With the nitrogen application rate of 180 kg/hm2

four nitrogen application ratios were designed in a field experiment

including base fertilizer:tillering fertilizer:panicle fertilizer=10:0:0 (T1)

4:3:3 (T2)

2:3:5 (T3)

0:3:7 (T4) respectively

to study the effect of postponing nitrogen application on rice yield formation and nitrogen dynamics in surface water of paddy field. The result showed that compared to T1

The treatments with 30% or even 50% of basal nitrogen fertilizer postponing to panicle fertilizer had no significant effect on rice grain yield

while the treatment with 70% of basal nitrogen fertilizer postponing to panicle fertilizer caused a significant decrease in rice grain yield. The peak concentrations of total nitrogen (TN) and total soluble nitrogen (DTN) in the surface water occurred after urea application for 1 day

NH4+-N reached the peak on 1 day after basal fertilizer and tillering fertilizer and 3 days after panicle fertilizer

and then these 3 forms of nitrogen gradually reduced to basically close to the treatment with no urea application. These days between basal and tillering fertilization

20 days after tillering fertilization and 9 days after panicle fertilization were the critical periods to control nitrogen loss from paddy field. After the urea application

DTN was the main part of the nitrogen in surface water of paddy field

and inorganic nitrogen (IN) was the major component in DTN

while NH4+-N accounted for more than 64.0% of IN. The concentrations of TN

DTN and NH4+-N in the critical periods for controling N loss during rice growth period were studied. Compared with T1

the concentrations of these 3 forms of nitrogen were decreased by 2.9%

1.6%

3.1% respectively in T2

decreased by 15.5%

14.7%

22.3% respectively in T3

and decreased by 16.1%

22.9%

34.1% respectively in T4. Considering the rice grain yield

T3 was determined to be the effective measure

as it could ensure rice grain yield and meanwhile effectively reduce the risk of nitrogen loss from paddy field.