Urease activity of microbial biomass in soils

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.