Measurement of ecosystem CO2 exchange
We measured net ecosystem CO2 exchange and ecosystem
respiration monthly during the growing season (June to October) in 2020.
To do so, we used a Li-6400 portable photosynthetic (Li-COR, USA)
instrument with the static chamber method. We collected measurements
between 8:00 a.m. to 12:00 p.m (Niu et al. 2008; Wu et al.2021). For measurements, we choose a clear, cloudless and windless day
as much as possible, at least three days after a rainfall. For
measurements we connected a leaf chamber (50 × 50 ×
50cm3 transparent plexiglass box) to the portable
photosynthetic instrument and installed small fan in each diagonal
direction at the upper end of the glass box so that gas was fully mixed.
We place the glass box on one of three aluminum sink frames (50 ×
50cm2) placed randomly within each plot to ensure a
smooth and airtight seal. We repeated measurements on each of the three
frames.
At each sample point, we collected measurements for 120 s and
CO2 concentration and water exchange flux values were
automatically recorded every 10 s. After these measurements, we
ventilated the leaf chamber to ensure it was filled with
convection-exchanged air, covered it with a black cloth to ensure no
light transmission and repeated the above procedure to determine
ecosystem respiration.
We measured soil respiration
using an open circuit Li-8100 soil carbon flux meter (Li-COR, Inc,
Lmcoln, NE, USA) at the same time as the net ecosystem exchange
measurements. We measured soil respiration within three PVC rings (10.5
cm in diameter and 8 cm in height) that were randomly placed 2 cm above
the ground surface in each plot. Prior to measurements, we clipped
plants inside the rings flush with the ground and removed debris.
We calculated net ecosystem CO2 exchange (NEE) and gross
ecosystem productivity (GEP) as follows: ∂C′ ∂t = INDEX(LINEST(Y1: Y12,
A1: A12),1);\(NEE=\frac{10VP(1-\frac{W}{1000})}{RS(T+273.15)}\frac{\partial C\prime}{\partial t}\);
NEE = GEP - ER.
Where ER is the measured ecosystem respiration. NEE, ER, GEP are in
μmol·m-2·s-1, Y1-Y12 is the
CO2 concentration value, A1-A1 is the measurement time,
V represents the volume of the box (cm3), P is the
atmospheric pressure inside the box (kpa), W is the water pressure
inside the chamber (mmol·mol-1), S is the bottom area
of the chamber (cm2), T is the temperature of the gas
inside the chamber (℃), and R=8.314 J-mol-1-K-1 (constant). We used
values of ecosystem CO2 exchange and soil respiration
during the growing season (June-October) to calculated the values of
NEE, GEP, ER, SR for each treatment.