Performance characteristics of an area distributed free air carbon dioxide enrichment (FACE) system

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Free air carbon dioxide enrichment (FACE) systems rely primarily on wind to distribute CO 2 across treatment plots. Most current FACE designs inject CO 2 at the perimeter of the plot, and low wind speeds can result in large horizontal gradients of CO 2 concentration. These gradients can be reduced by using blowers to inject CO 2 or a mixture of CO 2 and air into the plot, but this process can alter the microclimate enough to affect plant growth. This report describes a new FACE system in which CO 2 emitters are evenly distributed across a plot in a square array to provide an area-source of CO 2. A variable mixture of CO 2 and air is supplied continuously to each emitter at a constant low total volume flow. Tests in 13 m 2 plots of grassland and maize using set points of 1.4 times the ambient concentration showed good horizontal uniformity of CO 2 concentration except for a small radius around each emitter. Temporal variation in CO 2 concentration in the daytime at canopy height at the center of the plot was similar to that of other FACE designs, with 1 min average CO 2 concentrations within 10% of the target 86% of the time. At night, the CO 2 concentration achieved was more variable, but 1 min averages of CO 2 concentration were within 20% of the target 83% of the time. The frequency distribution of instantaneous measurements made with an open path CO 2 analyzer indicated a distribution strongly skewed toward low concentrations compared with the 1 min averages, which literature suggests may be a general characteristic of FACE systems. Canopy air temperatures on nights with low wind differed by less than 0.1 °C in plots with and without flow to the emitters, indicating minimal disturbance of the canopy microclimate. A square array FACE system could be scaled to any horizontal dimension without compromising the horizontal uniformity of CO 2 concentration, and could be useful in large-scale screening of germplasm for CO 2 responsiveness under field conditions.