2.5. Soil organic carbon measurement
Soil at the depth of 0~5cm, 5~20 cm, 20~80 cm, 80~140cm, 140~200cm was sampled using a 5cm diameter soil corer with 3 subsamples in each plot, mixed, air-dried, grinded and sieved for soil organic carbon (SOC) measurement. The SOC was measured by the potassium dichromate external heating method (Springer & Klee, 2010).