LAUSR

To test whether it is possible to change the nitrogen losses during top-dressing of urea in water-saving irrigated rice field by appropriate irrigation management, two-split surge irrigation (SI) was designed to bring more top-dressed urea into rhizosphere soil and investigate ammonia volatilization (AV) and nitrogen leaching compared with farmers’ flooding practice (FFP). Cumulative AV losses of 1 week after three top-dressing of urea under SI treatment were estimated as 4.92, 9.29, and 2.51 kg N ha−1, which was decreased by 27.75%, 18.10%, and 28.53%, respectively. Concentrations of ammonium, nitrate, and total nitrogen in soil solutions collected at 40–50 cm beneath soil surface under SI treatment, which were measured to evaluate the amount of nitrogen leaching, were increased by 0.46, 0.07 and 0.92 kg N ha−1, respectively. The SI treatment achieved higher grain yield by 0.33 t ha−1 over FFP treatment. Results indicated that the SI water regime during top-dressing of urea reduced AV losses greatly at the cost of increasing nitrogen leaching losses. Considering that nitrogen losses from AV after top-dressing of urea were generally 4–5 times of those from nitrogen leaching losses, the sum of nitrogen losses in volatilization and leaching were reduced in the SI treatment compared to the FFP treatment. In conclusion, two-split surge irrigation is helpful in reducing nitrogen losses after top-dressing of urea and improving grain yield.