Assay of urease activity using 14C-urea in stored, geologically preserved, and in irradiated soils

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Soils from adverse and diverse climatic and geographic areas, and stored for various lengths of time, were examined for their urease activities. During a period of 26 months of air-dry storage of Hawaiian and Puerto Rico latosols, one soil showed an increase in activity to 108·5 per cent and others a decrease to as little as 36 per cent. Urease activity found in 6–12 years stored, air-dry agricultural soils correlated with the organic matter content rather than with the numbers of microorganisms. In samples from the 60-years-old Hilgard soil collection, urease activity reflected both the organic carbon content and the numbers of organisms present. Measurable urease activities were detected in 8715 and 9550-years-old, radiocarbon dated, Alaska permafrost samples. No activity was detected in a 32,000-years-old sample. No urease activity was present in two high-salinity soils. Urease activity was detected in an alkali soil and in an acid soil, but highest activity was usually found in neutral soils. A high-energy electron-beam irradiation dosage of 4 Mrad increased urease activity in some soils, but decreased activity in two others. Doses of 8 Mrad decreased urease activity below their native levels in all soils. Evidently, an intracellular urease component becomes more accessible to substrate upon death of organisms. This effect is observable because of the strong resistance of soil urease to radiation inactivation. Urea-amended Dublin soil, subjected to incubation at 80 per cent relative humidity showed a measurable urease activity. It appears that an extracellular component of soil urease becomes associated with inorganic and organic soil colloidal particles and retains its activity for extended periods of time. A new method for urease activity determination in soils was devised, based on the detection of 14CO 2 release from 14C-urea-amended soils.