Biological integrity of mixed-grass prairie topsoils subjected to long-term stockpiling

Abstract

Surface mining often requires storage of topsoil in large piles for long periods of time (1–30 years). Such soil handling and storage results in physical and biogeochemical changes and may alter the soil biological integrity. Microbial activity regulates nutrient cycling and soil quality, and post-mining revegetation of most native forb species in the Northern Great Plains relies on a viable seedbank. Soil condition and biological viability influence the establishment and success of the aboveground plant community, and therefore should be considered in facilitating successful reclamation. In this study, we characterized soil biological integrity via the seed bank and microbial community structure in a topsoil stockpile (depths of 15 cm–750 cm). We measured microbial community structure using phospholipid fatty acid analysis and assessed the seed bank using the seedling emergence method under greenhouse conditions. We hypothesized with increasing depth, soil biota would decrease in abundance and perhaps exhibit a shift in community structure, and stockpiled soils would display different communities compared to an undisturbed reference site. Further, we hypothesized the soil seedbank would decrease with increasing depth and be less than the reference site. We found distinct shifts in microbial communities in terms of community structure and declines in overall abundance of organisms with increased depth using principal component analysis. Furthermore, overall microbial abundance within the top 15 cm of stockpiled soil was nearly one third less than our native undisturbed reference site, and soil at depth was increasingly depleted. Total seedling emergence from soil collected at all depths of the stockpile was lower (20 viable seeds) compared to the reference soil emergence (36 viable seeds). Our results demonstrate that stockpiling greatly affects soil microbial communities and that stockpiled topsoil is not a dependable source for forb seeds. Reclamation may require forb seeding and soil amendments to facilitate whole-system restoration.