Li Cheng
Ph.D. Completed May 2005

Contact Information:
Dr. Li Cheng

NSF AMS Accelerator Facility
Dept. of Physics
PAS Bldg.
University of Arizona
Tucson, AZ 85721

NSF AMS Accelerator Facility
Lab. of Tree-Ring Research
105 W. Stadium, Bldg. #58
University of Arizona
Tucson, AZ 85721
E-mail: lcheng@ltrr.arizona.edu

Ph.D project: Elevated Atmospheric CO₂ Impacts Carbon Dynamics in a C₄-Sorghum-Soil Agroecosystem- An Application of Stable Carbon Isotopes (õ¹³C) in Tracing the Fate of Carbon in the Atmosphere-Plant-Soil Ecosystem

Arizona Maricopa Free-Air Carbon Dioxide Enrichment (FACE) Experiment

Li Cheng

(Poster presented at Ecological Society of America Annual Meeting, Tucson, Arizona, 4-9 August, 2002 )

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 ( d¹³C) 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.

Free Air-CO₂ Enrichment (FACE) of C₄-Sorghum: Biochemical Composition and Decomposition of Sorghum Tissues Grown Under Elevated CO₂

Li Cheng

(Poster presented at ASA-CSSA-SSSA Annual Meeting, Denver, Colorado, 2-6 November, 2003

Abstract

Biochemical composition of sorghum tissue grown under elevated CO₂ was determined in Free-Air CO₂ Enrichment (FACE) experiment during the 1998-1999 growing seasons at Maricopa, Arizona, USA. Sorghum samples were collected from FACE (560 ppmv CO₂ ) wet (well-watered) and dry (water-limited), and Control (360 ppmv CO₂) wet and dry plots at the end of 1999 growth season. Elevated CO₂ increased cellulose and lignin by 40% and 5%, respectively, while total non-structural carbohydrates and amino acids were reduced by 8% and 7%, respectively. Phenolics were significantly higher in FACE roots, but not significant different in FACE leaves and stems. Water stress modified the CO₂ effects, especially for uronic acids that were increased in FACE-wet by 7% and reduced in FACE-dry sorghum tissues by 3%. The C/N ratio in sorghum tissues was not affected by CO₂, but was substantially lower under water stress. Laboratory incubation of soil amended with sorghum tissues showed the decomposition rate of FACE sorghum tissues was on average 14% lower than control sorghum tissues after 30 day incubation; consequently, 23% more new organic carbon was left in the incubated soil amended with FACE sorghum tissues based on isotope mass balance. The results confirm the slower decomposition of FACE plant tissue reported in literature apparently due to biochemical changes in the plant tissue.