The percolated NO3
− was minimal during the three rice seasons,
except in the short stage of wheat-rice alternation (Fig. 4a
and Table 2); at that time, it might actually have originated from
remnant NO3
− from the previous wheat season (Zhao et al., 2011).
The low NO3
− concentration in percolation water was due to that
flooding conditions during the rice seasons created an anaerobic
soil environment for denitrification. The fact of excessive accumulation
of NH4
+ in percolated water indicated that after quick
hydrolysis of applied urea with oxygen-limited nitrification under
flooding conditions, NH4
+ could still flow downward with infiltrated
water (Fig. 4a).