Nitrous Oxide Emission During Denitrification in a Flooded Field

Abstract Little field information is available on the amounts of nitrous oxide (N 2 O) released from soils during denitrification, or the proportion of the denitrification loss attributable to N 2 O production. The paper reports studies of N 2 O evolution and the simultaneous disappearance of nitrate (NO 3 ) in a flooded rice field which initially contained 4 g NO 3 ‐N m −2 in the top 0.08 m of soil. Measurements were made continuously for 18 days after the field was flooded. Nitrous oxide emission was calculated from the rate of increase of N 2 O in air circulating in a closed loop between a chamber installed in the field and an infrared gas analyzer. Emission rates as small as 1.8 ng N m −2 sec −1 could be measured. Nitrate disappearance was measured by chemical analysis of water samples. Nitrous oxide emission showed a diurnal cycle in phase with water temperature. Between the second and nineteenth day of flooding, 2.7 g NO 3 ‐N m −2 disappeared from the water and 0.038 g N 2 O‐N m −2 were produced, only 1.4% of the apparent N loss. In a supplementary, small‐bay experiment in which sodium nitrate and glycerol were added to the water, N 2 O production accounted for only 0.8% of the NO 3 disappearance. Even allowing for NO 3 removal through other mechanisms, the production of N 2 O in both experiments was very much less than the 7% commonly assumed for denitrification in current models of the global N 2 O budget.