Abstract:A field experiment was conducted to study dynamic changes of NH4+-N, TN, TP and DIP concentrations in surface water of paddy fields applied with different combination of N and P fertilizers. Results showed that TN, TP and DIP reached their respective peak values in concentration quickly after urea and superphosphate applied, and then soon decreased which fits an exponential equation, (Y=C0×e-kt), while NH4+-N peaked 2-4 days after urea applied, and decreased soon afterwards, and then leveled off after another 6-7 days. NH4+-N concentration rose much slower after basal dressing than after side-dressings at the tillering and milking stages, and the decrease of NH4+-N and TN concentrations was lower, too. With the same level of N fertilization, NH4+-N and TN concentrations in the surface water were higher in Treatment HP than Treatment OPT, while with the same level of P fertilization, TP and DIP concentrations were higher in Treatments HN and LN than in Treatment OPT, indicating that either N or P being too high or too low in application rate may accelerate loss of the other one. TN, TP and DIP could be cited as key parameters for N and P runoff loss within 3 days after urea application. A week after fertilization was the critical period for controlling loss of N and P from paddy fields.