1. We describe a field facility that controls CO2 along continuous gradients from superambient to subambient concentrations. The facility consists of two transparent, tunnel-shaped chambers, each 1-m wide and tall and 60-m long. Pure CO2 is injected into one chamber during daylight to initiate a superambient CO2 gradient (550–350 µmol mol−1). Ambient air is introduced to the second chamber to initiate a subambient CO2 gradient (350–200 µmol mol−1). CO2 concentrations at night are regulated at 150 µmol mol−1 above daytime values along each gradient. The CO2 gradients are maintained by varying the rate and direction (day/night) of air flow.
2. Air temperature and vapour pressure deficit are regulated near ambient values by cooling and dehumidifying air at 5-m intervals along chambers.
3. Desired CO2 gradients were regulated on grassland for virtually the entire 9-month growing season in 1998, including a 6-month drought. Consistent CO2 concentrations were maintained along gradients despite seasonal variation in species composition, leaf area, and temperature.
4. Daytime temperatures in chambers tracked the seasonal pattern in 1998. The polyethylene covering on chambers transmitted 90% of incident light, but usually increased the ratio of diffuse to direct light.
5. By enabling the study of trends in plant and ecosystem responses to CO2 over both subambient and superambient concentrations, elongated chambers fill a void in CO2 research facilities.