Denitrification of urea and sodium nitrate in some Manitoba soils

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Christianson, Carlyle Bruce
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The kinds and amounts of nitrogen gas lost during the oxidation of urea were determined. One thousand (1000) ppm-N as urea labelled with 52.4% 15N was uniformly mixed with soil to approximate the concentation of urea - N near the pellet site or fertilizer band. With Wellwood soil, gaseous losses as N2O and N2 amounted to approximately 25% of the added N at pH 6.1 in 2 months. Samples of this soil were shown to accumulate NO2-. The appearance of NO2- coincided with the appearance of gaseous N products in the soil atmosphere. Gaseous production declined as NO2- oxidized to NO3-. The majority of the N2 gas (which accounted for 40% of the N evolved) resulted from a van Slyke-type reaction in which one atom of N came from a soil source and one came from the fertilizer. When the pH of the Wellwood soil was increased to 7.6 by the addition of 10% CaCO3, the accumulation of NO2- occurred to a greater extend and lasted for the duration of the study. Liming slightly increased losses of nitrogenous gases. The rate of gaseous evolution slowly declined with time even though NO2- persisted for the duration of the study. In identical experiments, very little gaseous loss occurred from a Neuenberg soil (pH 7.1) even when the pH was lowered to 6.0 or raised to 8.0. Nitrate applied as Ca(NO3)2 to the Wellwood and Neuenberg soils was stable during aerobic incubation. The rate of gaseous evolution tended to decrease with increased soil pH and increased with increasing initial NO2- concentration when the Wellwood soil was treated with varying concentrations of NaNO2... Solution studies were performed using the amino acid glycine and NO2- between pHs 2 and 8 inclusive. The van Slyke reaction did not occur at any significant rate at pH values in excess of 5.0