Nitrogen
leaching loss from double cropping rice
fields was observed using a
lysimeter system during 1991-1994.The main results are summarized as follows:1. The basic form of
Nitrogen leached from rice fields-was nitrate nitrogen (NO-3-N)rather than ammonium nitrogen (NH+4-N),so the N fertilizer applied in rice fields could result in the nitrate pollution to underground water due to N leaching.2.N application showed an obvious effect on NH4-N concentrations in the percolateing soil depth of at 10-16cm,and a rather high NH+4-N concentration occurred in urea plot compared with the NH4 HCO3 plot,suggesting the possibility of more N-leaching in NH4HCO3 plot than in urea plot.3. The NO3-3-N level in the percolating at water soil depth of 90-96cm was consistently higher in the NH4HCO3 plot than in the urea plot,which may be ascribed to the obvious difference of transformation dynamics between NH4HCO3 and urea.4.Under the conditions of an application rate of 300 kg urea-Nha-1 and a percolation of 540 mm,a NO3-3-N leaching of about 23 kg ha-1 and a NO-3-N+NH+4-N leaching load of nearly 27 kg ha-1 were estimated for double-rice cropping paddy.5. Based on the observed peak levels of 9 to 11 mg NO-3-N L-1 during a certain period of time and the limit of 10 mg NO-3-N L-1 in drinking water derived from underground water, we recommended a application limit of 150 kg Nha-1 per rice-cropping season.