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The Release of Nonexchangeable Ammonium (¹⁵N Labelled) in Wetland Rice Soils¹

ABSTRACT

The release of nonexchangeable (fixed) NH⁺₄ was studied for three representative rice soils of the Philippines. Soil samples from these soils were treated with ¹⁵NH⁺₄ in the laboratory so that they contained appreciable amounts of nonexchangeable NH⁺₄ in ¹⁵N form. Special soil holders were filled with the labelled soil and incubated in the wetland rice soils from July to October at a depth of 0.2 m. During this period of incubation, highly significant amounts of labelled nonexchangeable NH⁺₄ were released from all three soils. For the Maligaya silty clay loam and Guadalupe clay, both containing vermiculite, the total release of nonexchangeable NH⁺₄ was higher than the release of labelled nonexchangeable NH⁺₄ showing that non-labelled NH⁺₄ was released as well. ¹⁵N analysis of the rice plants showed that the nonexchangeable labelled NH⁺₄ was subsequently available to the crop. Assuming a soil bulk density of 1 and a rooting depth of 0.2 m release of nonexchangeable NH⁺₄ from the Maligaya silty clay loam ranged from 80 to 100 kg N ha^–1, while for the Guadalupe clay and Santa Rita clay the net release of nonexchangeable NH⁺₄ amounted to about 20 to 40 kg N ha^–1, respectively. It is concluded that nonexchangeable NH⁺₄ may play a major role in N nutrition of rice crops grown in wetland rice soils rich in vermiculite.

¹ Contribution from the Dep. of Agron., The Int. Rice Res. Inst., Manila (mailing address: P.O. Box 933, Manila), Philippines, and Justus-Liebig University Giessen, Inst. of Plant Nutrition, D-6300 Giessen, West Germany.

² Postdoctoral Fellow, and Professor, Justus-Liebig University Giessen; and Agronomist and Head, Dep. of Agron., The International Rice Research Inst.

Received for publication January 19, 1983. Accepted for publication November 14, 1983.

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