Movement and Retention of NH4-N in Wetland Rice Soils as Affected by Urea Application Methods

A field experiment was conducted to investigate the dynamics of ammonium nitrogen (NH4+-N) in the soil during an entire rice-growing season. The NH4+-N dynamics were measured in paddy soils from two N application methods, namely, urea deep placement (UDP) and broadcast prilled urea (PU). The pore water samples from a 10 cm soil depth were collected using a “rhizon sampler.” The samples were collected at 0, 7, 10, 14, 20, and 22 cm from the urea briquette (UB) placement point at 7, 14, 21, 35, 64, and 83 days after transplanting (DAT) of rice. The NH4+-N in the floodwater sample was measured for a week after each split application of PU. UDP retained NH4+-N at the placement site (7–10 cm depth) until 64 DAT. A small amount of NH4+-N moved horizontally up to 14 cm from the placement site. It’s movement to the soil surface and floodwater was very low to negligible. In contrast, PU produced more NH4+-N in both the floodwater and in the soil surface. Therefore, broadcast urea had significantly (p < 0.05) higher ammonia volatilization (~15% of applied N) compared to UDP (<1%). UDP significantly (p < 0.05) increased grain yields, N uptake, and N recovery compared to broadcast urea. These results confirm that a single application of UDP could meet plants’ N demand throughout the rice-growing period, particularly for short- and medium-duration rice varieties.