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Direct Measurement of Dinitrogen and Nitrous Oxide Flux in Flooded Rice Fields

IFDC/IRRI Cooperative Project, IRRI, P.O. Box 933, Manila, Philippines

E. R. Austin

Agro-Economic Div., IFDC, Muscle Shoals, AL 35662

*Corresponding author.

ABSTRACT

A direct ¹⁵N method was used to measure N₂ and N₂O flux in flooded rice (Oryza sativa L.) fields. The method involved application of a highly ¹⁵N-enriched source to a 0.4-m by 0.4-m microplot, collection of evolved N₂ and N₂O in a chamber placed over the flood-water, removal of NH₃ from the air sample, conversion of some (N₂ + N₂O)-N in the air sample to NH⁺₄-N, and determination of ¹⁵N content with a mass spectrometer. The flux of (N₂ + N₂O)-¹⁵N was measured for 17 d following urea application by three methods, each replicated four times, on a Vertic Tropaquept. In all cases, the (N₂ + N₂O)-¹⁵N flux was much smaller than total gaseous ¹⁵N loss as estimated from unaccounted for ¹⁵N in the ¹⁵N balance after 18 d. The (N₂ + N₂O)-¹⁵N flux and total ¹⁵N loss were 1.1 and 40%, respectively, following basal broadcast and incorporation of 58 kg urea-N ha^–1 with 2-cm standing water; they decreased to 0.2 and 26%, respectively, when the urea basal incorporation was without standing water. The (N₂ + N₂O)-¹⁵N flux and total ¹⁵N loss were 0.5 and 46%, respectively, following application of 44 kg urea-N ha^–1 to 50-mm floodwater at 10 d after transplanting. The methodology could detect a flux of 10 g N ha^–1 d^–1 from the added 59 to 64 atom % ¹⁵N-labeled urea. Additional studies with ¹⁵N-labeled NO^-₃ evaluated the possibility that the low (N₂ + N₂O)-¹⁵N flux was due to incomplete recovery of N₂ and N₂O formed from added fertilizer. Direct recovery of (N₂ + N₂O)-¹⁵N following addition of labeled NO^-₃ ranged from 41 to 73% of the total ¹⁵N loss. No ¹⁵N was lost by leaching or runoff, and dissolved N₂O in the floodwater calculated from published solubility data was less than 1% of the added ¹⁵N.

Contribution from the Agro-Economic Div., International Fertilizer Development Center (IFDC), Muscle Shoals, AL 35662, and the Dep. of Agronomy, International Rice Research Inst. (IRRI), P.O. Box 933, Manila, Philippines.

Received for publication September 3, 1987.

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