坡向和坡位对小流域梯田土壤有机碳、氮变化的影响

坡向和坡位是影响土壤有机碳（SOC）和全氮（TN）变化的两大重要地形因素。研究其对小流域梯田SOC和TN变化的影响，对预测黄土高原地区坡改梯条件下土壤碳汇变化具有重要意义。本文以黄土丘陵沟壑区高泉沟小流域为例，区分东、西两种坡向，再根据海拔区分上（2220～2326 m）、中（2130～2220 m）、下（1938～2130 m）三种坡位，共采集213个土壤样品，研究坡向、坡位对梯田耕层（0～20 cm）SOC和TN的影响。结果表明，西向坡SOC和TN含量分别比东向坡高22.8%和13.6%（p0.1），东西向坡的中、下坡位SOC和TN含量均大于上坡位。在西向坡，SOC含量下坡位（8.78 g/kg）最高，中坡位（7.82 g/kg）次之，上坡位（7.46 g/kg）最低；与上坡位相比，中坡位和下坡位的TN含量提高了12.1%，24.2%。在东向坡，SOC含量中坡位（7.15 g/kg）最高，下坡位（6.28 g/kg）次之，上坡位（5.37 g/kg）最低；与上坡位相比，中坡位和下坡位的TN含量提高了37.3%，29.4%。坡向与坡位的交互作用对流域SOC的空间分布影响显著（p0.1）。坡向、坡位对土壤碳氮比值（C/N）也有显著影响（p0.1）。这一结果对准确估算流域梯田SOC和TN的变化提供了参考。

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 (p<0.1). 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 (p>0.1) and increased by 24.2% in the lower slopes (p<0.1). 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 (p<0.1) and increased by 29.4% in the lower slopes (p<0.1). 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 (p<0.1). These results are useful for accurately estimating the changes of SOC and TN in terraces of the small watersheds.