2.2 Experimental design
This experiment used a split-plot experimental design with four
replications. The main treatment factor was [CO2]
and subplot treatment factor was N application rate. There were two
[CO2] levels: ambient CO2 (400 μmol
mol-1) and eCO2 (600 μmol
mol-1). Since we have two independent growth chambers,
we set one chamber for ambient CO2 and the other for
eCO2 treatments, and repeated the whole experiment four
times. Each time was considered one replication. The
eCO2 concentration was achieved by pumping in industrial
grade pure CO2 to a partial pressure of 600 μmol
mol-1 (actual records range 595-640 μmol
mol-1), and the ambient condition had a
CO2 partial pressure range of 405 to 430 μmol
mol-1 during the growth period. As we focused on the
adaptation of rice plants to eCO2 at the early seedling
stage, we initiated eCO2 treatment from seeding. N
application rates included four levels: 0 (N0), 5 (N5), 10 (N10) and 15
(N15) kg N 666.7 m-2, which was comparable to the
general range of N fertilization rate in the region at this growth
phase. Half of the N fertilizer (in the form of urea, 46% N) was
premixed into the soil three days prior to seeding, and the remaining
portion was top-dressed in a water-solution form at true leaf-age 1, 2,
3 and 4, respectively, each at a 12.5% of the total N application rate.
Together, there were eight treatments, combined from two levels of
[CO2] and four levels of N application rate, and
each treatment had two plastic trays comprising 192 plants.