Biological nitrogen fixation offers a practical route to reduce reliance on synthetic fertilizers, and associative nitrogen fixation (ANF) may potentially satisfy a meaningful share of the nitrogen demand of many non-leguminous plants. We investigated the coupling of plant-derived carbon inputs to nitrogen fixation in the rhizosphere of switchgrass, an important bioenergy crop, by conducting multisubstrate DNA stable isotope probing using whole-plant mesocosms labeled with either ¹³CO₂ or ¹⁵N₂. In total, 772 taxa were labeled, including 365 that were ¹³C-labeled, 192 that were uniquely ¹⁵N-labeled, and 215 that were dual ¹³C- and ¹⁵N-labeled. These OTUs represented ~14.9% of the rhizosphere community. The pattern of isotope incorporation is consistent with multiple routes of fixation, driven by provisioning from plant photosynthate, and followed by rapid turnover and mineralization of N from microbial biomass. We also detect dual labeling in taxa typically associated with cyanobacterial or bryophyte-mediated soil crusts, suggesting that biocrust communities might establish on the soil surface below switchgrass plants. The data indicate that ANF in switchgrass is a community process driven by habitat conditions promoted by the host plant.