FREE-AIR CARBON DIOXIDE ENRICHMENT (FACE) EFFECTS ON THE ENERGY BALANCE AND EVAPOTRANSPIRATION OF SORGHUM

Authors: TRIGGS, JONATHAN - UNIV OF ARIZONA; Kimball, Bruce; Pinter Jr, Paul; Wall, Gerard - Gary; Conley, Matthew; BROOKS, TALBOT - ARIZONA STATE UNIV; LAMORTE, ROBERT - USWCL, PHOENIX AZ; Adam, Neal; OTTMAN, MICHAEL - UNIV OF ARIZONA; MATTHIAS, ALLAN - UNIV OF ARIZONA; LEAVITT, S - UNIV OF ARIZONA; CERVENY, R - ARIZONA STATE UNIV

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2004
Publication Date: 6/1/2004
Citation: Triggs, J.M., Kimball, B.A., Pinter Jr, P.J., Wall, G.W., Conley, M.M., Brooks, T.J., Lamorte, R.L., Adam, N.R., Ottman, M.J., Matthias, A.D., Leavitt, S.W., Cerveny, R.S. 2004. Free-air carbon dioxide enrichment effects on the energy balance and evapotranspiration of sorghum. Agricultural and Forest Meteorology. 124:63-79

Interpretive Summary: Previous studies conducted mostly using chambers have found evidence that the increasing atmospheric CO2 concentration may change the amount of water used by plants. Such a change in plant water use could impact regional water supplies and require farm managers to modify their management practices. In order to determine the magnitude of such changes under field conditions, plots of sorghum were exposed to elevated CO2 concentrations using free-air CO2 enrichment (FACE) apparatus. Devoid of walls, the FACE approach is the most natural technique available to conduct such research. Data were collected for two growing seasons at ample water and limited levels of water supply. The FACE treatment increased daytime foliage temperatures about 1.7 degrees C (3.1 degrees F) at ample water, which suggests optimal regions for sorghum production could shift in the future due to the elevated CO2 alone, regardless of any climate change. Daily evapotranspiration was consistently lower in the amply irrigated FACE plots, by about 13%, whereas there was little consistent change in the limited-water plots. These results suggest that future water use requirements will decrease slightly provided that future changes in climate are not adverse.

Technical Abstract: The effects of atmospheric carbon dioxide (CO2) enrichment on the energy balance and evapotranspiration (ET) of sorghum were determined using a residual energy balance approach. During the summer and autumn of 1998 and 1999, sorghum (Sorghum bicolor (L.) Möench) was grown under Free-Air CO2 Enrichment (FACE) conditions near Maricopa, Arizona. Relative differences in latent heat flux (LET) were estimated by subtracting soil heat flux (Go) and sensible heat flux (H) from net radiation (Rn) values in both Control CO2 plots (about 370 umol mol-1) and FACE plots (Control + 200 umol mol-1). Rn was observed using net radiometers. Go was measured with soil heat flux plates at a depth of 10 mm, then corrected for heat storage above the plates. H was determined using measurements of air temperature from aspirated psychrometers, leaf temperature from infrared thermometers, and wind data from a three-cup anemometer. Both FACE and Control plots were divided into semicircular halves to allow a well-watered (Wet) treatment and a drought-stressed (Dry) treatment. This allowed comparisons of the FACE effect on ET in normal and water-stressed conditions. Under Wet conditions, FACE decreased LET by 13.8% in 1998 and 11.8% in 1999, with standard errors (SE) of 1.8% and 1.9%, respectively. Drought-stress resulted in a reduction in LET of 8.5% (SE 3.7%) for the FACE treatments in 1998, but an increase in LET of 10.5% (SE 5.1%) in 1999. These data suggest that soil water availability may be a determining factor for the FACE effect. When soil water was readily available, midday leaf temperatures in the FACE plots were increased by 1.47 oC (SE 0.09 oC) in 1998, and 1.85 oC (SE 0.20 oC) in 1999. Increased foliage temperatures are an indication of increased stomatal resistance due to CO2 enrichment. In the Dry treatments, soil water was depleted more quickly in the Control plots, resulting in a reduction in transpiration relative to the FACE plots. Decreased transpiration caused average midday foliage temperatures and, consequently, H to be higher in the Control-Dry treatments in 1999. This accounts for the relative increase in LET in 1999 FACE-Dry. In 1998, the Dry plots received more irrigation water than planned, resulting in slightly warmer midday leaf temperatures in FACE than Control.