We examined the effects of growth carbon dioxide (CO2) concentration and soil nutrient availability on nitrogen (N) transformations and N trace gas fluxes in California grassland microcosms during early-season wet-up, a time when rates of N transformation and N trace gas flux are high. After plant senescence and summer drought, we simulated the first fall rains and examined N cycling. Growth at elevated CO2 increased root production and root carbon:nitrogen ratio. Under nutrient enrichment, elevated CO-2 increased microbial N immobilization during wet-up, leading to a 43% reduction in gross nitrification and a 55% reduction in NO emission from soil. Elevated CO2 increased microbial N immobilization at ambient nutrients, but did not alter nitrification or NO emission. Elevated CO2 did not alter soil emission of N2O at either nutrient level. Addition of NPK fertilizer (1:1:1) stimulated N mineralization and nitrification, leading to increased N2O and NO emission from soil. The results of our study support a mechanistic model in which elevated CO2 alters soil N cycling and NO emission: increased root production and increased C:N ratio in elevated CO2 stimulate N immobilization, thereby decreasing nitrification and associated NO emission when nutrients are abundant. This model is consistent with our basic understanding, of how C availability influences soil N cycling and thus may apply to many terrestrial ecosystems.