Nitrogen and phosphorus loss and optimal drainage time of paddy field under controlled drainage condition

In order to reduce agricultural non-point source pollution and improve water use efficiency, the research on the change in ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N) and total phosphorus (TP) load in surface water at each growth stage was conducted by use of lysimeter based on experimental test in flooding paddy field at different leakage rates. Results showed that in the early period of paddy flooding, the nitrogen (N) and phosphorus (P) loads were at a relatively high level, but decreased gradually as the flooding lasted. Compared with the flooding on the first day, the loss of NH4+-N, NO3--N, and TP decreased by 58.90-97.75 %, 14.09-98.06 %, and 39.00-94.28 % respectively at the end of flooding. Different leakage (2 mm day(-1) and 4 mm day(-1)) had a certain influence on the change in NH4+-N, NO3--N and TP load, but the difference was not significant; therefore, drainage should be avoided in the early period of paddy flooding, and immediate drainage should be avoided after rainstorm and surface irrigation. A multi-objective controlled drainage model based on pollution reduction and water saving was established in order to obtain the optimal drainage time at each growth stage and to better guide the drainage practices of farmers. The optimal time for surface drainage were respectively the fourth, seventh, seventh, and fifth day at the stage of tillering, jointing-booting, heading-flowering, and milking after flooding. The innovative point of this study was to get rid of the concentration of N and P as an only visual angle, but taking the specific amount of N and P loss into consideration illustrated the controlled drainage pollution-reduction effect, taking the cumulative evapotranspiration as the index of water-use efficiency illustrated the water-saving effect, and considering together the two aspects determined the optimal drainage time.