Trace N gas losses and N mineralization in Mojave desert soils exposed to elevated CO2

We examined N cycling processes in desert soils exposed to elevated CO2 to better understand how some features of aridland soil N and C cycling may respond to an altered atmospheric composition. We measured rates of denitrification, potential denitrification, N2O fluxes, NH3 volatilization, and net mineralization in an intact Mojave Desert ecosystem with elevated CO2 (Free Air Carbon Enrichment technology) over 2 y. All measurements were performed on soil under four different cover types: Larrea tridentata; Lycium spp.; Pleuraphis rigida; and plant interspaces. The mean rate of denitrification was 161±96 μg N m−2 d−1. Field fluxes of N2O occurred sporadically, with a mean of 30±20 μg N m−2 d−1. Rates of NH3 volatilization experienced less variability than did N2O fluxes, with a mean of 120±45 μg N m−2 d−1. Mean potential denitrification enzyme activity (DEA) was 146±8 mg N m−2 d−1. Rates of net mineralization were highest in soil under L. tridentata and Lycium spp. (398±108 mg N m−2 d−1) and lowest in the plant interspaces (129±28 mg N m−2 d−1). There was no effect of elevated CO2 on N2O fluxes or mineralization rates. There was a 39% increase in NH3 volatilization with elevated CO2 during March 2000. Potential DEA increased by 193% with elevated CO2 in October 1999 and decreased by 45% in March 2001. These results suggest that NH3 volatilization may be a more important component of aridland gaseous N emissions than previously thought, and that the potential for high DEA does not necessarily induce large fluxes of N2O under natural conditions, especially in aridlands where rainfall primarily occurs in winter when soil temperatures can limit microbial activity. This study also suggests that the effects of elevated CO2 on soil microbial activity may not be consistent for all seasons.