During 2015, net nitrogen mineralization, net nitrification, soil nitrogen fixation, and root nitrogen fixation was measured in the Switchgrass Nitrogen Rate Experiment, at both Kellogg Biological Station (MI, USA) and Arlington Agricultural Research Station (WI, USA). All replicates of 3 fertilizer treatments (0 kg N/ha/yr, 56 kg N/ha/yr, and 196 kg N/ha/yr) were measured 4 times: pre-fertilizer (May), post-fertilizer (June), at peak biomass (late July) and post-senescence (October). This data file contains the data on root N fixation, measured with 7-day lab incubations with 15N2 and glucose.