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Effects of Elevated CO2 on Nitrogen Gas Emissions from Desert Soil
Reference
McCalley, C.K., Strahm, B.D., Sparks, K.L., Eller, A.S.D. and Sparks, J.P. 2011. The effect of long-term exposure to elevated CO2 on nitrogen gas emissions from Mojave Desert soils. Journal of Geophysical Research 116: 10.1029/2011JG001667.

Background
The authors state that in earth's arid regions, "emissions of nitrogen (N) gases are important to long-term soil fertility and regional atmospheric chemistry," which fact makes any alterations in N gas emissions in response to atmospheric CO2 enrichment a subject of considerable concern.

What was done
Working at the Nevada Desert FACE Facility northwest of Las Vegas, Nevada (USA), McCalley et al. measured soil fluxes of reactive N gases (NO, NOy, NH3) and N2O in plots receiving long-term fumigation with ambient (380 ppm) and elevated (550 ppm) CO2. These treatments were begun in April 1997; and reactive N gas flux measurements were made under these conditions several years later in April 2005, July 2005, July 2006, January 2007 and March 2007, as well as after the termination of CO2 fumigation in July 2007, October 2007, January 2008 and April 2008.

What was learned
The five researchers report that "long-term exposure to elevated CO2 decreased reactive N gas emissions from Mojave Desert soils," and that it did so primarily "in islands of fertility created by the dominant shrub Larrea tridentata," and especially "in the spring and fall when recent precipitation, either natural or artificial, created soil conditions that are optimal for biological activity." Emissions of N2O, on the other hand, were "a very small component" of gaseous N loss and were "largely insensitive to elevated CO2."

What it means
The insensitivity of N2O emissions to elevated CO2 is a positive thing, in that McCalley et al. say that "nitrous oxide is a potent greenhouse gas with 180 times the global warming potential of CO2," citing Lashof and Ahuja (1990), while Crutzen et al. (2007) say that N2O has "a 100-year average global warming potential 296 times larger than an equal mass of CO2." In addition, the five researchers state that the greater-than-60% reductions in reactive N gas fluxes during periods of peak N demand imply that elevated CO2 is "increasing the retention of biologically available N during critical growth periods," which is a major benefit for desert ecosystems.

References
Crutzen, P.J., Mosier, A.R., Smith, K.A. and Winiwarter, W. 2007. N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. Atmospheric Chemistry and Physics Discussions 7: 11,191-11,205.

Lashof, D.A. and Ahuja, D.R. 1990. Relative contributions of greenhouse gas emissions to global Warming. Nature 344: 529-531.

Reviewed 16 November 2011