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Residual Fertilizer Nitrogen in a Flooded Rice Soil¹

ABSTRACT

Residual effects of ¹⁵N-labelled fertilizer (ammonium sulfate) nitrogen were measured during the second cropping season of rice (Oryza sativa L.). During the first cropping season, labelled fertilizer N was applied at 100 kg N/ha using different methods of N application. At the end of harvest the rice straw obtained during the first cropping season was incorporated back into the respective plots. During the second season half the plots were treated with an additional 100 kg N/ha of nonlabelled fertilizer N as an early season top dressing. The uptake of residual labelled N (that present in the native soil organic matter and in the incorporated rice straw) was measured during the growing season and at harvest time under fertilized and nonfertilized conditions.

The recovery of residual labelled N in grain and straw was 2.7 to 3.1 kg N/ha, with a large fraction of N (34.3 to 40.1 kg N/ha) still remaining in the soil. Approximately 3 to 5 kg of the residual labelled N/ha present at the beginning of the season was not accounted for in any fraction and was assumed to be lost from the system. Over a 2-year period, about 66 to 82% of the applied labelled fertilizer N could be accounted for by plant uptake or in the soil with the remainder lost from the plant-soil system.

¹ Contribution from Laboratory of Flooded Soils and Sediments, Dep. of Agronomy, Louisiana Agric. Exp. Stn., Louisiana State Univ., Baton Rouge, LA 70803. This study was conducted cooperatively with Tennessee Valley Authority, Soil and Fertilizer Research Branch, Div. of Agric. Develop.

² Research Associate and Professor, respectively. The senior author is presently working as post-doctoral research associate, Dep. of Biological and Agric. Eng., North Carolina State Univ., Raleigh, NC 27607.

Received for publication May 10, 1977. Accepted for publication December 15, 1977.