Temperature effects on soil urease activity

Abstract

Accurate estimates of the activation energy for soil urease are needed to model how changes in soil temperature quantitatively change the hydrolysis rate of urea applied to soil. Activation energy values reported in the literature for soil urease range from 16.8 to 95.1 kJ mol −1. The wide variation in these values could be due to confounding of temperature effects by other rate-controlling factors in some of these studies. Therefore, we evaluated the effects of temperature on urea hydrolysis rates under laboratory conditions with other rate-controlling factors held constant. Temperatures of 5, 15, 25, 35 and 45°C were used with soil samples from areas mapped as Kahola and Smolan. The soils were limed to pH 8.2 and THAM buffer was adjusted to the same pH to ensure high and stable pH during experiments. A solution to soil ratio of 2:1 was used to maintain a nonlimiting water content. To ensure a reaction rate independent of substrate concentration, a urea concentration of 1000 mg N kg −1 soil was used, and not more than 10% of it was allowed to hydrolyze at each temperature. Increasing temperature from 5 to 45°C increased the urea hydrolysis rate. The mean activation, energies for urease were 49.4 and 53.6 kJ mol 1̄ for Kahola and Smolan soils, respectively. These values correspond to a Q 10 of approx. 2 in the temperature range studied. They differed from some reported values, probably because of differences in assay procedures.