Effects of slope aspect and position on soil organic carbon and nitrogen of terraces in small Watershed

Slope aspect and position are two important factors which affect the changes of soil organic carbon (SOC) and total nitrogen (TN). It is important for studying the influences of two terrain factors on the SOC and TN and forecasting the changes of carbon sink in watersheds of the Loess Plateau. Based on two slope aspects (west-facing slope and east-facing slope) and three slope positions (upper slope, mid slope and lower slope), 213 soil surface samples were collected to investigate effect of slope aspects and positions on SOC and TN (0～20 cm) in terraces at a typical watershed of Gaoquangou in hilly regions of the Loess Plateau. SOC and TN in the 0～20 cm soil layer on the west-facing slopes are improved by 22.8% and 13.6% compared with those of the east-facing slopes (p0.1). In the same slope aspect or slope positions, SOC and TN in the 0～20 cm soil layer in the mid slopes and lower slopes are higher than those of the upper slopes. In the west-facing slopes, the contents of SOC in the lower slopes (8.78 g/kg) are the highest, followed by the mid slopes (7.82 g/kg), and the contents of SOC in the upper slopes (7.46 g/kg) are the lowest. Compared with the upper slopes, the contents of TN are increased by 12.1% in the mid slopes (p0.1) and increased by 24.2% in the lower slopes (p0.1). In the east-facing slopes, the contents of SOC in the mid slopes (7.15 g/kg) are the highest, followed by the lower slopes (6.28 g/kg), and the contents of SOC in the upper slopes (5.37 g/kg) are the lowest. Compared with the upper slopes, the contents of TN are increased by 37.3% in the mid slopes (p0.1) and increased by 29.4% in the lower slopes (p0.1). The interaction of slope aspect and position affects spatial distribution of SOC in the watershed significantly (p 0.1). Slope aspect and position have significant influences on soil C/N ratio (p0.1). These results are useful for accurately estimating the changes of SOC and TN in terraces of the small watersheds.