CO2 exchange between the biosphere and the atmosphere is one of the most important components of the global carbon cycle. The CO2 exchange should be monitored continuously and at multiple geographical points because of great temporal and spatial variations. A Multi-channel automated chamber system was developed for continually monitoring CO2 exchange between the agro-ecosystem or the soil and the atmosphere. This system consisted of an automated chamber subsystem and a CO2 concentration analysis and data logging subsystem. Both subsystems were under the control of a programmable logic controller (PLC). The automated chamber subsystem contained 18 chambers and a compressor. The chambers, 50 cm×50 cm×50 cm, were constructed of clear PVC fixed to an aluminum alloy frame. The chambers had PVC lids hinged at the sidewall and each lid was closed and opened automatically by the push and pull of a pneumatic cylinder mounted on the opposite sidewall. The pneumatic cylinders were controlled by high pressure air from a compressor regulated by the PLC. Fans were fixed on each pneumatic cylinder to mix the air inside the chamber completely when their lids were closed. A buffer pine (L=1.5m) with an inner diameter of 0.4 mm was inserted through the lid to keep the air pressure balanced between the inside and the outside of the chamber. During measurement, one of the 18 chambers was closed for measuring and the others were kept open to allow precipitation and leaf litter to reach the enclosure surface, to maintain the soil conditions as natural as possible. Three minutes of closure time was needed for each chamber for measurement at separate locations. Regulated by the PLC, measurements for the 18 chambers were completed in 54 minutes, and another cycle of measurement began after a six-minute interval. The CO2 concentration analysis and data logging subsystem was composed of a CO2 analyzer, a multi-channel gas valve and a data logger. The multi-channel gas valve was controlled by the PLC to switch gas between the chambers and the CO2 analyzer. During the analysis, one chamber was closed and the air inside it was continuously withdrawn by a pump through a multi-channel gas valve into the CO2 analyzer. After the CO2 concentration was measured, the air was returned into the chamber through another multi-channel valve to minimize changes of the air within chamber. The results of the CO2 concentration were recorded by the data logger at intervals of 10 seconds. In addition, environmental variables were simultaneously measured by sensors and these results were recorded by the data logger. The CO2 exchange was calculated as the slope of change in CO2 concentration within chamber, adjusted for air temperature and pressure.
The reliability of the multi-channel automated chamber system was tested; the system was used to monitor the CO2 exchange between a wheat ecosystem and the soil respiration of a wheat field and an apple orchard with the atmosphere. The results showed that the equilibrium of the system could be reached within 60 seconds and the turbulence of the fans had no significant effect on this CO2 exchange. The changes in air and soil temperature and soil moisture inside the chambers due to enclosure of the chambers were within the degree of acceptability for field study. The net ecosystem CO2 exchange for the wheat ecosystem was -2.35 μmol•m-2•s-1and the soil respiration was 3.87 μmol•m-2•s-1in the wheat field and 6.61 μmol•m-2•s-1in the apple orchard. In conclusion, the system was reliable for monitoring CO2 exchange continuously and automatically at multiple points, and had little influence on natural conditions.