Effect of temperature and nitrogen source on nitrification in a sandy soil

The effect of soil temperature, nitrogen (N) source, and indirectly soil pH, on the nitrification of (NH4)-N-15-N, was studied in a sandy, moderately acidic soil. Four N sources, including a N-15-labeled urea solution and mixtures of a N-15-labeled urea and a HNO3-N solution were added to soil which was incubated at 20degreesC, 16degreesC, 12degreesC, 10degreesC, 8degreesC, and 5degreesC. The N sources adjusted the soil pH to 8.3, 7.1, 6.7, and 5.4 within 4 days of application. These treatments enabled an evaluation of the interactions between soil pH and temperature as they affect nitrification in sandy soil. The content of (NH4)-N-15-ions and (NO3)-N-15-ions in 1 M KCl extracts was determined by steam distillation and mass spectrometry. Maximum rates of nitrification decreased as temperature was reduced from 20degreesC to 5degreesC and as the input of alkalinity in the N sources declined. The relationships between temperature and relative maximum rates of nitrification (r(rel)) were best described by the Arrhenius equation. However, the temperature coefficient, or Q(10), determined from the fitted Arrhenius equation was similar to5, a value which is higher than that typically used to describe the effect of temperature on nitrification. The production of NO3-ions after 56 days declined as soil temperature was reduced from 20degreesC to 5degreesC. Nitrification was limited by pH in soil treatments which were amended with urea and incubated at 20degreesC. In contrast, nitrification ceased at soil pH 5.0, 5.8, and 6.3 when soil amended with urea was incubated at temperatures of 16degreesC, 12degreesC, and 10degreesC, respectively. Nitrite (NO2-ion) was found to accumulate in urea-amended soil incubated at temperatures below 12degreesC, implying that the growth and activity of nitrite oxidizing bacteria were preferentially restricted at temperatures below 12degreesC. The peak concentrations of NO2-ions preceded the inhibition of nitrification at pH 5.8 and pH 6.3 in soil incubated at 12degreesC and 10degreesC, respectively. These findings suggest that nitrification is not always limited by the growth and activity of NH4-ion oxidizers as has been often assumed in models of nitrification.