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

Abstract

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 20°C, 16°C, 12°C, 10°C, 8°C, and 5°C. 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 20°C to 5°C 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 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 20°C to 5°C. Nitrification was limited by pH in soil treatments which were amended with urea and incubated at 20°C. In contrast, nitrification ceased at soil pH 5.0, 5.8, and 6.3 when soil amended with urea was incubated at temperatures of 16°C, 12°C, and 10°C, respectively. Nitrite (NO₂-ion) was found to accumulate in urea-amended soil incubated at temperatures below 12°C, implying that the growth and activity of nitrite oxidizing bacteria were preferentially restricted at temperatures below 12°C. The peak concentrations of NO₂-ions preceded the inhibition of nitrification at pH 5.8 and pH 6.3 in soil incubated at 12°C and 10°C, 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.

Acknowledgments

The authors would like to acknowledge the technical assistance of Anne McMurdo, Tom Turi, and Michelle Fleming. Assistance given by Dr. Lyn Abbott, Department of Soil Science and Plant Nutrition, University of Western Australia, during aspects of the Honours study is gratefully acknowledged.