Nutrient availability to plants is regulated by rates of nutrient release and immobilization and by nutrient transport processes in soils. Nitrogen is released from soil organic matter into plant-available forms by the process of N-mineralization, and it is removed from the plant-available pool during N immobilization by soil microbes; however we have a very poor understanding of how environmental factors regulate these processes to produce seasonal changes in N availability. We used ¹⁵N-isotope dilution to measure gross rates of N mineralization and immobilization at 2-wk intervals during two growing seasons in the surface soils of an Idaho big sagebrush—crested wheatgrass ecosystem to quantify seasonal patterns in N-cycling rates.  We also measured soil moisture, temperature, inorganic N, net N mineralization, and soil respiration rates to determine what factors regulate gross and net N cycling rates in the field.  During both a wet and a dry year, seasonal patterns in net N mineralization were similar: net N mineralization dominated in spring and fall, and net N immobilization dominated in summer. This pattern in net mineralization could be largely explained by differences in the sensitivities of gross N mineralization and immobilization to temperature and moisture: gross N mineralization rates were largely insensitive to changes in soil temperature, but were strongly regulated by soil moisture. In contrast, gross N immobilization rates were more sensitive to temperature and less sensitive to soil moisture. Seasonal changes in the quality of microbial substrates (i.e. C:N) appeared to play a trivial role in regulating seasonal patterns in net N mineralization. Biogeochemical models may need to be modified to account for the differing sensitivities of N mineralization and immobilization to temperature and moisture.