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Michaelis Constants of Urease in Soils and Soil Fractions¹

ABSTRACT

Studies to determine the Michaelis constant (K_m value) for urease activity in Iowa surface soils showed that the value obtained for the activity of this enzyme was different for different soils. When the method used to determine K_m did not involve shaking of the soil-substrate mixtures, the K_m value for urease activity ranged from 1.3 x 10^-3M to 7.0 x 10^-3M. Shaking the soil-substrate mixtures during incubation decreased the K_m value obtained, and statistical analysis showed that this value was significantly correlated with organic C and total N. There was no significant correlation, however, between K_m value of the urease activity and other soil properties studied (pH, percentage clay, percentage silt, percentage sand). The maximum enzyme reaction velocity (V_max value) for urease activity in soils was markedly different for different soils and always increased when the soil-substrate mixtures were shaken during incubation. The V_max values obtained from static and shaking incubation techniques were significantly correlated with organic C, total N, and clay content.

Results reported show that the K_m value of urease in soils is similar to that of urease in soil fractions and is nearly two orders of magnitude lower than the previously reported K_m value for this enzyme in soils.

¹ Contribution from the Iowa Agr. & Home Econ. Exp. Sta., Ames as Journal Paper no. J-7480. Project no. 1845.

² Assistant Professor, Dep. of Agronomy, Iowa State Univ., Ames.

Received for publication January 19, 1973. Accepted for publication May 17, 1973.