Tillage Effects on Intracellular and Extracellular Soil Urease Activities Determined by an Improved Chloroform Fumigation Method

The intracellular urease activity in soils determined by the chloroform fumigation method is underestimated because of urease degradation by soil protease and denaturation by chloroform. The objectives of this study were to correct this underestimation and to investigate the responses of intracellular, extracellular, and total urease activities to no-tillage, reduced tillage, and moldboard plowing treatments. Surface layer soils (0-20 cm) after 8-year different tillage treatments were collected. First, the soil samples were fumigated with ethanol-free chloroform in the presence of a protease inhibitor. Subsequently, a urease addition experiment was conducted to determine what percentage of soil urease was denatured during the fumigation procedure. Our results showed that the corrected intracellular urease activities in soils under no-tillage, reduced tillage, and moldboard plowing accounted for 58.2%, 62.3%, and 51.7% of the total urease activity, respectively. Both no-tillage and reduced tillage significantly increased the intracellular, extracellular, and total urease activities compared with moldboard plowing. The ratio between intracellular and extracellular urease activities was not significantly different between no-tillage and reduced tillage probably because the soil ammonia content was similar under these treatments. The three urease activity pools were significantly correlated with soil organic C, microbial biomass C and N, and available P. These results indicate that both the intracellular and extracellular urease activities were sensitive indicators for monitoring soil quality under different tillage practices.