Rainfall was the significant factor in the leachate amount during
wheat season. The precipitation amount, intensity, and frequency
(Fig. 5) probably had important effects on the N concentration in
percolation water (Fig. 4b) and might have caused great variations
(0.53–10.4 kg N ha−1; Table 2) in the amounts of N leaching. Despite
great variations in the amount of N leaching, the significant downward
migration of NO3
− in the soil profile during wheat seasons
suggests a risk of groundwater pollution in paddy soil. An investigation
carried out in the 1990s in the TLR found that the concentration
of NO3
− averaged 7.89 mg N L−1 among 40 well water samples,
and revealed that NO3
− in well water in the TLR was commonly
associated with N leaching from farmland (Xing et al., 2001).