Urease activity of microbial biomass in soils

Separation of the different pools of enzyme activity in soils (intra- and extracellular) is needed to assess the contribution of the microbial community 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 chloroform fumigation. The activities of total urease (intra- and extracellular; after chloroform fumigation without toluene treatment), extracellular urease (before chloroform fumigation without toluene treatment) and intracellular urease (activity of the microbial biomass), i.e. (total activity minus extracellular activity) were significantly correlated with microbial biomass 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 urease activity of the microbial biomass, expressed as a percent of total urease activity, ranged from 37.1 to 73.1% (avg=54.0%). The remaining activity (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 increased from 21.4% to 65.9% (avg=46.6%). When expressed mg−1 microbial biomass C (specific activity), the urease activity of the microbial biomass ranged from 80 to 419 (avg=164) μg NH4+–N mg−1 Cmic 2 h−1. The total urease activity and the activity of the microbial biomass were underestimated, 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 the 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).