Stability of urease in soils

Abstract

Studies with surface samples of Iowa soils selected to obtain a wide range in properties showed that the following treatments of field-moist soils had no effect on urease activity: leaching with water ; drying for 24 h at temperatures ranging from 30 to 60°C ; storage for 6 months at temperatures ranging from −20 to 40°C; incubation under aerobic or waterlogged conditions at 30 or 40°C for 6 months. No loss of urease activity could be detected when field-moist soils were air-dried and stored at 21–23°C for 2yr, but complete loss of urease activity was observed when they were dried at 105°C for 24 h or autoclaved (120°C) for 2h. Inactivation of urease in moist soils was detected at temperatures above 60°C. Treatment of field-moist soils with proteolytic enzymes which cause rapid destruction of jackbean urease did not decrease urease activity, but jackbean urease was destroyed or inactivated when added to sterilized or unsterilized soils. Although no decrease in urease activity could be detected when field-moist soils were air-dried, an appreciable (9–33%) decrease in urease activity was observed when air-dried soils were incubated under aerobic or waterlogged conditions. This decrease occurred within a few days, and prolonged incubation or repetition of the drying-incubation treatment did not lead to a further decrease in urease activity. Treatment of incubated air-dried soil with urease or glucose initially increased urease activity to a level exceeding that of the undried soil, but this activity decreased with time and eventually stabilized at the level observed for the undried soil. The work reported supports the conclusions from previous work that the native urease in Iowa soils is remarkably stable and that different soils have different levels of urease activity determined by the ability of their constituents to protect urease against microbial degradation and other processes leading to inactivation of enzymes.