Soil emissions of nitrogen trace gases in the Mojave Desert under elevated carbon dioxide: implications for regional tropospheric chemistry.

Understanding the exchange and transformation of compounds at the biosphere - atmosphere boundary is critical to predict the consequences of global change on future atmospheric composition. We present here observations of nitrogen trace-gas efflux from Mojave Desert soils under ambient and elevated CO2 conditions. Measurements were conducted at the Nevada Desert Free Air CO2 Enrichment Facility (FACE) north of Las Vegas, Nevada, USA. Previous results from this site have demonstrated that elevated CO2 has increased microbial functional diversity and, therefore, potentially altered the magnitude and composition of trace nitrogen gas efflux. A shift in emission between the ozone precursor NO and aerosol-producing ammonia under elevated CO2 would have profound influences on atmospheric composition and global temperature forcing. Nitrogen effluxes from the soil were quantified using a custom-built reduction system and traditional electron-capture chromatography to quantify fluxes of NH3, NO, NOy, and N2 O. Under ambient CO2 conditions, soil emissions were dominated by emissions of NO (0.35 nmol m-2 s-1) with smaller emissions of non-NO NOy compounds and NH3 (0.074 and 0.053 nmol m-2 s-1, respectively). After a seven-year elevated CO2 treatment of 550 ppmv, soil emissions are now dominated by ammonia (0.22 nmol m-2 s-1) with smaller emissions of NO (0.13 nmol m-2 s--1) and non-NO NOy (0.05 nmol m-2 s-1). If the shift from NO to NH3 soil emission under elevated CO2 observed in this study were common across arid lands globally, the potential would exist, depending on the ensuing atmospheric chemistry, to shift regional emissions from precursors of tropospheric ozone production to precursors of secondary organic aerosols. Such a shift could potentially alter the regional atmosphere from a positive (NO induced tropospheric ozone production) to a negative global temperature forcer (increased aerosol production and cloud formation) independent of carbon dioxide concentrations.