This dataset contains data supporting the paper: DeCrappeo, N.M., DeLorenze, E.J., Giguere, A.T., Pyke, D.A., and Bottomley, P.J. Fungal and bacterial contributions to nitrogen cycling in cheatgrass-invaded and uninvaded native sagebrush soils of the western USA (accepted at the journal Plant and Soil). The purpose of the study was to evaluate the relative contributions of soil bacteria and fungi to inorganic nitrogen (N) cycling in sagebrush and cheatgrass-invaded soils using a 15N isotope dilution experiment. Soils were collected from sagebrush and cheatgrass rhizospheres at six paired sites in southwest Idaho and southeast Oregon. In order to partition the contribution of each microbial group to N cycling, soils were treated with isotopically labeled N sources and protein synthesis inhibitors. Bronopol and cycloheximide block protein synthesis in bacteria and fungi, respectively; nitrogen can still be taken up by the organisms, but the organisms are unable to assimilate the nutrient into biomass. Laboratory incubations were carried out to study the partitioning of N to microbial biomass and dissolved inorganic nitrogen pools, which were then used to calculate the following nitrogen transformation rates: gross mineralization, net mineralization, ammonium consumption, and net nitrification.