Cheatgrass (Bromus tectorum L.) is an invasive annual grass that is associated with greater bioavailability of soil nitrogen (N). Plant interactions with the soil microbiota affect rates of N cycling, including N fixation and saprotrophism. This project asks a) if cheatgrass shows genotypic variation in the relative enrichment of isotopic 15N, a proxy for microbial N fixation. We also explore the effect of competition with crested wheatgrass (Agropyron cristatum), the most broadly successful restoration grass. To do this, we performed a full-factorial randomized complete block experiment with rhizoboxes of 15 genotypes with intraspecific or interspecific (versus crested wheatgrass) competition. The homogenized shoot tissue was combusted to quantify isotopic nitrogen (Δ 15N), which associates with the relative contributions of N fixation. Preliminary results indicate that genotype, but not competitor, affects Δ15N. Next steps will test for associations with adaptive life history strategies or habitat conditions of the original genotype. Additional repetitions of the experiment are ongoing.  Insights into the 15N content of cheatgrass shoot tissue can inform the prediction of invasions and support decision making in land management.