Soil survey investigations of freshwater subaqueous soils

Jonathan M Bakken, University of Rhode Island

Abstract

Subaqueous soils are those that occur under permanently flooded or ponded environments. Few studies have investigated freshwater subaqueous soil systems. In this study, I investigated freshwater subaqueous soils in three created (impoundments) and three natural lakes. The primary objectives were: (i) to characterize, classify, and map subaqueous soil resources; (ii) to begin to develop an understanding of soil organic carbon (SOC) and metal (Pb and As) distributions in lakes; (iii) to estimate SOC and sediment accumulation rates; and (iv) to assess phosphorus (P) concentration-invasive plant distribution relationships. Traditional and modified soil survey techniques were used to conduct the subaqueous soil inventories. Bathymetric maps were effective for delineating freshwater subaqueous soil-landscape units. The primary units identified were lakeshores, coves, lakebeds, shoals, and deepwater. Ground penetrating radar (GPR) was effective at identifying the presence of stones and boulders, stratified mineral layers, water depth, and organic soil material thickness. Over 100 pedons were sampled and described; 81 were characterized. Soils classified as Sapric Frasiwassists (34); Fluventic, Thapto-histic, Psammentic, and Typic Frasiwassents (32); and Histic, Fluvaquentic, and Typic Humaquepts (15). Subaqueous Inceptisols (Frasiwassepts) are being proposed as an addition to Soil Taxonomy to accommodate the Humaquepts. Five soil series were established and developed into map units to construct soil maps for each of the ponds. ^ Elevated concentrations of Pb and As were found in the upper 30 cm of most of the subaqueous soils. Historic use of Pb-arsenate pesticide and leaded gasoline (atmospheric deposition of Pb) are the most probable sources of the trace metals. Out of 315 samples tested, five samples had As and Pb levels above those considered by national standards to have an effect on benthic biology (70 and 218 µg g−1, respectively). Sedimentation rates were calculated using depth to elevated Pb and/or As concentrations as a stratigraphic marker of the year 1920 (the beginning of widespread use of leaded gasoline and Pb-arsenate pesticides). Sedimentation rates were similar among water bodies; ranging from 0.02 to 0.27 cm yr−1. Natural lakes had significantly lower sedimentation rates (0.11 cm yr −1) than reservoirs (0.25 cm yr−1) (p = 0.006) suggesting that the freshwater subaqueous soil environment is a sink for contaminants and that these contaminants persist over time in the soils. ^ SOC:SOM ratios varied by horizon, site, and soil type, ranging from 0.25 to 0.70. Mean SOC pools for mineral soils ranged from 56 Mg C ha−1 for the sandy families of Frasiwassents to 204 Mg C ha−1 for the coarse-loamy families. Organic soils (Sapric and Terric Frasiwassists) had mean SOC pools ranging from 388 to 427 Mg C ha−1. Average C-sequestration rates for the last 90 years ranged from 0.26 (natural) to 0.33 Mg C ha−1 yr−1 (created). SOC pools and sequestration rates for lakes should be considered in future carbon accounting efforts. ^ Vegetation in the three created systems was dominated by the invasives variable milfoil and fanwort. Only one of the natural lakes contained invasive species and these were in isolated locations. Soil samples were collected from the common habitats in the four ponds with aquatic invasive plant species and the samples analyzed for extractable P. Average concentrations from the sequential extraction were 3.05 µg g−1 (porewater), 44.38 µg g−1 (NaHCO3), and 42.54 µg g−1 (NaOH) with a mean total extractable P value of 89.97 µg g−1. Three significant (p = 0.05) relationships were identified between P concentrations and invasive vegetative cover. Logistic regression analysis suggested that total extractable P concentrations >200 µg g−1 would likely have a significant effect on the occurrence of an aquatic invasive plant species. Total extractable P concentrations varied significantly among the soils mapped in the ponds suggesting that subaqueous soil maps can be helpful in understanding P distribution relative to invasive species distribution. My studies showed that subaqueous soil surveys may assist scientists and resource managers in various ways: by providing a better understanding of how the freshwater subaqueous soil environment serves as a sink for contaminants that persist over time; by allowing estimation of SOC pools which provides a useful interpretation to aid in the accounting of subaqueous SOC pools; and by providing greater understanding of invasive species distribution and P concentrations.^

Subject Area

Agriculture, Soil Science

Recommended Citation

Jonathan M Bakken, "Soil survey investigations of freshwater subaqueous soils" (2012). Dissertations and Master's Theses (Campus Access). Paper AAI1516157.
http://digitalcommons.uri.edu/dissertations/AAI1516157

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