TESTING NATIVE GRASSES FOR SURVIVAL AND GROWTH IN LOW pH MINE OVERBURDEN

Abstract

Overburden piles at the Molycorp molybdenum mine in north central New Mexico contain neutral rock types as well as mixed volcanic rocks, which are highly weathered materials with low pH and high salinity from pyrite oxidation. The mixing of rock types during overburden pile construction has resulted in heterogeneous substrates with a range of pH and soluble salt levels. An experiment to determine grass species more likely to survive and grow in these low pH overburden materials used substrate treatments consisting of an unadulterated acid rock, an acid:neutral overburden mixture ratio of 9: I, and an acid:neutral overburden mixture ratio of 3:1. Containerized grass seedlings of 54 species/ecotypes, primarily cool-season natives of the western U.S, were transplanted into these substrates. Species grown from seed collected at the Mo\ycorp site having superior performance included Muhlenbergia montana (2 ecotypes), Blepharoneuron trieholepis, Festuea species (3 ecotypes), and a Poa species. A number of commercially available grass varieties had good survival and growth in these substrates: Deschampsia caespitosa 'Peru Creek Festuea ari:::onica 'Redondo', Festuca ovina 'Covar Festuea ovina 'MX-86', Festllea sp. 'Shorty', Poa compressa 'Reubens', Paseopyrum smith;; 'Arriba, Barton, and Rosana and Elymus traehycaulus 'San Luis'. Other native grass species that showed superior survival and growth in these acid rock substrates included Elymus canadensis, Danthonia interrnedia, Sporobolus wrightii, Poa nemoralis, and Hesperostipa cornata. Additional