Effect of temperature and nitrogen source on nitrification in a sandy soil
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The effect of soil temperature, nitrogen (N) source, and indirectly soil pH, on the nitrification of 15NH4-N, was studied in a sandy, moderately acidic soil. Four N sources, including a 15N-labeled urea solution and mixtures of a 15N-labeled urea and a HNO3-N solution were added to soil which was incubated at 20C, 16C, 12C, 10C, 8C, and 5C. 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 15NH4-ions and 15NO3-ions in 1 M KCl extracts was determined by steam distillation and mass spectrometry. Maximum rates of nitrification decreased as temperature was reduced from 20C to 5C and as the input of alkalinity in the N sources declined. The relationships between temperature and relative maximum rates of nitrification (rrel) were best described by the Arrhenius equation. However, the temperature coefficient, or Q10, determined from the fitted Arrhenius equation was 5, 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 20C to 5C. Nitrification was limited by pH in soil treatments which were amended with urea and incubated at 20C. In contrast, nitrification ceased at soil pH 5.0, 5.8, and 6.3 when soil amended with urea was incubated at temperatures of 16C, 12C, and 10C, respectively. Nitrite (NO2-ion) was found to accumulate in urea-amended soil incubated at temperatures below 12C, implying that the growth and activity of nitrite oxidizing bacteria were preferentially restricted at temperatures below 12C. The peak concentrations of NO2-ions preceded the inhibition of nitrification at pH 5.8 and pH 6.3 in soil incubated at 12C and 10C, 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.
