Urea losses in flooded soils with established oxidized and reduced soil layers

Laboratory batch incubation experiments were conducted to determine in fate of urea-¹⁵N applied to floodwater of four rice soils with established oxidized and reduced soil layers. Diffusion-dependent urea hydrolysis was rapid in all soils, with rates ranging from 0.0107 to 0.0159 h^-1 and a mean rate of 0.0131 h^-1. Rapid loss of 53%-65% applied urea-¹⁵N occurred during the first 8 days after application, primarily by NH₃ volatilization. At the end of 70 days, an additional 20%-30% of applied urea-¹⁵N was lost, primarily through nitrification-denitrification processes. The soil types showed significant differences in total applied urea-¹⁵ recovery. Conversion of urea-¹⁵N to N₂-¹⁵N provided direct evidence of urea hydrolysis followed by nitrification-denitrification in flooded soils.