Assessment of soil development following a sub-alpine mine reclamation

Abstract

The Summitville Mine is located at 3,500 m in the San Juan Mountain Range, Rio Grande County, Colorado. Open-pit mining for gold created 223 hectares of waste rock and drastically disturbed land. The waste rock had acidic pH and high metal concentrations, so reclamation plans were enacted to accelerate soil formation and establish a self-sustaining plant community. Treatments were applied to the waste rock to develop a soil with adequate pH, nutrients, organic matter, and a functioning microbial community. The waste rock treatments were mushroom compost and lime (t-M), limed stockpiled topsoil (t-SM), and non-stockpiled topsoil (t-NM). Six years after reclamation, the objectives of this study were to assess soil development in the treated waste rock by measuring microbial community characteristics in relation to physicochemical properties and plant community measures and comparing these soil characteristics to native reference sites. The native reference sites were a sub-alpine meadow, forest, and a clear-cut forest. Physicochemical properties included soil pH, organic matter, nitrogen, and total metals. Plant characteristics were cover and species richness. Microbial properties included active (viable) biomass and community structure (Phospholipid Fatty Acid analysis), fungal and bacterial total biomass (Microscopy), community function measured as carbon-substrate utilization and richness (Biolog analysis), and litter decomposition. The combination of soil microbial community, physicochemical, and plant characteristics indicated soil development in the waste rock treatments. Both t-NM and t-M were the most effective in reclaiming the waste rock. These waste rock treatments had plant cover, soil organic and inorganic N, pH, and total metals that were similar to the native reference sites. Either t-M, t-NM, or t-SM provided a functioning microbial community and litter decomposition similar to the native reference sites. However, the waste rock treatments had lower microbial active and total biomass than the native reference sites. Only t-NM was similar to the sub-alpine meadow in microbial community richness, which included bacteria, fungi, and actinomycetes. From the various soil characteristics studied, substrate utilization richness, bacterial total biomass, and soil organic matter had many significant correlations with other soil characteristics, which included inorganic N and total metal concentrations, fungal total biomass, microbial community richness, and plant species richness. Future projects could use one of these soil characteristics with their associated correlations to assess soil development after reclamation.