Through-flow chamber CO2/H2O canopy gas exchange system—Construction, microclimate, errors, and measurements in a barley (Hordeum vulgare L.) field

An automated, portable, and non-steady-state through-flow canopy-chamber assembly for measurements of CO2 and water vapour fluxes on field crops and for simultaneous recording of environmental variables within the chambers and in the field is described. The system comprises of four units operating independently of each other. Each unit consists of (i) a central measurement device containing the CO2 and water vapour sensors, gas valves and pumps and a PC-controlled data acquisition system, (ii) an air conditioner for cooling and drying of incoming air, and (iii) the canopy chamber. Within each chamber, sensors for air, leaf, and soil temperature as well as for soil moisture are installed. All operations of the system are controlled by a PC. Chamber effects on the microclimate were carefully studied. Downwelling and upwelling total radiation, its balance, and incoming photosynthetically active radiation (PAR) inside the chamber were reduced by 2%, 3% and 10%, respectively. Diffuse fraction of PAR on clear days at noon was about 5% higher than in the open field, whereas it decreased by about 10% under cloudy conditions. Maximum over-temperatures of the air inside the chamber were less than 3 K. Based on an analysis of the dynamic properties of the CO2 and H2O sensors and of the chamber, a data analysis approach was developed that allows for bias compensation in CO2 and H2O gas exchange rates occurring under non-steady-state measurement conditions in the field. The system was used successfully over several years in studies on canopy gas exchange of barley (Hordeum vulgare L.) and oilseed rape (Brassica napus L.).