| nitrogen balance and groundwater nitrate contamination: comparison among three intensive cropping systems on the north china plain. | the annual nitrogen (n) budget and groundwater nitrate-n concentrations were studied in the field in three major intensive cropping systems in shandong province, north china. in the greenhouse vegetable systems the annual n inputs from fertilizers, manures and irrigation water were 1358, 1881 and 402 kg n ha(-1) on average, representing 2.5, 37.5 and 83.8 times the corresponding values in wheat (triticum aestivum l.)-maize (zea mays l.) rotations and 2.1, 10.4 and 68.2 times the values in apple ... | 2006 | 16364521 |