PARENT SESSION
Poster Session #11: Elevated CO₂.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Perturbations to nitrogen cycling in response to elevated CO₂ in an intact Mojave desert ecosystem.

Billings, Sharon¹, Schaeffer, Sean¹, Charlet, Terri², Zitzer, Stephen ³, Smith, Stanley², Evans, R. Dave¹, ^{1 2 3}

ABSTRACT- Elevated CO₂ may result in increased carbon (C) substrate in soils, altering ecosystem C and nitrogen (N) balances. In ecosystems where N is limiting, such as deserts, this could have large effects on ecosystem productivity. We measured the N isotope composition of vegetation and soil at the Nevada Desert Free-Air Carbon Enrichment Facility. This approach allowed us to determine if N cycling is altered in elevated CO₂. ¹⁵N values of soil N from the top 5 cm of elevated and ambient plots were not different (5.8±0.3 vs. 6.1±0.3‰). In contrast, ¹⁵N values from leaves of three perennial shrubs in elevated CO₂ were enriched in ¹⁵N compared to those in ambient plots (9.1±0.5 vs. 6.7±0.7‰ in Larrea tridentata). These data could be explained by several mechanisms. Plants in elevated CO₂ plots could be obtaining their N from deeper in the soil profile. Higher losses of gaseous N from elevated CO₂ soils also could be responsible for higher ¹⁵N values in treatment plots. Another important mechanism resulting in enriched pools of inorganic N in elevated plots could be increased microbial turnover. We suggest that although higher rates of N loss in elevated CO₂ plots may contribute in part to large differences in vegetation ¹⁵N, increased microbial activity with elevated CO₂ is an important cause of altered N dynamics that may ultimately affect desert productivity.

KEY WORDS: elevated CO₂ , ¹⁵N , nitrogen cycling, desert soils