To aim the problem of
nitrogen fertilizer overused to cause lower efficiency of fertilizer-N utilization and environment pollution, the field and lysimeter experiments under different amount of fertilizer-N applied were conducted in the
Taihu lake area to seek the
optimum nitrogen rate for a high productive rice-wheat system and reducing nitrate leaching. The preliminary results showed fertilizer-N rate used increased with yield of rice & wheat. Optimum rates of fertilizer-N
ranged from 225 to 270 and 180 to 225 N kg hm -2 for rice and
wheat in the two main paddy soils, respectively. With both high percentage of earbearing tiller and setting percentage, yields of rice & wheat were higher at optimum rate of fertilizer-N used than at other treatments. Nitrogen was leached away from field mainly in the form of NO- 3-N, not NH+ 4-N, and it occurred in the season of growing wheat and the early stage of rice pouding and transplanting. Nitrate-N concentrations in the
leachate increased gradually with the increasing of fertilizer N applications. Which in the lysimeter leachate ranged from 5.4 to 21.3 mg L -1 from the field receiving 225 kg hm -2, with 60% leachate being over the standard of 10 mg L -1 NO- 3-N standard and in groundwater ranged from 1.9 to 11.0 mg L -1 from the fields receiving 270 to 315 N kg hm -2,with 20% samples being nearly and 10% over the standard, respectively. Although NO- 3-N levels in groundwater from fertilized fields are usually well below the standard, cumulating of high NO- 3-N leachate will be a potential risk of water contamination in Taihu Lake area.
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