Wildlands of the United States' Intermountain West contain recurring interspersed plant-community types; namely native sagebrush (Artemisia tridentata spp. wyomingensis Nutt.), non-native invasive cheatgrass (Bromus tectorum L.), and crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.]. Soil nitrogen (N) cycling in these water and N co-limited ecosystems shows very strong spatial and temporal variability, but the mechanism(s) by which these semi-arid plant communities control soil N transformations are not well understood. Over two growing seasons, we conducted field and laboratory incubations of intact soil cores (0-10 cm) with and without water added, and created a mass balance model to predict N mineralization. We found that soils under cheatgrass had the highest net N mineralization, net nitrification and soil moisture compared to soils from under the other two plant communities. Moreover, water additions to field-incubated soil cores under cheatgrass more than doubled net N mineralization (0.18 +/- 0.02 vs 0.07 +/- 0.01 mg N kg(-1) d(-1)). Temperature had a small effect on net N mineralization and net nitrification, with both rates increasing by < 0.005 mg N kg(-1) d(-1) per degrees C. The model's ability to predict N mineralization was relatively low (R-2 = 0.33). However, both our model and the data themselves strongly support plant community regulation of soil N cycling through modification of soil moisture.