Bacterial and eukaryal diversity in soils forming from acid mine drainage precipitates under reclaimed vegetation and biological crusts

Low-pH and low-nutrient conditions make the establishment of vegetation in mining-impacted environments particularly challenging. This study was conducted on soils forming from secondary Fe mineral precipitates in experimental plots at a 50-year-old acid mine drainage (AMD) barrens in Central Pennsylvania, U.S.A. Our objective was a qualitative assessment of bacterial and eukaryal diversity in reclaimed precipitates supporting dense successional vegetation six years after a single addition of lime (to raise pH from 2.5 to 4.5) and a single incorporation of compost. At the time of sampling, the pH of reclaimed precipitates had declined to initial values (2.5-2.7) and was similar to that of control precipitates supporting indigenous biological crusts. Microbial diversity was compared using 454 pyrosequencing of 16S rRNA genes (V1-V5) and 18S rRNA (V4-V5) in upper layers (adhering to roots and crusts) and lower layers (below roots and crusts). A total of 1721 bacterial OTUs at 97% similarity and 307 eukaryal OTUs at 97% similarity were recognized in the entire dataset. Despite similar pH, reclaimed precipitates had more diversity than control precipitates. Proteobacteria and Actinobacteria were the most abundant bacterial phyla in reclaimed and control precipitates, respectively. Acidobacteria were more abundant in root-and crust-enriched layers than in the underlying precipitates containing less carbon. Basidiomycota fungi were the most abundant classified eukaryotes in reclaimed precipitates, while Bryophyta dominated control precipitates. Glomeromycota were observed in reclaimed but not in control precipitates, where Ascomycota were the most abundant fungi. After the one-time reclamation approach, bacterial richness in AMD precipitates was three-fold lower than that determined for acid sulfate soils using closed reference OTU picking. These results suggested the need for further lime and compost incorporation to increase soil functionality.