Spatial and temporal distribution of nitrogen in a puddled rice soil following application of urea-based fertilizers by different methods

Summary Field experiments were conducted on transplanted rice (Oryza sativa L., var. Ratna) in a submerged soil, in order to study the distribution of N applied at 100 kg/ha. N was introduced as prilled urea or neem cake coated urea broadcast on the soil, and as urea supergranules, 1 or 2.5 g in size, point-placed at 5 cm depth. The surface-broadcast N was found mostly in the top 0–5 cm layer of soil and there was only a small vertical movement of applied N to 5–10 cm depth. With point placement of 1-g urea granules, the fertilizer N was found mostly at 5–10 cm depth and within 2.5 cm horizontal distance from the point of placement, compared with 5–10 cm depth and within 5 cm horizontal distance from the point of placement for the 2.5-g granules. With the two broadcast applications, the NH₄ ⁺-N content reached peaks of 40–68 mg/kg soil in the top 0–5 cm of soil within the first 3–6 days, decreased at a faster rate from the 6th to 12th day and then at a slower rate up to the 32nd day. In contrast, the NH₄ ⁺-N content around the points of placement of the urea supergranules reached peaks of 570–900 mg/kg soil during the first 3–12 days after placement, then decreased rapidly during the next 6–9 days, after which the values remained more or less unchanged but were still higher than the untreated control value.