Effect of Placement Methods on Floodwater Properties and Recovery of Applied Nitrogen (15N‐Labeled Urea) in Wetland Rice

Abstract

AbstractStudies at the International Rice Research Institute (IRRI) during the 1981 dry and wet seasons evaluated the effects of placement methods on recovery of fertilizer N by rice (Oryza sativa L.) using 15N‐labeled urea in field microplots. Uniform placement and point placement of urea supergranules (USG) gave the highest yield (6.4 Mg/ha) with efficiency of 51 kg rough rice/kg N in the dry season. In the wet season, point placement by USG produced the highest yield (4.4 Mg/ha) with efficiency of 40 kg rough rice/kg N. Higher floodwater pH and total N (urea + NH4‐N) concentration in floodwater were recorded after fertilization with split application and band placement of urea indicating the greater potential for NH3 volatilization and denitrification losses from these methods. Lower floodwater pH and low total N concentration in floodwater (5 mg/kg) were detected with uniform placement of prilled urea and point placement of USG, demonstrating that these methods can reduce N losses in lowland rice. Most fertilizer N (15N‐labeled urea) uptake occurred during the early crop growth stages. However, fertilizer N uptake was sustained with point placement of USG and uniform placement of prilled urea. Native soil N was taken up by the rice crop during the entire growing season, and was the major N source at the later part of crop growth in the wet season. Fertilizer 15N as high as 75% in the dry season and 65% in the wet season were recovered by the rice plant at harvest (grain and straw) with point placement of USG.