2.2 OTC system
Open-top chambers (OTCs) were established to simulate atmospheric CO2 concentration [CO2] enrichment from 2016-2019. Two treatments of elevated [CO2] were set up. One treatment was a stepwise increase (SI) of +40 µmol mol-1 per season, the other was a constant increase (CI) of +200 µmol mol-1 for all seasons. The target [CO2] in SI was CK+160µmol mol-1in 2019. Four OTCs was designated as replicates for each treatment and ambient [CO2] (CK).
The regular octagonal prismatic OTCs (height 3 m and have a bottom area ~10 m2) were lined with aluminum alloy frame and transparent glass, with the top opening slopes inclined inward by 45°. Pure CO2 was stored in several cylinders and sprayed from emission tubes surrounding the glass wall of OTCs. CO2 concentration in the OTCs were monitored by sensors (GMM 222, Vaisala Inc., Helsinki, Finland) handing in the center of chamber and regulated by computer program in real time. Temperature sensor was applied to detect and record temperature in OTCs. The photosynthetic photon flux density (PPFD) was monitored in real time by micro meteorological station (AWS800; Campbell Scientific, Inc., USA) installed in experiment field. Information on the CO2concentration increment, temperature and PPFD over the entire growth season was summarized in Table 1.