The Australian grains free-air carbon dioxide enrichment (AFACE) system is located 7 km west of Horsham, Victoria, Australia (36°45'07"S, 142°06'52"E, 127 m elevation) on a 7.5 ha site, where eight 12-m diameter FACE rings (elevated CO2) and eight experimental areas with atmospheric CO2 (controls) were established in June 2007. Subsequently, these rings were increased to 16-m diameter in 2009 which enabled researchers to accommodate twice as many experimental plots for a 33% increase in gas use compared with 12-m rings. Rings are spaced 78 m centre to centre, providing a clearance of 62 m between the edges of adjacent rings (Fig. 1). This facility has provision for the storage and reticulation of bulk CO2 to supply the AGFACE rings as well as electricity and water to service each part of the experiment.
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Carbon dioxide is only enriched from the time of solar rise to solar set when the top of the solar disc is at the horizon. Enrichment occurs from crop emergence until physiological maturity. Crop emergence is when the first true leaves emerge from the soil and physiological maturity is when the grain moisture content declines below 40%.
The main components (Fig. 1) of the direct injection AGFACE design are a bulk supply of CO2 gas to the field, power supply to each ring, a central control system for each ring and eight 16 m rings, each consisting of eight individually controlled stainless steel fumigation tubes arranged in an octagon.
The major components of central control box are:
a) a manual cut-off valve at the bottom the control box allowing the gas supply to each ring to be turned on or off independently,
b) next in the gas line is a filter ensuring that any moisture or particulate matter is trapped and will not foul the operation of subsequent soft start valve and proportional valves (electro-pneumatic regulators),
c) a manual pressure regulator is next in the gas line, this helps to stabilise the subsequent manifold pressure irrespective of the supply line pressure and enables a different pressure to be used at the manifold than the supply line,
d) from the manual pressure regulator, the gas goes through an electrically controlled cut-off valve / soft start valve. This valve enables the gas flow to be turned on and off by the microcontroller / microprocessor. It also ensures that when the gas flow is turned on, the pressure rise is dampened slightly, thus giving some protection to the following electro-pneumatic regulators, e) the gas is then flows from the ‘soft start valve’ to an eight port manifold. Eight electro-pneumatic regulators, one for each side of the octagonal ring, are connected to the manifold, f) next are ‘electro-pneumatic regulators’. These regulators control the pressure and therefore the gas flow rate through the fumigation tubes in proportion to an input current signal (4-20 mA) to the regulators, and g) microcontroller and microprocessor.
Figure 1. Major components of the AGFACE system
