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Effect of Temperature on the Gaseous Nitrogen Products of Denitrification in a Silt Loam Soil¹

ABSTRACT

The amount and relative proportion of N₂ and N₂O evolved from a nitrate-amended surface soil under an atmosphere of He in the range of 7 to 75°C was examined. The objective was to evaluate the effect of temperature on N₂O evolution and investigate the optimal temperature for denitrification. The soil had 2.43% organic C and pH of 6.8. Incubations were performed in all-glass systems which were sealed throughout the incubation period. Denitrification products (N₂O, NO, and N₂) were determined periodically by gas chromatography using an ultrasonic detector. At the end of the incubation period, inorganic N remaining was determined.

Above 15°C, the rate of denitrification increased with temperature, with an apparent optimum of 60 to 67°C. The percentage as N₂O of the total (N₂O + N₂)-N declined with time, but was still 44 to 50% of the gas produced at 16 days with the 7 and 15°C experiments. Nitrous oxide comprised up to 88% of the gas produced at 40°C after 2 days. At 50°C, most of the gas was N₂O at one day, but at 60 to 67°C, N₂O had disappeared by 1 day. Denitrification was still occurring at 70°C, but at a reduced rate. More than 85% of the N evolved at 70°C was as N₂O. No denitrification occurred at 75°C.

At 50 to 67°C, much more N was recovered as gaseous N than was initially present in the system as NO₃-N, and no (NO₂^-+NO₃^-)-N remained at 4 days. These results are explained by the hypothesis that thermophilic Bacillus sp. (and perhaps others) rapidly reduces NO₃^- to NO₂^-, with the NO₂^- reacting with oxidized N functional groups to form nitrogenous gases. This hypothesis was partially confirmed by experiments involving treatment of steam-sterilized soil with NO₂^- at 40 and 60°C. Our results suggest that the commonly-stated optimal temperature for denitrification may be too high for true biological denitrification due to the dominance of temperature-dependent chemical reactions at >50°C.

¹ Research supported by The College of Agricultural and Life Sciences, University of Wisconsin-Madison, and by NSF (DEB-7817404).

² Professor, Research Assistant, and Undergraduate Student, respectively, Department of Soil Science, University of Wisconsin-Madison, 53706. Present address of I. R. Fillery is Soil Scientist, International Fertilizer Development Center, Muscle Shoals, AL, 35660, and of G. P. Marx is Research Assistant, Department of Agronomy, Colorado State University, Fort Collins, CO, 80521.

Received for publication January 24, 1979. Accepted for publication July 11, 1979.

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