PARENT SESSION
Oral Session #42: Carbon and Nitrogen Cycling. Presiding: S. Hobbie.
Wednesday, August 8, 2001. 8:00 AM to 11:30 AM. Hall of Ideas E.

Carbon and nitrogen limitations on nitrogen cycling in an arid Colorado Plateau ecosystem.

SCHAEFFER, SEAN¹, EVANS, DAVE¹, ¹

ABSTRACT- Nitrogen (N) is thought to be the second most limiting resource, next to water, controlling net primary production in arid and semi-arid ecosystems. Nitrogen cycling by soil microbes may be limited by available carbon (C). We investigated the effects of C and N additions on ammonia (NH₃) volatilization, ammonification, nitrification, and microbial activity in a Colorado Plateau Desert ecosystem. Additions were made in a full factorial design at approximately four times ambient levels of soil C and N at three spatial locations: under the canopies of a C3 shrub, C4 shrub, and in the interspace. After 24 hours, NH₃ volatilization in the +N treatment (325.5 ±74.7 g N-NH₃ m^-2 h^-1) was higher relative to the control (21.9 ±10.2 g N-NH₃ m^-2 h^-1). Conversely, volatilization in the +C+N and +C treatments was not different from the control (P>0.1). Ammonium (NH₄) levels were 50% lower in C additions compared to controls. Soil CO₂ fluxes were 300% greater in C additions compared to controls. Decreases in NH₃ volatilization, NH₄, and increases in soil CO₂ flux with added C suggest an increase in microbial immobilization of inorganic N, indicating a potential C limitation for microbial activity. By characterizing the response of these ecosystem processes to potential changes in C and N availability, we gain an improved understanding of the function of arid ecosystems and how they may respond to perturbations.

KEY WORDS: nutrient cycling, nitrogen, Colorado Plateau, carbon