The effect of long-tem exposure to elevated CO2 on NO and NH3 efflux from Mojave Desert soils

Background/Question/Methods

Increases in atmospheric CO₂ can alter terrestrial N dynamics, potentially changing the efflux of reactive nitrogen compounds to the atmosphere. Nitric oxide (NO) and ammonia (NH₃) are both volatile products of the soil N cycle and have opposing effects on the radiative forcing potential of the atmosphere. Therefore, there is potential for a positive or negative feedback between the Earth’s surface and the atmosphere mediated through nitrogen trace gases. Further, in arid ecosystems gaseous N fluxes can comprise a substantial proportion of annual N loss, thus shifts in emissions could have important consequences for the overall N budget. To assess the effect of future atmospheric CO₂ concentrations on reactive N gas production we measured soil emissions of NO and NH₃ at the Nevada Desert FACE Facility in rings that had been fumigated with 550 ppm CO₂ for approximately 10 years and in blower control rings. Soil fluxes were measured in soils beneath the evergreen shrub Larrea tridentata and in plant interspaces before and 36 hrs after a 30 mm artificial precipitation event. Measurements were made four times during the year, March, July, October and January, with the CO₂ fumigation ending just prior to the July measurements.

Results/Conclusions
In dry soils, NO and NH₃ fluxes were generally small with fluxes ranging from 0.06 – 0.2 and 0.15 – 0.9 ng N m^-2 s^-1 respectively. Further, there was no effect of long-term exposure to elevated CO₂ on the composition or magnitude of emissions from dry soils. Irrigation resulted in 15 to 60-fold increases in reactive N gas emissions. During the winter and spring, emissions of both NH₃ and NO were lower in plots exposed to elevated CO₂. The reduction in NO and NH₃ efflux under elevated CO₂ was strongest in the spring in soils under the dominant evergreen shrub Larrea tridentata, with these soils exhibiting a 3-4 times lower post-wetting N gas pulse compared to control plots. Reductions in N gas losses under elevated CO₂ are consistent with patterns observed following experimental additions of labile C to Mojave Desert soils. This and other studies indicate that microbial activity in this system is C limited and increases in soil C availability resulting from long-term exposure to elevated CO₂ may decrease this limitation. During periods when microbial activity is high this appears to increase the amount of N that is immobilized as opposed to lost the atmosphere as NO or NH₃.