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Nitrous Oxide Production by Nondenitrifying Soil Nitrate Reducers¹

ABSTRACT

Of 214 soil bacterial isolates able to reduce NO₃^-, 209 produced nitrous oxide (N₂O), even though only 46 were respiratory denitrifiers (competent to carry out complete reduction of NO₃^- to N gases). Nitrite or NH₄⁺ was the major product of NO₃^- reduction by the nondenitrifying organisms, but typically about 5 to 10% and up to 34% of the NO₃^- reduced by them was released as N₂O during a 2-week incubation. Bacillus and Enterobacter were the most commonly observed genera of nondenitrifying N₂O producers. Fermentative NO₂^- reduction and N₂O production by a Bacillus sp. and a Citrobacter sp. were characterized in pure culture studies. Dinitrogen (N₂) was not produced in detectable quantities by these organisms. When added to autoclaved soil, they accumulated more N₂O than two denitrifying pseudomonads, since the latter consume and produce N₂O. In tryptic soy broth (TSB), which allows active fermentative growth, NH₄⁺ was apparently the major product of NO₃^- reduction. In nutrient broth (NB), NO₂^- accumulated. Added NH₄⁺ did not inhibit N₂O production or apparent reduction to NH₄⁺, indicating that these processes are not assimilatory. The effect of added glucose on N₂O production varied with the organism and media composition. Nitrous oxide production from NO₂^- by these organisms was shown to be at least partially a biochemical reaction. The N₂O evolved slowly in batch cultures and mostly after apparent growth ceased. This is apparently a novel mechanism of N₂O generation which differs significantly from respiratory denitrification.

¹ Contribution from the Kentucky Agric. Exp. Stn. Journal Article no. 81-3-38.

² Assistant Professor and former Graduate Student, respectively, Dep. of Agron., Univ. of Kentucky, Lexington, KY 40546.

Received for publication February 19, 1981. Accepted for publication April 28, 1981.

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