Nitrogen‐15 Balance and Residual Effects of Urea‐N in Wetland Rice Fields as Affected by Deep Placement Techniques

Abstract

Abstract The 15 N balance at harvest, the residual effects of applied N fertilizer, and the distribution of N by depth in a wetland rice ( Oryza sativa L.) soil (Alfisol) were determined in field experiments using 15 N‐labeled urea. Nitrogen‐15 balance data show that with point, deep placement of urea supergranules (USG), only 4 to 5% of the applied N was unaccounted for after harvest of both the dry‐season and wet‐season crops. In contrast, when prilled urea was basally broadcast and incorporated and topdressed at panicle initiation (PI), 11 to 35% of the applied N was not recovered at harvest, indicating substantial loss of fertilizer N, especially when poor incorporation resulted in high floodwater mineral‐N levels. Uniform deep placement also resulted in high recovery of 15 N (92–107%), confirming that the high total 15 N recovery obtained with point deep‐placement of USG was largely a function of depth of placement. Band application of urea gave complete 15 N recovery (102%) in the dry season and low recovery (61%) in the wet. Nitrogen‐15 balance data showed that 22 to 56% of the applied urea‐N still remained in the soil after the final harvest. The uptake by the wet season crop of the residual labeled soil N from the dry season experiment was about 5% of the initial applied N (87 kg N/ha) for all treatments. Placement method appeared to influence the actual recovery of residual soil 15 N; the percentage recovery from the USG treatment was highest and that from the split urea treatment lowest. In neither the dry season nor the wet season was 15 N enrichment significant in soil below 30 cm. Generally, the distribution of the residual 15 N in soil reflected the placement pattern of urea.