Timing of Nitrogen Fertilizer for Rice in Relation to Paddy Flooding

Nitrogen use efficiency of rice (Oryza sativa L.) may be related to timing of N application relative to paddy flooding. A field experiment was conducted for three years to determine if surface broadcast applications of urea N applied at any of four times prior to, or two times after, paddy flooding affected N use efficiency in rice. Plant height, flag leaf N content at heading, N uptake, lodging, and yield were used as indicators of N use efficiency. Application of N to a dry paddy prior to flooding, resulted in better N utilization by the rice—regardless of timing—than when N was topdressed into a flooded paddy. This probably resulted from movement of urea into the soil as the paddy was flooded. Movement of urea into the soil with the wetting front would place the N below the soil surface and would minimize the loss of N through NH3 volatilization from the floodwater. It also would place the N below the oxidized soil zone found at the soil‐water interface, and minimize any nitrification of NHS in the oxidized soil layer, and resultant losses from denitrification in the anaerobic zones in soil or flood‐water. Applying the N immediately prior to flooding resulted in greater utilization of the N than if it were applied several days prior to flooding. Time of application prior to flooding was especially significant when the soil surface was moist and light showers occurred after urea application. A moist soil surface would favor rapid urea hydrolysis and ammonia volatilization prior to flooding, reducing the amount of N moved into the soil with wetting. When the soil was dry and no precipitation occurred following fertilizer application, urea applied as early as 5 d prior to flooding gave good performance. This probably was due to slower rates of granule dissolution and slower hydrolysis of urea, leaving substantial amounts of urea N present for “incorporation” with the floodwater.