Assay of urease enzyme activity in an ammonium‐fixing soil

Abstract

Abstract 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 NH4 fixation by the buffer; without tris, NH4 production rates amounted to only 35% of coresponding urea hydrolysis rates. Equal rates of urea disappear‐ ance and NH4 formation occurred in the non‐fixing soil irrespective of buffer amendment. Tris‐inhibition of NH4 fixation during 4h incubation at 37°C, however, depended on NH4 Cl rate and buffer strength. 0.025–0.10 M tris (pH 9.0) reduced NH4 fixation to negligible amounts at < 0.03 M NH4C1 whereas, at 0.06–0.24 M NH4Cl, substantial NH4 fixation occurred in the presence of 0.05 M tris; NH4 fixation in unbuffered soil, however, always exceeded that in tris‐buffered soil. Borate buffer (...