Surface mine reclamation strategy alters the microbial community function and composition in switchgrass (Panicum virgatum) soils.

Since the 1970’s, bioenergy crop production has steadily increased, initiating demand to find alternative growing land. An innovative option is the use of marginal lands, such as reclaimed mine lands, for bioenergy crop agriculture. Switchgrass (Panicum virgatum) is a promising bioenergy crop that can be grown on marginal lands due to its robust growth in various soil types and climates. However, little is known regarding plant-microbe interactions among switchgrass systems within reclaimed mine lands. A study conducted in 2008 grew switchgrass on high- and low- quality reclaimed mine sites (Hampshire and Hobet, respectively) in West Virginia to examine the feasibility of switchgrass as a reclamation-friendly bioenergy crop. Switchgrass yields at Hampshire were nearly an order of magnitude higher than Hobet (8.4 Mg ha−1 vs 1.0 Mg ha−1). Within Hampshire, the Cave-in-Rock cultivar yield was approximately 2-fold greater than that of Shawnee (12.9 Mg ha-1 vs. 7.6 Mg ha-1). Here, I sought to identify plant-microbial interactions that may account for this shift in cultivar yield by combining enzymatic activity analyses with shotgun metagenomics. I tested two hypotheses: i) that the microbial community’s ability to acquire C, N, and P as well as the abundance of functional genes encoding enzymes associated with C, N, and P acquisition will be greatest in Hampshire soils compared to that of Hobet and ii) that there will be a cultivar-specific microbiome that may drive previously observed greater, but differential yields across switchgrass cultivars at Hampshire. Hampshire soils showed significant increases in extracellular enzyme activities associated with the mineralization and acquisition of C and N compared to Hobet, whereas the activity of an enzyme associated with P acquisition was variable across sites and cultivars. Metagenomic analyses revealed significant increases in carbon, nitrogen, and phosphorus metabolism-associated gene abundances within Hampshire’s microbiome, but showed no differences in housekeeping genes across sites. Further, a taxonomically-unique bacterial community was found between sites but not cultivars, with Hampshire soils having a greater abundance of copiotrophic bacteria while Hobet soils had a greater abundance of oligotrophic bacteria. Similarly, a taxonomically-unique fungal community was found between sites but not cultivars, with Hampshire soils having a greater abundance of fungi that preferentially degrade labile C compounds while Hobet soils had a greater abundance of fungi that preferentially degrade recalcitrant C compounds. Throughout, there were no cultivar-specific microbiome differences observed. Together, these data suggest that differences in soil quality due to different reclamation strategies foster a compositionally and functionally unique soil microbiome but they do not suggest microbiome-influenced differences in yield across cultivars.