Interactive effects between lime, organic matter, and bacteria in the establishment of Leymus cinereus in mine tailings

Abstract

The landscape legacy of historical metal-mining activity can persist for decades. The most frequent strategies used for the remediation of contaminated soils include: the use of synthetic membranes to isolate contaminants (>$0.5 million/acre), direct revegetation (less expensive but difficult to sustain), or lime amendments ($5000/acre). Looking for more cost-effective bioremediation approaches, we performed a set of greenhouse studies to determine what combinations of soil amendments would lead to the best vegetative response, and potentially associated reductions in soil arsenic (As) levels, in "slickens" collected from the Lampert Ranch along the upper Clark Fork near Warm Springs, MT. In our first greenhouse experiment, we planted Leymus cinereus (basin wildrye) and compared (after 12 weeks) plant growth and foliar metal concentrations across treatments. Amendments included single or factorial additions of 5% lime, organic matter (+OM), and an arsenic-oxidizing (+oxbact) strain of Agrobacterium tumefaciens (Agtu). Surprisingly, the OM+oxbact treatment revealed among the best plant growth and arsenic uptake response. We then performed a second greenhouse experiment with two levels of OM (1.5% and 5%) and an additional treatment: a mutant (reducing strain) of Agtu. Basin wildrye grown in soils amended with 5% OM generally did better than those grown in soils amended with 1.5% OM. At the same time, foliar As uptake (biomass multiplied by As concentration) was unexpectedly high (0.020 mg pot-1) for plants grown in soils amended with 5% OM + oxbact, 3 times greater than foliar arsenic uptake in plants grown in soils amended with 5% OM+ lime and 4 times greater than foliar uptake by plants grown in soils amended with 5% OM and the reducing strain of Agtu. These results suggest the combination of OM and Agtu oxbact strain could provide a potentially cost- effective approach to remediating As-contaminated soils. Finally, our study results imply that soil restoration approaches could be improved through a greater consideration of microbial communities supported by these re-establishing vegetation communities, which could lead to more sustainable ecosystem successional trajectories.