NO3
− predominated over NH4
+ in the runoff during wheat seasons
(Fig. 6b; Table 4). One important reason might be that a great
amount of NO3
− from N fertilizer accumulated in the cultivated
layer (0–15 cm) could be transported horizontally into the 15 cm
drainage ditches and flow out of the field on rainy days (Fig. 2). Chen
et al. (2003) investigated the characteristics of nitrate horizontal
transport in paddy soil, and found that the horizontal transport of
NO3
− was strongly correlated with the soil moisture content. Using
the lysimeters (Fig. 3c), we were able to separate N runoff on the
soil surface from that at 15 cm soil depth during two consecutive
wheat growing seasons. The results interestingly revealed that on
average, 84% of N runoff occurred at the 15 cm depth, with only 16%
on the surface. This is because the horizontal transport of water
in the top 15 cm soil layer preceded the surface flow during the
gradual saturation of soil by rainwater. Obviously, the use of conventional
drainage ditches greatly increased the risk of runoff loss
for accumulated N in the cultivated layer, especially NO3
−, even
though this practice was certainly effective in preventing waterlogging
injury to wheat plants after frequent rain events. Further
investigations are needed on this phenomenon, since it has not
been mentioned in most previous studies. The quantities of accumulative
N runoff (33.4–58.7 kg N ha−1; Table 4) from the three
wheat seasons were far greater than those from the corresponding
rice seasons (2.65–21.8 kg N ha−1), even though N applied in
wheat season was 100 kg N ha−1 less than that in rice season. The
data were also higher than the field measurements reported by Guo
et al. (14.7 kg N ha−1; 2004b) and Tian et al. (5.2–38.6 kg N ha−1;
2007) in the same region. These results suggest that runoff driven
by precipitation and drainage ditches is a very important pathway
for N export from paddy soil during wheat season.