Zinc toxicity thresholds for important reclamation grass species of the western United States

Heavy metal toxicity thresholds for plant species that are used in reclamat ion activities in Western North America have not been established. As a res ult, ecological risk assessments must rely on toxicity thresholds establish ed for agronomic species, which may differ from those of species used in re clamation. As a result, risk assessors have the potential for classifying s ites as phytotoxic to perennial, nonagronomic species and calling for inten sive remediation activities that may not be necessary. The objective of thi s study was to provide a better estimate of Zn toxicity thresholds for five grass species that are commonly used in reclamation efforts in the western United States. We used a greenhouse screening study where seedlings of red top (Agrostis alba Roth), slender wheatgrass (Elymus trachycaulus [Link] Go uld ex Shinners var. Pryer), tufted hairgrass (Deschampsia caespitosa (L.) Beauv.), big bluegrass (Poa ampla J. Presl var. Sherman), and Great Basin w ildrye (Leymus cinereus [Scribn. & Merr] A. Love var. Magnar) were grown in sand culture and exposed to concentrations of soluble Zn ranging from 0.6 (control) to 500 mg/L. We determined six measures of toxicity: the 50-d LC5 0, 50-d EC50-plant, 50-d EC50-shoot, 50-d EC50-root, PT50-shoot, and PT50-r oot. Results suggest that these grass species have higher Zn phytotoxicity thresholds (PT50s) than agronomic species reported in the past. Roots appea red to be more susceptible to the toxic effects of zinc (as indicated by re duced growth) than shoots. This observation suggests that measures of EC50- shoot alone may be misleading for these,species where growth reduction is m anifested primarily in roots. Values of EC50-plant for these reclamation gr asses were between 84 and 222 mg/L. Measured PT50-shoot values were between 2,449 and 5,026 mg/L. These thresholds should be more useful for risk asse ssors than those based on agronomic crops that are currently used.