[Nitrogen balance and its effects on nitrate-N concentration of groundwater in three intensive cropping systems of North China].

Selecting three main intensive cropping systems of North China, i.e., wheat-maize rotation, plastic greenhouse vegetable, and apple orchard as test objectives, this paper studied their nitrogen (N) budget, soil nitrate-N accumulation, and year-round dynamics of groundwater nitrate-N concentration. The results showed that in plastic greenhouse vegetable cropping system, the annual N input from chemical fertilizers, manure, and irrigation was 1358, 1881 and 402 kg x hm(-2), being 2.5, 37.5 and 83.8 folds of the corresponding items in wheat-maize cropping system, and 2.1, 10.4 and 68.2 folds in orchard, respectively, and its total N input amounted to 3656 kg x hm(-2), being 5.8 times of the wheat-maize cropping system, and 4.2 times of the orchard. The wet deposition N in the three cropping systems ranged from 14.2 kg x hm(-2) to 18.9 kg x hm(-2). The N output by wheat-maize, greenhouse vegetable and orchard was 280,329 and 121 kg x hm(-2), the N surplus was 349, 3327 and 746 kg x hm(-2), and the remained nitrate-N after harvest amounted to 221-275, 1173 and 613 kg x hm(-2) in 0-90 cm soil layer, and 213-242, 1032 and 976 kg x hm(-2) in 90-180 cm soil layer, respectively. Crop field had a comparatively even distribution of nitrate N in its 0-180 cm soil profile, and a sharp increase of nitrate N throughout the soil profile were found in both greenhouse vegetable and orchard fields. There was an evident nitrate leaching in all three cropping systems. The groundwater in shallow well (< 15 m) was severely contaminated in greenhouse vegetable area, with the nitrate-N concentration in 99% of the samples exceeding the maximum permissible limit for drinking water (10 mg x L(-1)), while 5% of the samples in deep well in vegetable area and in shallow well in orchard and 1% of the samples in deep well in wheat-maize field were exceeded the limit. The nitrate-N concentration exponentially decreased with well depth (m) in greenhouse vegetable area.