Ammonia volatilization and nitrogen leaching following top-dressing of urea from water-saving irrigated rice field: impact of two-split surge irrigation

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 1week after three top-dressing of urea under SI treatment were estimated as 4.92, 9.29, and 2.51kgNha(-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 40-50cm beneath soil surface under SI treatment, which were measured to evaluate the amount of nitrogen leaching, were increased by 0.46, 0.07 and 0.92kgNha(-1), respectively. The SI treatment achieved higher grain yield by 0.33tha(-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.