done on a 400 MHz (9.4 T) Bruker BioSpin AVANCE II spectrometer fitted with a 4 mm magic angle spinning probe at the College of Sciences Major Instrumentation Cluster (COSMIC) facility at Old Dominion University (Norfolk, VA). The instrument's performance is validated regularly by analysis of glycine, adamantane, and/or hexamethylbenzene. One-dimensional 13C NMR spectra were acquired using the nearly quantitative multipulse cross-polarization (multiCP) pulse program (Johnson & Schmidt-Rohr, 2014). Samples were spun at the magic angle at 14 kHz and analyzed using a relaxation delay of 1 s, 3000 scans, five cross-polarization segments, and a total contact time of 3.30 ms. The obtained spectra were phased, calibrated to an external glycine standard, and multiplied by an exponential window function (EM) of 200 Hz. Spectra were then baseline-corrected and integrated in the following ranges: 0−45 ppm (methyl and methylene); 45−60 ppm (αC in peptides); 60−95 (O-alkyl); 95−110 ppm (anomeric C), 110−145 ppm (aryl), 145−165 ppm (aryl-O), and 165−215 ppm (CO). All data were processed using the Bruker TopSpin 4.0.7 software. 

This method yielded the NMR spectra reported in the manuscript.