The effect of long-term exposure to elevated CO₂ on nitrogen gas emissions from Mojave Desert soils

In arid regions, emissions of nitrogen (N) gases are important to long-term soil fertility and regional atmospheric chemistry, making alterations in N gas emissions an important aspect of ecosystem response to climate change. Studies at the Nevada Desert FACE Facility suggest that rising atmospheric CO₂ concentrations impact ecosystems N dynamics in the Mojave Desert; our objective was to identify whether those responses translate into changes in trace N gas emissions. We measured soil fluxes of reactive N gases (NO, NO_y, NH₃) and N₂O in plots receiving long-term fumigation with ambient and elevated (550 ppm) CO₂. Reactive N gas emissions were significantly lower under elevated CO₂ during high soil moisture conditions in the spring and fall. The strongest responses occurred in the islands of fertility created by the dominant shrub Larrea tridentata, where fluxes were 3-5 ng N m^-2 s^-1 lower in elevated CO₂ plots. Changes in total N gas emissions were driven by reduced NO and NH₃ emissions, with smaller changes in NO_y efflux and little to no production of N₂O. Lower N gas emissions under elevated CO₂ reflect changes in plant and microbial demand for N, suggesting increased uptake or immobilization coupled with decreased rates of N mineralization and nitrification. This response of N gas efflux to elevated CO₂ in the growing season suggests that in deserts, elevated CO₂ promotes ecosystem retention of N during periods of peak biological demand. Concomitantly, exposure to elevated CO₂ alters inputs of new reactive gases into the atmosphere, potentially impacting local atmospheric processes.