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This article in SSSAJ

  1. Vol. 51 No. 5, p. 1355-1362
     
    Received: Nov 4, 1986
    Published: Sept, 1987


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doi:10.2136/sssaj1987.03615995005100050047x

Floodwater Properties, Nitrogen Utilization, and Nitrogen-15 Balance in a Calcareous Lowland Rice Soil1

  1. S. K. De Datta,
  2. I. R. P. Fillery,
  3. W. N. Obcemea and
  4. R. C. Evangelista2

Abstract

Abstract

Large areas of lowland rice (Oryza sativa L.) soils in South and Southeast Asia are calcareous. Field studies in the 1983 dry and wet seasons evaluated the effects of N source and application method on N loss and N utilization in a calcareous clay loam soil using the 15N-balance technique. In the 1983 wet season and the 1984 dry season, crop recovery of the residual 15N remaining after the first crop was determined. When urea supergranules (USG) were deep point-placed, the partial pressure of ammonia (pNH3) in the floodwater was negligible. High floodwater pNH3 was calculated with split application of fertilizer. Farmers' split of prilled urea (PU) gave the highest pNH3 (0.44 Pa in the wet season). pNH3 in the floodwater peaked on the third day and dropped to negligible thereafter. Deep placement (10 cm) of USG had the highest grain yield in the dry season. Ammonium sulfate produced a yield similar to that of urea. The highest 15N recovery by the crop (straw + grain) was 78% in the dry season and 64% in the wet season with deep point placement of USG. Nitrogen-15-balance data showed that with point placement of USG, 3% N was not accounted for after harvest in the dry season and 10% in the wet season. In contrast, when urea or ammonium sulfate was split-applied using improved timing and water management (researchers' split), 26 to 28% of applied N was not recovered at harvest in the dry season and 36 to 41% in the wet season. Soil N fractionation showed that 14 to 26% of 15N-labeled fertilizer was immobilized into the acid-hydrolyzable organic N fraction in the dry season and 9 to 18% in the wet season. With point placement of USG and band placement of urea, the decrease in the value of hydrolyzable organic N fraction at harvest indicated N release for plant use during the cropping season. Applied N immobilized into the nonhydrolyzable organic N and fixed in the nonexchangeable N was not significant.

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