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Evidence for Clostridia as Agents of Dissimilatory Reduction of Nitrate to Ammonium in Soils¹

ABSTRACT

Reduction of NO₃^- to NH₄⁺ was studied in fresh and air-dried soils which were amended with ¹⁵NO₃^- and glucose, acetate, or water and incubated anaerobically. Soils were either untreated or heat-shocked at 68°C for 1 hour prior to amendment. Ammonium was produced rapidly after the onset of anaerobiosis, and thereafter was incorporated into organic matter. ¹⁵NH₄⁺ plus ¹⁵N-organic matter production was observed in glucose-amended fresh soil in quantities up to 43% of added ¹⁵NO₃^- in untreated samples and 55% in heat-shocked samples after 5 days incubation. In soils unamended with organic carbon, NO₃^- reduction to NH₄⁺ was minor. Pretreatment of the soils either by air-drying or heat-shocking increased to a similar extent the amount of NO₃^- reduced to NH₄⁺. The activity of clostridia during the NO₃^- reduction was indicated by the absence of any effect exerted by heat-treatment, the production of H₂ and CO₂, and the presence of higher numbers of anaerobic sporeforming bacteria relative to denitrifying bacteria in the air-dried soil. Also, the most common isolate capable of reducing NO₃^- to NH₄⁺ was a Clostridium spp. The addition of washed spores of a NO₃^--reducing Clostridium isolated from the soil increased the formation of ¹⁵NH₄⁺-N plus ¹⁵N-organic N fourfold, an accumulation equivalent to 83% of added ¹⁵NO₃^--N.

The reduction of NO₃^- to NH₄⁺ in soils was not inhibited by NH₄⁺ or glutamine, indicating that the mechanism of reduction was dissimilatory. This conclusion is supported by studies with several Clostrdium spp. isolated from the soils. The isolates were capable of reducing NO₃^- to NH₄⁺ and exhibited increased cell yields when NO₃^- was included in the growth medium. These studies suggest that the potential for significant dissimilatory NO₃^- reduction to NH₄⁺ exists in most soils, principally in the sporeforming genera of Clostridium and Bacillus, but that this potential is likely expressed only when soils become anoxic and are rich in C.

¹ Contribution from the Dept. of Microbiology and Public Health and the Dept. of Crop and Soil Sci., Michigan State Univ., East Lansing, MI, 48824. Journal Article no. 8883 of the Mich. Agric. Expt. Stn. The research was supported by NSF Grant DEB 77-19273 and Regional Res. Proj. NE-39.

² Former Graduate Research Assistant and Professor, respectively. Current address of senior author is Institute of Ecology, Univ. of Georgia, Athens, GA 30602.

Received for publication January 22, 1979. Accepted for publication April 2, 1979.

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