Emergence and growth of seven grass species across a gradient of metals and arsenic in lime-amended contaminated soils
Martin, Tara Noel.
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Montana's Upper Clark Fork River Basin contains hundreds of square kilometers of land impacted by mine wastes and/or smelter emissions from decades of copper mining and related activities. Contaminated soils in the Basin are often acidic and highly enriched with the trace elements cadmium, copper, arsenic, lead, zinc, and others. Natural plant colonization is often impaired, as evidenced by barren areas that are so phytotoxic that normal germination and establishment cannot occur. One reclamation strategy being used is in-place treatment with soil amendments including lime and other products. This provides a more hospitable substrate for plants by raising pH and lowering the mobile and bioavailable fraction of metals. Since contaminants are not removed with in-place treatment, short-term and long-term effectiveness of the soil amendments and the vegetative cover continue to be debated. Several experimental plots within the Basin have been treated in-place, but have developed plant communities of limited diversity where some seeded species failed to establish or persist. A greenhouse pot study was used to determine site-specific toxicity thresholds across a dilution of total metals and arsenic that significantly reduced plant growth. Two sets of contaminated and reference soils collected from the Basin were mixed to obtain metal and arsenic concentration gradients from 244 to 5885 and 250 to 7521 mg/kg, respectively. Five native and two non-native grasses were grown in separate trials. Percent emergence, shoot height, total biomass and root mass ratio were analyzed. Sensitivity of the seven grasses varied according to the response measured and dilution series. Most species showed significant reductions in total biomass and shoot height when the sum of total metals and arsenic was 559 to 1900 mg/kg. Redtop (Agrostis gigantea Roth) was the most tolerant species, not displaying significant decreases in total biomass until the sum of total metals and arsenic reached 5783 mg/kg. Because the study used contaminated environmental samples and nonagricultural species, the results may better estimate site-specific ecological risk and toxicity thresholds for in-place treated soils in the UCFRB over studies performed in sand with inappropriate surrogate species.