Response of soil nitrogen dynamics to elevated CO2 in a Mojave Desert ecosystem.
BILLINGS, S.A.*, S.M.SCHAEFFER and R.D.EVANS
University of Arkansas-Fayetteville, AR 72701 USA 1
Growth and reproduction of many desert plant species are limited by the availability of water and plant-available nitrogen (N). It is theorized that atmospheric CO2 may increase plant water-use efficiency, and thus increase net primary productivity (NPP) in deserts; however, we do not know how higher CO2 levels will affect soil N availability. We are investigating the effects of elevated CO2 (550 ppm) on soil N transformations at the Nevada Desert FACE (Free-Air Carbon Enrichment) facility in an intact Mojave Desert ecosystem. We examined soil N2O and CO2 fluxes, ammonia volatilization, and N-mineralization under three plant species and in plant interspaces, under both ambient and elevated CO2. N2O fluxes were undetectable in all seasons tested. Nitrogen mineralization was significantly higher under shrubs (27.9 4.4 g gsoil-1), and lowest in plant interspaces (8.0 2.8 g gsoil), where plant litterfall is minimal. Losses of N via NH3 volatilization were appreciable (0.73 0.12 vs. 0.65 0.07 g NH3-N m-2 d-1 with ambient and elevated CO2, respectively). These data suggest a trend of lower losses under plant canopies than in interspaces, and lower losses with elevated CO2. Soil CO2 flux rates were significantly higher with elevated CO2 (49.2 12.5 vs. 17.6 6.5 g m-2 h-1, P < 0.05); this data can serve as a proxy for underground microbial activity, especially in plant interspaces where roots are scarce. In conjunction with NH3 losses and soil CO2 data, this suggests microbial activity is highest where carbon supply is least limited. Our data indicate that soil microbial activity, and thus N-immobilization, may increase in an atmosphere of elevated CO2. This change in N availability to plants may counteract any potential gains in NPP caused by increases in water-use efficiency.
This abstract is being presented at: 11:45 AM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.