Free-air carbon dioxide enrichment (FACE): effects on sorghum evapotranspiration under well-watered and water stressed irrigation treatment

Sorghum evapotranspiration (ET) was evaluated for well-watered (Wet) and water-stressed (Dry) irrigation treatments at ambient (370 µmol mol-1) and enriched (550 µmol mol-1) CO2 concentrations during the summer growing seasons of 1998 and 1999. In order to examine sorghum growth in actual field conditions, the Free-Air Carbon dioxide Enrichment (FACE) apparatus was employed. FACE treatments were replicated four times, each replication was conducted inside a 25 meter diameter ring. Each ring perimeter consisted of a 4.7 centimeter diameter PVC pipe with vertical 2.5 meter CO2 vent pipes spaced at intervals of 2 meters. Air enriched with CO2 was blown into the rings, exiting through tri-directional jets along vertical pipes spaced at elevations across the vertical crop canopy profile. Wind direction, wind speed, and CO2 concentration were measured at the center of each ring. These data were used in a fibre-driven computer controlled system delivering a 3-dimensional plume of CO2-enriched air which permeated the above ground vegetative area. The system maintained CO2 concentrations at 200 µmol mol-1 above ambient, 24 hours a day throughout the two growing seasons. The Four Control rings were identical in design and air flow to the four FACE rings, however, Control ring blowers emitted ambient air only. Level basin (flood) irrigation was utilized to create two irrigation regimes. Wet treatments were supplied with ample water based on predicted (AZSCHED computer model), and measured (volumetric soil moisture), soil water depletion. Dry treatments were irrigated twice, first after planting and second before flag leaf emergence. Therefore, there were two levels of CO2 and two levels of soil water supply, each replicated four times, creating sixteen semi-circular plots. Soil water was measured prior to and after each irrigation through neutron moderation. Resulting data were used for calculation of ET based on the soil water balance method. The average cumulative ET for sorghum in 1998 and 1999 results: 1) FACE-Wet plots showed an 18% reduction in ET when compared to Control-Wet plots; 2) FACE-Dry plots exhibited only 1% less ET than Control-Dry plots. Average treatment ET by year is as follows: 1) 1998, FACE-Wet 355 mm, Control-Wet 444 mm, difference of 20%; FACE-Dry 242 mm, Control-Dry 240 mm, difference of -.6%; 2) 1999, FACE-Wet 661 mm, Control-Wet 784 mm difference 16%; FACE-Dry 382 mm, Control-Dry 392 mm, difference of 2%. Under well watered conditions, CO2 enrichment caused partial stomatal closure thereby reducing leaf conductance and ET. However, sorghum grown in dry treatments experienced leaf bundle cell leakyness, where CO2 leaked out of the Kranz structure. This impaired carbon fixation and mitigated the differences between FACE and Control plot conductance and subsuquently ET. Data suggests that irrigation requirements for sorghum may be less in the future high-CO2 world.