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Nitrous oxide (N₂O) emission-suppressing activity of some electron-transport inhibitors of the Hill reaction system was investigated. The Hill reaction inhibitors&#8212;paraquat, isouron, bromacil, diquat, and simazine&#8212;all of which have been or are currently being used as herbicides in farming activity are expected to inhibit the electron-transporting pathways of nitrate respiration in denitrifying bacteria. Using N₂O-emitting soil bed (5.0 g of fresh weight) from a continuously manured Andisol corn farmland in Hokkaido, Japan, which was autoclaved and further supplemented with an active N₂O-emitter, <i>Pseudomonas</i> sp. 5CFM15-6D, and 1 mL of 100 mM NH₄NO₃ or (NH₄)₂SO₄ solution as the sole nitrogen source (final concentration, 0.2 mM) in a 30 mL gas-chromatography vial, the effects of the five herbicides on N₂O emission were examined. Paraquat and isouron (each at 50 &#181;M) showed a statistically significant suppression of N₂O emission in both the nitrification and the denitrification processes after a 7-day-incubation, whereas diquat at the same concentration accelerated N₂O emission in the presence of NO₃^&#8722;. These results suggest that paraquat and isouron inhibited both the nitrification and the denitrification processes for N₂O generation, or its upstream stages, whereas diquat specifically inhibited N₂O reductase, an enzyme that catalyzes the reduction of N₂O to N₂ gas. Incomplete denitrifiers are the key players in the potent emission of N₂O from Andisol corn farmland soil because of the missing <i>nosZ</i> gene. The electron relay system-inhibiting herbicides&#8212;paraquat and isouron&#8212;possibly contribute to the prevention of denitrification-induced nitrogen loss from the farming soil.