Distribution, salinity, kinetic and thermodynamic characteristics of urease activity in a vertisol profile

Abstract

The distribution, and the effects of temperature, moisture and organic C substrate on urease activity of a vertisol up to 120 cm depths were studied. The kinetic and thermodynamic characteristics of urease activity in the soil were also studied. Urease activity (measured at the natural soil pH) was found to be higher in soil at 20-60 cm than in the top soil (0-10 cm). It remained relatively constant for up to 900 days, when the soil was kept either field moist at 4°C or air-dried at 25°C. However, partial drying of the soil or rewetting of air-dried soil, especially at elevated temperatures (40°C), and glucose addition, affected substantially but differentially the urease activity of soil obtained from different depths. The distribution in soil urease activity assayed at an optimum pH (using tris-HCl buffer, pH 9.0) became relatively uniform at all depths. From the effect of different buffer pH's on soil urease and from kinetic and thermodynamic characteristics, it is suggested that lower urease activity in the top soil was primarily a consequence of suboptimal pH, resulting in greater configurational or steric hindrance in formation and/or breakdown of urea-urease complex in the soil. Differences in seasonal variation in temperature, moisture, organic substrate and microbial activity may also affect distribution and stability of urease at different depths.