Wednesday, November 7, 2007
324-11
Sulfide Distribution in Subaqueous Soil Systems.
Margot K. Payne, USDA-NRCS, 60 Quaker Lane, Suite 46, West Warwick, RI 02886 and Mark Stolt, University of Rhode Island, Dept. of NRS Univ. of Rhode Island, 112 Kingston Coastal Institute Bldg, Kingston, RI 02881.
Sulfidization, or the accumulation of sulfides, is an important soil forming process in estuarine and marine soils. Sulfidic materials in subaqueous soil are important in determining areas ideal for growth of submerged aquatic vegetation and placement of dredged material. Prior to 1992, Soil Taxonomy defined sulfidic materials based on sulfide and CaCO3 levels. The definition has since changed to a classification based on incubation pH measurements. The objectives of this research were to determine the distribution of sulfides in subaqueous landscapes and to evaluate sulfide levels in relation to other soil properties. Fifty-two soil samples from three shallow estuaries in Rhode Island were analyzed for acid volatile sulfides (AVS), chromium reducible sulfur (CRS), incubation pH, and salinity after oxidation with peroxide. Total sulfide measurements (CRS+AVS) ranged from 20 – 11592 μg g-1 with an average of 2965 μg g-1. In most cases, CRS represented nearly 100% of the total sulfides measured. Nearly half of all samples measured (47%) had incubation pH readings ≤4.0. Only two out of 17 pedons (7 of 52 soil samples) contained sulfidic materials as previously defined by sulfide content, while incubation pH measurements showed 11 of 17 pedons (27 of 52 soil samples) to meet current criteria for sulfidic materials. Soils that reached an incubation pH ≤4 showed no significant difference in carbon content from those that had an incubation pH >4, and samples with higher incubation pH values had significantly higher levels of calcium carbonate than those with low incubation pH. Carbon content and particle size appear to be the two factors that were the most strongly related to the distribution of sulfides. Total dry weight of salts after oxidation with hydrogen peroxide showed a stronger relationship with total sulfides (R2=0.70) than incubation pH (R2=0.19). Sulfide content and incubation pH showed similar trends among landscape units with high sulfide levels, low incubation pH, and high carbon and calcium carbonate contents on low energy, silty landscape units.