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

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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.
Funding Information
  • National Science Foundation (DEB 77‐19273)