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.