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

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.