Fractionation and Characterization Methods

Four grams of each soil were weighed into 50 mL centrifuge tubes with four replicates and 40 mL of DI water was added to each tube prior to shaking overnight. After shaking, each soil suspension was allowed to settle undisturbed for approximately two hours before the dispersed colloidal fraction (3mL) was sampled by slowly pipetting at a depth of 2.5 cm below the solution surface in manner similar to Miller and Miller (1987)(Miller and Miller, 1987). The dispersed clay present in each tube was quantified by placing the pipetted suspension in preweighed, oven-dried aluminum tins and heating at 110° C to dry the sample prior to reweighing the pan. Eighty air-dried soil samples reflecting a range of soil properties were initially screened using the above procedure and 33 soils were selected for further characterization and identified as ìproject soilsî. Subsequent WDC measurements were conducted on field-moist samples of the thirty-three project soils selected from the initial screening (Table 1).

Analysis of Suspension Properties

The pH and electrical conductivity of the suspensions remaining after sampling for WDC were measured for each suspension prior to filtering through a 0.2µm pore size polycarbonate membrane filter. Dissolved organic carbon (DOC), dissolved cation and anion analyses were performed on the filtrates. The DOC content for each sample was determined using a Shimadzu Organic Carbon Analyzer. The concentrations of the major solution cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) and anions (Cl^-, NO₃^-, SO₄^2- , PO₄^3-, F^-) were determined by ion chromatography using a CD20 conductivity detector (Dionex, Inc.). The sodium adsorption ratio, a property often correlated with the relative dispersiveness of a given solution (Frenkel et al., 1978; Khilar, 1984), was calculated as:

for the average extract cation data with all components expressed in terms of meq L^-1(Bohn et al., 1985; Sposito, 1989). The reported SAR value reflects the mean sodium adsorption ratio for the four individual replicates for each soil.

Dissolved and Colloid-Associated Organic Carbon

Samples for the analysis of organic carbon associated with dispersed colloidal material were collected using a modified version of the procedure described above. Twenty gram samples of each soil were weighed into three 250 mL centrifuge tubes. Two-hundred milliliters of DI water was added to each tube prior to shaking the sample overnight. After shaking, the samples were allowed to settle for 2 hours before the top 2.5 cm of the suspension present in each sample tube was slowly removed using a 50 mL syringe. The three replicate suspensions for each soil were combined, quick frozen with liquid N₂, and subsequently freeze dried. The total organic carbon (TOC) for bulk samples of each soil (<2mm fraction) and the colloid-associated carbon were determined by dry combustion using the LECO furnace method (Nelson and Sommers, 1982).

Organic Matter Characterization