Distribution Patterns of Ammonium Nitrogen and 15N Uptake by Rice after Deep Placement of Urea Supergranules in Wetland Soil

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Abstract
In‐situ distribution patterns of ammonium nitrogen (NH+4‐N) and 15N uptake by wetland rice (Oryza sativa L.) were studied on the farm of the International Rice Research Institute (IRRI) during the 1979 dry season. Individual 2‐g urea supergranules were placed at 5‐, 10‐, and 15‐cm soil depths in wetland rice plots transplanted at spacings of 20 by 20 cm and 10 by 40 cm. The ammonium (NH+4) concentration gradients near placement sites showed that the NH+4 movement was slow; in the transplanted plots, NH+4 concentrations decreased with time largely due to plant uptake. Distribution patterns of NH+4 were influenced by the depth of supergranule placement and the transplanting geometry. In general, the greater the distance from the point of placement of supergranule to the plant, the slower was the depletion of NH+4‐N at the placement site. During the dry season, the NH+4 almost completely disappeared during the 40 days after transplanting, whereas during the wet season, NH+4 persisted for up to 40 days after transplanting.The 15N uptake patterns were s‐shaped. During the first 20‐day period after transplanting, deep‐placed N was presumably physically or spatially unavailable, and negligible plant uptake occurred. After 20 days, the 15N uptake was retarded by placement of supergranules at lower depths and away from the rice hills. However, at final harvest more 15N was taken up from urea placed at the 15‐cm depth than from the 5‐cm depth. A marked lower recovery of split‐applied urea‐N (25 to 34%) than that of deep‐placed urea‐N (50 to 61%) was observed.

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