FACE FACILITY CO2 CONCENTRATION CONTROL AND CO2 USE IN 1990 AND 1991

CO2 treatment level control and CO2 use are reported for free-air carbon dioxide enrichment (FACE) facility operations at the University of Arizona's Maricopa Agricultural Center in 1990 and 1991. These are required for evaluation of the validity of biological experiments conducted in four replicates of paired experimental and control plots in a large cotton field and the cost-effectiveness of the plant fumigation facility. Gas concentration was controlled to 550 mumol mol-1 at the center of each experimental plot, just above the canopy. In both years, season-long (April-September) average CO2 levels during treatment hours (05:00-19:00 h Mountain Standard Time) were 550 mumol mol-1 measured at treatment plot centers when the facility was operating. Including downtime, the season average was 548 mumol mol-1 in 1991. In 1990, the season averages for the four elevated CO2 treatments varied from 522 to 544 mumol mol-1, owing to extended periods of downtime after lightning damage. Ambient CO2 concentration during treatment was 370 mumol mol-1. Instantaneous measurements of CO2 concentration were within 10% of the target concentration of 550 mumol mol-1 more than 65% of the time when the facility was operating, and 1 min averages were within 10% of the target concentration for 90% of the time. The long-term average of CO2 concentration measured over the 20 m diameter experimental area of one array at the height of the canopy was in the range 550-580 mumol mol-1 during July 1991, with the higher values near the edges. In 1991, CO2 demand averaged 1250 kg per array per 14 h treatment day, or 4 kg m-2 of fumigated plant canopy. The FACE facility provided good temporal and spatial control of CO2 concentration and was a cost-effective method for large-scale field evaluations of the biological effects of CO2.