PARENT SESSION
Poster Session #45: Elevated CO₂ I.
Wednesday, August 7. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Seasonal soil respiration and soil CO₂ under elevated CO₂ in the sorghum agroecosystem.

Cheng, L.^*,1, Leavitt, S.¹, Brooks, T.^1,2, Matthias, A.¹, Williams, D.¹, Thompson, T.¹, Kimball, B.², Pinter, P.², ¹ The University of Arizona, Tucson, AZ² Water Conservation Lab., USDA-ARS, Phoenix, AZ

ABSTRACT- Soil respiration directly transfers soil organic carbon (SOC) to atmosphere and may determine success of soil carbon sequestration schemes. However, the effects of elevated CO₂ on soil respiration are still poorly understood, especially in the open-field ecosystems. Our study investigated the variations of soil respiration under elevated CO₂ during 1998 and 1999 sorghum growing seasons at the Free-Air CO₂ Enrichment (FACE) experiment at the University of Arizona Maricopa Experimental Farm. Soil respiration was measured at the soil surface with a Licor-6200 and soil air was sampled at depths of 15 cm and 30cm at biweekly intervals. Two-year average soil CO₂ efflux from elevated CO₂ (ambient + 200 mol mol^-1) plots was 3.3 mol m^-2s^-1, about 13% greater than that from control (ambient CO₂ concentration ~360 mol mol^-1) plots, but the seasonal pattern in 1998 was different from 1999. Soil CO₂ efflux under elevated CO₂ in 1998 increased from 3% to 107% significantly throughout the growing season, whereas in 1999 large increases in soil CO₂ efflux occurred during later stages of growth, and before sorghum heading the CO₂-enriched plot had lower soil CO₂ efflux than the control. To further understand the belowground C cycling, isotopic carbon (¹³C) analysis of soil air collected from the soil profile was used to resolve root respiration and old SOC decomposition in the soil CO₂ efflux. Our results indicate that although elevated CO₂ increased soil CO₂ efflux significantly, about 55% of the increased CO₂ flux was derived from root respiration and only 45% came from old SOC decomposition. Compared with control over two growing seasons, elevated CO₂ increased root respiration by an average of 18%, but the effect on old SOC decomposition was not constant---stimulating SOC decomposition by 28% in 1998 and reducing it by 3% in 1999.

KEY WORDS: FACE, soil CO₂, carbon isotopes, sorghum