Changes in the quantity and quality of plant detrital inputs to soil may be an important mechanism controlling soil C and N dynamics in sagebrush rangelands affected by annual grass invasion. We investigated the seasonal fate of soil NH₄ and NO₃ consumption processes in response to stimulated substrate pools in mesocosms taken from soils beneath cheatgrass stands and sagebrush canopies. Mesocosms were harvested 5 times over 17-months, and soil C and N cycling rates in intact cores were determined during 72-h laboratory incubations. Soil moisture ranged from 9 to 28% across sampling dates, and strongly affected rates of C mineralization, N cycling, and net N₂O flux. Baseline N₂O fluxes were low (<0.34 μg N/kg/d) in soils below field capacity, were correlated with gross nitrification, and increased by 1000% in moist spring soils. NH4 addition stimulated net NH4 consumption and net nitrification rates, and increased N₂O flux by 50-500%. NO₃ addition had no effect on N₂O flux until moisture exceeded 25%, when both NH4 and NO3 stimulated N₂O flux. High C mineralization rates, rapidly growing microbial biomass, and high Cmin:NO₃ ratios in moist spring soils likely alleviated C-limitation to denitrification in anaerobic microsites. Cheatgrass soils typically had greater N₂O fluxes than soils from beneath sagebrush-canopies, and larger N₂O response to N additions. This is consistent with greater nitrification in response to increasing ammonification rates observed in cheatgrass compared to sagebrush soils. These results suggest that N₂O flux in these soils is primarily due to trace gas loss during nitrification. Denitrification in anaerobic microsites may be C-limited during much of the year, but NO₃ limited during the spring when plant C inputs are high.