ELEVATED CO2 AND NUTRIENT ADDITION ALTER SOIL N CYCLING AND N TRACE GAS FLUXES WITH EARLY-SEASON WET-UP IN A CALIFORNIA ANNUAL GRASSLAND

We examined the effects of growth carbon dioxide (CO2) concentration a nd soil nutrient availability on nitrogen (N) transformations and N tr ace 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 fir st fall rains and examined N cycling. Growth at elevated CO2 increased root production and root carbon:nitrogen ratio, Under nutrient enrich ment, elevated CO2 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 immobili zation at ambient nutrients, but did not alter nitrification or NO emi ssion. Elevated CO2 did not alter soil emission of N2O at either nutri ent level. Addition of NPK fertilizer (1:1:1) stimulated N mineralizat ion and nitrification, leading to increased N2O and NO emission from s oil, The results of our study support a mechanistic model in which ele vated CO2 alters soil N cycling and NO emission: increased root produc tion and increased C:N ratio in elevated CO2 stimulate N immobilizatio n, thereby decreasing nitrification and associated NO emission when nu trients are abundant. This model is consistent with OUT basic understa nding of how C availability influences soil N cycling and thus may app ly to many terrestrial ecosystems.