Document: SHA-3-59-105

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

Abstract:
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 g_soil^-1), and lowest in plant interspaces (8.0 2.8 g g_soil), 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.

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This abstract is being presented at: 11:45 AM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.