Assay of urease enzyme activity in an ammonium-fixing soil

Rates of substrate disappearance and product formation were compared as measures of urease enzyme activity in an NH₄-fixing and in a non-fixing soil under tris-, borate- or non-buffered assay conditions over 4h at 37°C. Tris-buffered urease activity of the NH₄-fixing soil was 119 μg urea-N hydrol./g/h or 116 μg (KCl-extractable) NH₄-N/g/h indicating prevention of NH₄fixation by the buffer; without tris, NH₄production rates amounted to only 35% of corresponding urea hydrolysis rates. Equal rates of urea disappearance and NH₄formation occurred in the non-fixing soil irrespective of buffer amendment. Tris-inhibition of NH₄fixation during 4h incubation at 37°C, however, depended on NH₄Cl rate and buffer strength. 0.025-0.10 M tris (pH 9.0) reduced NH₄fixation to negligible amounts at < 0.03 M NH₄C1 whereas, at 0.06-0.24 M NH₄C1, substantial NH₄fixation occurred in the presence of 0.05 M tris; NH₄fixation in unbuffered soil, however, always exceeded that in tris-buffered soil. Borate buffer (0.06M, pH 10) did not influence the extent of NH₄fixation. Tris significantly enhanced urea hydrolysis in the slightly acid, non-fixing soil but not in the moderately alkaline NH₄-fixing soil indicating an effect of soil type on pH optima of urease enzyme activity. The urease activities of both soils in borate were considerably lower than in tris, possibly because of the combined effects of excess alkalinity and high substrate concentration.