Separation of the different pools of enzyme activity in soils (intra- and e
xtracellular) is needed to assess the contribution of the microbial communi
ty to specific enzyme reactions. Microbial biomass C and N were determined
by the chloroform fumigation method, and urease activity was assayed at the
optimal pH (THAM buffer, pH 9.0) in 10 surface soils before and after chlo
roform fumigation. The activities of total urease (intra- and extracellular
; after chloroform fumigation without toluene treatment), extracellular ure
ase (before chloroform fumigation without toluene treatment) and intracellu
lar urease (activity of the microbial biomass), i.e. (total activity minus
extracellular activity) were significantly correlated with microbial biomas
s C (r = 0.84**, 0.97*** and 0.84***, respectively) and N (r = 0.76**, 0.80
** and 0.66*, respectively). The C-to-N ratios of the soils ranged from 8.6
to 18.1, and those of the microbial biomass ranged from 5.2 to 30.3. The u
rease activity of the microbial biomass, expressed as a percent of total ur
ease activity, ranged from 37.1 to 73.1% (avg = 54.0%). The remaining activ
ity (26.9-62.9%, avg = 46.0%) was extracellular. When toluene was used as a
plasmolytic agent in the non-fumigated soils, the urease activity increase
d from 21.4% to 65.9% (avg = 46.6%). When expressed mg(-1) microbial biomas
s C (specific activity), the urease activity of the microbial biomass range
d from 80 to 419 (avg = 164) mu g NH4+ -N mg(-1) C-mic 2 h(-1). The total u
rease activity and the activity of the microbial biomass were underestimate
d, because the activity of purified Jack bean (Canavalia ensiformis) urease
was decreased by 29% after fumigation with chloroform. Using the activity
of 1 mg of protein of Jack bean urease as a reference and correcting for th
e 29% of the urease denatured by chloroform fumigation, the urease protein
equivalent in soils ranged from 1.9 to 12.0 mg kg(-1) of soil (avg = 5.3 mg
kg(-1) of soil). (C) 1998 Elsevier Science Ltd. All rights reserved.