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

Heavy metal toxicity thresholds for plant species that are used in reclamation activities in Western North America have not been established. As a result, ecological risk assessments must rely on toxicity thresholds established for agronomic species, which may differ from those of species used in reclamation. As a result, risk assessors have the potential for classifying sites as phytotoxic to perennial, nonagronomic species and calling for intensive remediation activities that may not be necessary. The objective of this 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 redtop (Agrostis alba Roth), slender wheatgrass (Elymus trachycaulus [Link] Gould ex Shinners var. Pryor), tufted hairgrass (Deschampsia caespitosa (L.) Beauv.), big bluegrass (Poa ampla J. Presl var. Sherman), and Great Basin wildrye (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 LC50, 50-d EC50-plant, 50-d EC50-shoot, 50-d EC50-root, PT50-shoot, and PT50-root. Results suggest that these grass species have higher Zn phytotoxicity thresholds (PT50s) than agronomic species reported in the past. Roots appeared to be more susceptible to the toxic effects of zinc (as indicated by reduced growth) than shoots. This observation suggests that measures of EC50-shoot alone may be misleading for these species where growth reduction is manifested primarily in roots. Values of EC50-plant for these reclamation grasses 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 assessors than those based on agronomic crops that are currently used.