2.3. Soil carbon dioxide sampling and
measurement
The carbon dioxide concentration was measured with a classical gas well
method (Fig. 1). We selected 3 representative points for sampling in 5
plots, drilled holes with soil drills corer at a distance of 200, 140,
and 80, 20, 5cm underground. After that, we
buried
alternately gas collectors , which were inserted one end of a rigid
plastic tube with an inner diameter of 4 mm. the joint was sealed with
silicone, the other end was led to the ground with the rubber hose
inserted and a rubber stopper sealed. Finally, the soil was backfilled
uniformly.
The sampling was started after the backfilled soils tend to
coincide with the surrounding
soil. Soil CO2 samples were collected at the middle ten
days of each month from September 2012 to September 2013, 5 mL of gas
was taken from each collector, and the concentration of carbon dioxide
was determined by gas chromatography. Before each measurement, pure
CO2 gas was used as the standard curve.