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.