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

SoilScience • Volume 175, Number 11, November 2010 Urease is one of the most thoroughly studied soil enzymes because it greatly influences the fate and performance of a widely used fertilizer (urea) (Bremner and Mulvaney, 1978; Ma et al., 1999; Tate, 2002). Urea was reported to be taken up by microorganisms, such as Corynebacterium glutamicum and Paxillus involutus (Jahns and Kaltwasser, 1989; Beckers et al., 2004), or by natural bacterial assemblages (Jorgensen, 2006) and used as their nitrogen source. Urea can be hydrolyzed by both intracellular and extracellular ureases. The intracellular and soluble ureases are considered to be more labile than the stabilized extracellular urease (Gianfreda et al., 1994; Qin et al., 2010). Therefore, differentiating the intracellular from the extracellular urease activity could provide a better understanding of the mechanisms of microbial urea hydrolysis. The chloroform fumigation method (soil samples are fumigated with chloroform at 25°C for 24 h in the dark) is widely used to lyse microbial cells in soils for microbial biomass determination (Brookes et al., 1985). Klose and Tabatabai (1999) successfully used this method to separate the intracellular urease activity from the extracellular urease activity. They assumed that the urease activity in fumigated soil represents the sum of intracellular and extracellular urease activities, whereas that in nonfumigated soil represents only the extracellular urease activity (Klose and Tabatabai, 1999,2000). The intracellular urease activity was obtained by subtracting the extracellular activity from the total activity values (Klose and Tabatabai, 1999,2000). It was observed that the intracellular urease activity ranged from 37.1% to 73.1% (averaged 54%) of the total urease activity in 10 soil types in Iowa (Klose and Tabatabai, 1999). However, the intracellular and total urease activities determined by the chloroform fumigation method could be underestimated because of urease denaturation by chloroform (Dilly and Nannipieri, 1998) and urease degradation by soil proteases (Renella et al., 2002). The objectives of this study were to (i) improve the chloroform fumigation method for intracellular and extracellular urease activity determination; and (ii) study the responses of the intracellular and extracellular urease activities to 8-year no-tillage, reduced tillage, and moldboard plowing in the North China Plain.