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Effect of Water Depth on Nitrogen Use Efficiency and Nitrogen-15 Balance in Lowland Rice¹

Basic studies to understand and delineate N loss mechanisms and to develop management practices to minimize N losses are vital to increasing rice (Oryza sativa L.) productivity and N use efficiency in lowland rice. In 1984, two field studies in lowland rice evaluated the effect of water depth on potential N loss and N use efficiency on Maahas clay (Andaqueptic Haplaquoll) and ¹⁵N balance on Maligaya clay (Vertic Tropaquept). In the first experiment on Maahas clay, the partial pressure of NH3 (NH₃) in the floodwater was lower when fertilizer was incorporated without standing water than when incorporated with 50 mm water. However, when fertilizer was topdressed 10 days after transplanting (DT), NH₃ was lower with 100 and 150 cm standing water than with 50 mm water due to dilution and lower floodwater pH and temperature. In the second experiment conducted on Maligaya clay, grain yield from urea applied basally and incorporated without and with 25 mm standing water was similar to that from point-placed urea supergranules. Yields from fertilizer topdressed 10 DT with 0, 50, and 100 mm standing water were similar but significantly lower than when fertilizer was applied basally and incorporated at all water depths. Nitrogen-15 balance data show that fertilizer applied basally and incorporated without standing water gave a total I5N recovery of 66% and crop (grain + straw) recovery of 39%. Standing water at depths of 25 and 50 mm during incorporation decreased total ¹⁵N recovery by 5 to 7%. Topdressing fertilizer 10 DT without standing water resulted in 59% total N recovery and 38% plant recovery.

Key Words: Partial pressure of ammonia • Ammonia volatilization • Grain protein • Oryza sativa L

² Principal scientist and head, senior research assistant, former visiting research fellow, and research assistants, respectively, Dep. of .Agronomy, IRRI, Los Baiios, Laguna, Philippines.

Received for publication December 11, 1985.

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