Effects of Stockpiling on Topsoil Biogeochemistry for Semiarid Mine Reclamation

Stockpiling and storage of topsoil for use in reclamation and revegetation are common practices for many mining operations.However, stockpiling can lead to significant changes in topsoil physical and biogeochemical properties that may be detri-mental to reclamation. The objective of this research was to assess the effect of long-term stockpiling on soil biogeochemicalproperties in a semiarid region. We hypothesized that soil properties would change systematically with depth reflecting a shiftto anaerobic conditions and resulting in a general decrease in soil health. To address this hypothesis, boreholes > 20-m deepwere drilled into a 14-year-old topsoil stockpile at a copper mine in Arizona and samples collected every ~ 75 cm. Sampleswere analyzed for soil DNA biomass, texture, general agronomic properties, mineral composition, oxalate and dithioniteextraction of active mineral phases, and total elemental composition. Depth profiles revealed non-systematic changes inbiogeochemical variables with depth, including variation in soil DNA biomass, organic matter (OM), extractable nitrate(NO 3 -N) and ammonium (NH 4 -N) nitrogen, plant-available manganese (Mn) and iron (Fe), and oxalate-extractable Mn andFe. Differences in biogeochemical properties were associated with zones of variable redox state mediated by OM content andlayer depth. Anaerobic zones were observed at depths greater than 4 m where OM > 1%, and aerobic zones were observedat depths up to 15 m where OM < 1%. This study demonstrates the importance of stockpile composition on biogeochemicalprocesses during storage and contributes to improved understanding of topsoil management as a resource for reclamation ofdegraded mine lands in semiarid environments.