Abstract: With the sandy farmland in the marginal oasis in middle reaches of Heihe River Basin, Northwest China as test object, this paper studied soil NO3--N accumulation and leaching under effects of different farmland use type and winter irrigation. The results showed that the mean NO3--N concentration in 0〖KG-*2〗-〖KG-*7〗300 cm soil profile in different farmlands ranged from 127 mg·kg-1 to 8360 mg·kg-1. Soil NO3--N concentration was higher in 0〖KG-*2〗-〖KG-*7〗40 cm and 135〖KG-*2〗-〖KG-*7〗300 cm layers, but lower in 40〖KG-*2〗-〖KG-*7〗135 cm layer. Greenhouse vegetable field had a significantly higher soil NO3--N concentration than the other farmland use types. The accumulated amount of soil NO3--N decreased in the order of greenhouse vegetable field>tomato field>cotton field>seed maize field>maize-wheat rotation field>maize-wheat stripe intercropping field>alfalfa field>jujube plantation. The NO3--N accumulation in 0〖KG-*2〗-〖KG-*7〗300 cm soil profile in greenhouse vegetable filed reached 217145 kg·hm-2, which would be a serious menace to groundwater quality, followed by tomato field and cotton field. Lesser accumulation of soil NO3--N was found in seed maize field, maize-wheat intercropping field, maize-wheat rotation field, alfalfa field, and jujube plantation, but its pollution potential would not be neglected. After winter irrigation, soil NO3--N concentration decreased in 0〖KG-*2〗-〖KG-*7〗80 cm layer but increased in 80〖KG-*2〗-〖KG-*7〗300 cm layer, indicating that winter irrigation caused NO3--N leaching into deeper soil depth. The leached amount of soil NO3--N to deeper layers increased with increasing amount of winter irrigation. To mitigate soil NO3--N leaching and groundwater contamination, a comprehensive consideration should be made on the rational arrangement of farmland use type, proper decrease of planting N-accumulated crops, and reasonable winter irrigation.

Key words: ecological river, vegetation, eco-hydraulics, resistance coefficient, numerical simulation.