Switchgrass root endophytic diazotrophs Raw sequence reads. Switchgrass root endophytic diazotrophs

The use of synthetic nitrogen fertilizers to grow biofuel crops adds to the economic and environmental costs of biomass production. Biological nitrogen fixation (BNF) provides an alternative, eco-friendly source of nitrogen for leguminous plants and some non-legumes. With the objective of characterizing and eventually harnessing nitrogen-fixing bacteria in switchgrass roots, we assayed nitrogenase activity and surveyed nifH-expressing bacteria in roots of switchgrass grown under agricultural conditions. Plants were cultivated at two separate locations in Oklahoma in different soil-types for five years with three nitrogen fertilizer regimes (0, 90 and 180 kg N. ha-1. yr-1). Nitrogenase activity associated with roots cleaned of soil was found to be highest in plants grown with no N-fertilizer and lowest or not detectable for plants grown at the highest level of N-fertilizer. A total of 454 abundant nifH Operational Taxonomic Units (OTUs) were identified in the switchgrass roots. Diversity analysis of active nifH-expressing bacteria showed that the most common orders were Burkholderials (171 OTUs), Desulfovibrionales (55 OTUs), and Rhizobiales (44 OTUs). OTUs belonging to Pseudacidovorax, Desulfovibrio, Dechloromonas, and Bradyrhizobium were considerably enriched in roots of plants grown under low nitrogen conditions. Analysis of significant OTUs showed specific nifH-expressing microbial communities in most switchgrass root samples that were different/unique for low nitrogen fields at each location.