Free-air CO 2 enrichment effects on the energy balance and evapotranspiration of sorghum

Abstract

Increasing atmospheric carbon dioxide (CO 2) 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 C 4 grain crop) using a residual energy balance approach. During the summer and autumn of 1998 and 1999, sorghum was grown under free-air CO 2 enrichment (FACE) conditions near Maricopa, Arizona. Latent heat flux ( λET) was determined by subtracting soil heat flux ( G 0) and sensible heat flux ( H) from net radiation ( R n) values in both Control CO 2 plots (about 370 μmol mol −1) and FACE plots (Control + 200 μmol mol −1). R n was observed using net radiometers. G 0 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 CO 2 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.