Growth chambers and other enclosures used in plant physiology and growth studies tend to introduce chamber effects that alter the microclimate around the plants compared with the natural environment. A free-air (chamberless) carbon dioxide enrichment (FACE) system has been developed by Brookhaven National Laboratory (BNL) to provide controlled fumigation conditions while minimizing the potential to impose a discernible chamber effect. This system is capable of exposing large numbers of field-grown plants to elevated levels of atmospheric carbon dioxide (CO2) from seedling emergence until physiologic maturity. A FACE User Facility was established at the Maricopa Agricultural Center, University of Arizona, for continuous enrichment of CO2 at a set point of 550 mumol mol-1 during daylight hours throughout the cotton crop growing seasons of 1989-1991. The facility consisted of four circular BNL FACE arrays and associated equipment placed in a commercial cotton plantation. FACE array diameters of 23, 25, and 27 m were tested. The FACE facility included the ability to operate the experimental plots under two watering regimes using an automated, sub-surface irrigation system. CO2 was stored in a 48 000 kg receiver and vaporized with a heat exchanger that used water at ambient temperature as the energy source. The 1 min average CO2 concentration was held to within +/- 20% of the set point more than 98% of the time that the arrays were operating during all three seasons. In 1991, the long term average CO2 concentration measured at 63 points throughout the volume of a 20 m diameter experimental plot (ground to canopy top) centered within a 25 m diameter FACE array was 568 mumol mol-1. All of the FACE arrays operated for more than 99% of the planned experimental period in 1991. These 3 years of operation have demonstrated that the BNL FACE technology can be used as a basis for a large scale facility devoted to studying the fate of carbon in the terrestrial environment.
