Our field site (35 m × 30 m) had a slope with an average angle of 9° (range: 2°–19°) and was surrounded by other similarly sized terraced fields. The site covers an area of 721 m2 and its altitude ranges from 723 m to 730 m (Fig. 1). The annual temperature is 9.2 °C and the average annual precipitation reaches 500 mm with 70% occurring from July to September (Li et al., 2003 and Li et al., 2007). The soil type is Calcaric-Cambisols with an average texture involving 6% sand, 63% silt and 25% clay. Corn (Zea mays L.) and winter wheat (Triticum aestivum L.) are the major crops cultivated in the study area.