Mine reclamation using biofuel crops: Insights into the microbial ecology of the switchgrass (Panicum virgatum) microbiome

Bioenergy crop production has steadily increased due to growing political support for renewable energy, thus initiating a demand to find alternative agricultural land. An innovative option is the use of marginal soils, such as reclaimed mine lands, to produce bioenergy crops. 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 resilience 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 illuminate plant-microbial interactions that may account for this drastic shift in cultivar yield by assessing the soil microbial community’s function and composition. I tested two hypotheses: i) that the microbial community’s ability to acquire C, N, and P will be greatest in Hampshire soils compared to that of Hobet and ii) that there will be a cultivar-specific root-associated microbiome that may drive previously observed greater, but differential yields across switchgrass cultivars at Hampshire. I found that reclamation strategy substantially impacts the switchgrass microbiome’s composition as well as its ability to acquire critical nutrients like carbon, nitrogen, and phosphorus. I also found that a functionally, but not necessarily compositionally, unique microbiome exists in the root-associated soils compared to that of the bulk soil. Additionally, there were indicators that organic amendments to the topsoil may induce cultivar-specific soil microbiomes that mediate or facilitate differential yields within Hampshire. Taken together, I suggest that organic amendments to the topsoil during reclamation selects for a cultivar-specific microbiome more adept to acquiring critical nutrients and thus, increases aboveground productivity