Free-air CO₂ enrichment effects on the energy balance and evapotranspiration of sorghum

Increasing atmospheric carbon dioxide (CO₂) likely will affect future water requirements of most plants, including agricultural crops. This research quantifies such effects on the energy balance and evapotranspiration (ET) of sorghum (Sorghum bicolor (L.) Möench, a C₄ grain crop) using a residual energy balance approach. During the summer and autumn of 1998 and 1999, sorghum was grown under free-air CO₂ enrichment (FACE) conditions near Maricopa, Arizona. Latent heat flux (λET) was determined by subtracting soil heat flux (G₀) and sensible heat flux (H) from net radiation (R_n) values in both Control CO₂ plots (about 370 μmol mol^-1) and FACE plots (Control + 200 μmol mol ^-1). R_n was observed using net radiometers. G₀ 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 λET by 13.8±1.8% in 1998, and 11.8±1.9% in 1999. Drought-stress resulted in a reduction in λET of 8.5±3.7% for the FACE treatments in 1998, but an increase in λET of 10.5±5.1% in 1999. When soil water was readily available, midday canopy temperatures in the FACE plots were increased by 1.47±0.09°C in 1998, and 1.85±0.20°C in 1999, indicative of increased stomatal resistance due to CO₂ enrichment. These data suggest that soil water availability is a determining factor for the FACE effect. Water use efficiency (WUE) increased about 28% due to elevated CO ₂ under Wet conditions due to a savings of water for about the same growth, whereas under Dry conditions it increased about 16% due to much greater relative growth on only a slightly higher amount of water.