Design and performance of a new FACE (free air carbon dioxide enrichment) system for crop and short vegetation exposure

• A new FACE technology is presented that injects CO2 within the canopy.
• The system is low in CO2 consumption costs.
• System performance in terms of achieving desired [CO2] is very good.
• The system is modular and flexible to be adapted for various types of short vegetation.
• The system does not create major microclimatic artifacts.

Current atmospheric CO2 levels are unprecedented for the last 800,000 years and require assessments of their potential effects. Free Air Carbon dioxide Enrichment (FACE) is regarded as the most appropriate exposure system for experimental field investigations. While the early FACE technology involved fans for pre-dilution of CO2, now systems both with fans and without fans operating with pure CO2 are employed. The most often used construction in FACE technology involves peripheral injection from a circular or octagonal plenum with CO2 release pipes and jet holes surrounding the experimental plot which can create [CO2] gradients across the plot. Here, we present a new FACE design for short vegetation (crops and grassland) that does not involve peripheral injection but distributes CO2 directly within the plot canopy. The system involves numerous CO2 release holes in porous tubes that hang down from the upper canopy surface and stretch into the canopy. They can be adjusted in height according to crop growth. Plot size was 2 m × 2 m which allows for a high number of replicates at low CO2 consumption costs (6.5 tons CO2 in one season at five replicate plots = 3.5 kg CO2 per m2 usable plot area and day during 24hr operation). System performance in terms of CO2 control was very good both in time and space, with 93% of all instantaneous readings during daylight hours matching ±20% deviation from the set point, and low variation of [CO2] at a given moment within the exposed canopy. Some shading (c. 4% at full canopy development) resulted from the FACE structure at the plots but hardly any other microclimatic artifacts were observed. The FACE system is of modular construction and highly flexible and adaptable to various types of short vegetation.