Batch sorption experiment 61

The selection of the mineral and organic materials used in the batch sorption experiments was made with the goal of simulating phenomena observed in the field as directly as possible, while increasing the likelihood of observing C saturation. For this reason, soil samples from sub-surface horizons were selected instead of neat and pure minerals present in soils (e.g., kaolinite, illite, goethite and gibbsite), recognizing that soil

samples contain a significant amount heterogeneity in composition. Sub-surface soils were selected rather than surface soils because of low initial C concentrations that enable them to sorb greater amounts of organic C inputs as well as avoiding issues of

distinguishing newly sorbed versus exchanged organic matter.

Four soils (referred to here by their soil series names: Edgemont, Drummer, San Ysidro and Towaliga) were selected for use in the batch sorption experiments (Table 2.1). The Edgemont soil was collected from the Stroud Water Research Center. Three other soils were selected from a set of 213 subsurface samples used in a previous study on DOM sorption (Mayes et al., 2012). These soils represent three soil orders, and were selected on the basis of having a maximum DOC sorption capacity (Qmax) in the upper

25th percentile of their respective soil order. While clay and Fe oxide contents have been found to be the strongest predictors of Qmax (Kothawala et al., 2009; Mayes et al., 2012),

the selected samples used in the current study varied in Fe oxide content and mineralogy, but did not vary widely in clay content (Table 2.1). Based on Natural Resources

Descriptions), the dominant clay mineralogy was smectite for the Drummer and San Ysidro soils, and kaolinite for the Towaliga and Edgemont soils.

Soils were ground to pass through a 500-µm sieve and stored air-dry prior to analysis and use in the batch sorption experiments. Soil mineral specific surface area was measured using the N2-BET method after removing organic matter by muffling at 350 °C

for 18 h. Samples were de-gassed at 325 °C for 4 h with N2 and He to remove adsorbed

water. Nitrogen was dosed on the surfaces at 77 K with different gas pressures in a Tristar 300 surface area and porosity analyzer (Micromeritics, Norcross, GA). The multi-point Brunauer-Emmett-Teller (BET) method was used to calculate SSA values under the relative pressure between 0.05 and 0.3 atm.

Similar to the approach used to select the mineral component, a high C

concentration, chemically complex DOM solution derived from leaf litter was selected instead of commercial humic acids or single, model compounds. A stock DOM solution was prepared by mixing surface leaf litter with water (10:1 water volume to litter mass ratio) in a 40 L plastic carboy. The litter was collected from a mixed oak-maple stand, with dominant species consisting of Quercus palustris, Quercus rubra, Quercus alba, Acer rubrum, Liriodendron tulipifera, and Fagus grandifolia at the Stroud Water

Research Center, in the same area from where the Edgemont soil sample was collected. The litter was kept field moist, cut into small pieces (1-3 cm), added to the water, and allowed to soak for 48 hours at room temperature with occasional stirring to invert the mixture. The resulting suspension was decanted, passed through glass fiber filter (GF/F, 0.45 µm diameter), and the DOM filtrate was stored at 4 °C until subsequent use. The resulting stock DOM solution was found to have a dissolved organic carbon (DOC)

concentration of 614 ± 7.6 mg C L-1.

The batch sorption experiments were conducted by mixing 0.25 g of soil with 100 mL of DOM in a 250 mL Erlenmeyer flask. DOM of differing concentrations was

prepared by diluting the stock DOM solution with deionized water to target DOC concentrations of 50, 100, 150, 200, 300, 350, 400, 450, 500, 600 mg C L-1. The batch sorption experiment for each soil included one blank sample consisting of DOM (~600 mg C L-1) without soil to determine DOM degradation during the experiment, and a second blank composed of 0.25 g soil mixed with deionized water (0 mg C L-1). Thus, a total of 12 Erlenmeyer flasks for each experiment were shaken for 24 h on a reciprocal shaker (180 rpm) placed inside a constant temperature chamber set to 20 (± 0.2) °C. To maximize DOM sorption, we did not use a biocide and used room temperature instead of the more typical temperature of 4 °C during DOM sorption. Previous experiments showed that microbial activity actually enhanced sorption (A. K. Aufdenkampe,

unpublished work). After 24 h, the suspensions were filtered through a 0.45 µm cellulosic membrane (GE Osmonics, Minnetonka, MN). The organo-mineral complexes retained on the filters were washed into aluminum pans and oven-dried at 50 °C. The filtrates were stored at 4 °C in glass vials for a maximum of 24 hours before DOC determination. Batch sorption experiments were replicated three times for each soil, and each replicate set of experiments was run independently.

The original stock DOM solution, the initial dilution series and the filtrates after the batch sorption experiments were analyzed for DOC using a persulfate digestion and spectrophotometric method (Method 10173, Hach Company, Loveland CO). Carbon and nitrogen concentrations, δ13C, and δ15N of the organo-mineral complexes retrieved after

DOM sorption were analyzed using an elemental analyzer coupled to a stable isotope ratio mass spectrometer. Organic C loadings of the organo-mineral complexes were then determined by dividing the solid-phase C concentrations (mg C g-1 sample) after DOM sorption by mineral SSA (m2 g-1 sample).