Characteristics of nitrate leaching from hilly cropland of purple soil
紫色土坡耕地硝酸盐淋失特征

Nitrate leaching from hilly cropland of purple soil was studied by monitoring and analyses of nitrate accumulation in soil profiles. Experiments were conducted from 2003 to 2005 in artificial small catchments with subsurface flow monitoring. Results showed that nitrate accumulated in dry seasons and diminished in rainy seasons on the hilly cropland. Subsurface flow is the main pathway of nitrate leaching in the rainy season. Nitrate leaching through subsurface flow was influenced by a shallow soil layer (60 cm), low infiltration parent rock beneath the soil, and heavy rainfall in summer. Average annual discharge of subsurface flow from 2003 to 2005 was 169.7 mm, which amounted to 52.42% of total runoff in the rainy season. The content of NO-3-N in subsurface flow was persistently high with an average of 14.92 mg·L-1 between 2003 and 2005. Annual average loss of NO-3-N through subsurface flow was 27.98 kg·hm-2, and accounted for 10.0% of total fertilizer nitrogen applied within a year. NO-3-N loss rates through subsurface flow showed seasonal patterns. It peaked during maize elongation and stamen stages, which accounted for 69% of the annual NO-3-N loss flux. Nitrate loss through subsurface flow results in in situ groundwater pollution and downstream pollution. Pollution by long-distance travel of soluble nitrate could become a major threat to the water quality in the region of the Three Gorges Reservoir.