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.