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 fertilizer treatments SWF1 (0 kg N/ha/yr), SWF3 (56 kg N/ha/yr), and SWF8 (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 net mineralization and net nitrification, measured with 28-day lab incubations at constant moisture and temperature, and soil N fixation, measured with 7-day lab incubations with 15N2 and glucose.