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 grasslandmicrocosms during early-season wet-up, a time when rates of Ntransformation and N trace gas flux are high. After plant senescenceand summer drought, we simulated the first fall rains and examined Ncycling. Growth at elevated CO2 increased root productionand root carbon:nitrogen ratio. Under nutrient enrichment, elevatedCO2 increased microbial N immobilization during wet-up,leading to a 43% reduction in gross nitrification anda 55% reduction in NO emission from soil. ElevatedCO2 increased microbial N immobilization at ambientnutrients, but did not alter nitrification or NO emission. ElevatedCO2 did not alter soil emission of N2O ateither nutrient level. Addition of NPK fertilizer (1:1:1) stimulatedN mineralization and nitrification, leading to increased N2Oand NO emission from soil. The results of our study support a mechanisticmodel in which elevated CO2 alters soil N cycling and NOemission: increased root production and increased C:N ratio in elevatedCO2 stimulate N immobilization, thereby decreasingnitrification and associated NO emission when nutrients are abundant.This model is consistent with our basic understanding of how C availabilityinfluences soil N cycling and thus may apply to many terrestrial ecosystems.