In southwest Montana, more than a century of copper mining and smelting has left a significant biogeochemical legacy of metal/metalloid-contaminated terrestrial and aquatic ecosystems. In historically forested terrestrial ecosystems, long-term deposition of contaminants from smelter emissions, coupled with forest-clearing for fuel and grazing, have shifted the vegetation from forest to grassland-dominated, expanded the range of invasive species, and produced large areas of bare ground, which has caused extensive erosion and secondary deposition of organic matter and metal(loid)-contaminated sediment from the steep slopes and saddles of mountain uplands. Current steep slope restoration initiatives stem from the hypothesis that soil metal(loid) concentrations and nutrients have been eroded from poorly vegetated areas over time, and therefore, it is nutrient limitation rather than metal(loid) toxicity that is preventing natural revegetation post-smelter aerosol cessation. Here, we compared watershed-wide patterns of soil physicochemical and vegetation characteristics of 6 communities (3 grassland, 2 aspen, and 1 conifer) in Muddy Gulch, a smelter aerosol-affected watershed in southwest Montana, to test the hypothesized patterns and barriers to restoration, as well as provide insight into the relationship between existing soil and biological conditions prior to restoration.
Results/Conclusions
We observed strong patterns in soil physicochemical properties and vegetation community characteristics across Muddy Gulch. Total metal(loid)s (sum of arsenic, copper, lead, and zinc), soil organic matter (SOM), and vegetation cover all increased as slope decreased. Values of all three variables peaked in the valley bottom vegetation communities and decreased as one moves from the toe slopes to the side-slopes and saddles of the watershed. Interestingly, plant diversity did not necessarily increase with increasing plant cover. Bioavailable metal(loid)s (sum of easily extractable arsenic, copper, lead, and zinc) tended to be highest in communities with the highest total metal(loid)s. Soil pH drove bioavailable metal(loid)s, with small increases in pH coinciding with large decreases in copper and lead bioavailability, while small decreases in pH resulted in decreased arsenic bioavailability. We found fewer differences in bioavailable metal(loids)s between communities, likely because small differences in soil pH had sizeable impacts on bioavailability that possibly obscured differences between communities. Our results suggest that extensive soil erosion of poorly vegetated and bare areas does not necessarily indicate low levels of contaminant availability and may continue to have implications for restoring vegetation and promoting ecosystem development in southwest Montana.