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Amidase Activity in Soils: III. Stability and Distribution¹

ABSTRACT

Activities of soil enzymes are affected by methods of handling, storing, and pretreating the sample before enzyme assay. Amidase was recently detected in soils, and this study was carried out to assess the factors affecting its stability and distribution. Results showed that storage of field-moist samples at 5°C for 3 months decreased the activity in five soils by an average of 4%. Air-drying field-moist samples resulted in decreases of amidase activity ranging from 14 to 33% (avg = 21%). Freezing of field-moist samples at –20°C for 3 months resulted in activity increases ranging from 3 to 16% (avg = 9%). Heating of field-moist and air-dried samples for 2 hours before assay of amidase activity showed that this enzyme is inactivated at temperatures above 50°C. The effects observed were similar for the three substrates (formamide, acetamide, and propionamide).

Studies of distribution of amidase in soils showed that it is concentrated in surface soils and decreases with depth. Statistical analysis indicated that the activity of this enzyme is significantly correlated with organic C in surface soils (r = 0.74***) and in soil profiles (r = 0.89**). Amidase activity also was significantly correlated with percentage N (r = 0.74***), percentage clay (r = 0.69***), and urease activity (r = 0.73***) in the 21 surface soil samples studied. There was no significant relationship between amidase activity and soil pH or percentage sand. Amidase activity and microbial counts obtained with acetamide or propionamide as a substrate in the absence of toluene indicated that these substrates induce production of this enzyme by soil microorganisms.

¹ Journal Paper no. J-9879 of the Iowa Agric. Home Econ. Exp. Stn., Ames, Iowa. projects 2082 and 2112.

² Graduate Research Assistant and Professor of Soil Biochemistry respectively, Dept. of Agron., Iowa State Univ., Ames, IA 50011. The senior author currently is Assistant Professor, Dept. of Soil & Environ. Sci., Univ. of California, Riverside, CA 92521.

Received for publication May 27, 1980. Accepted for publication November 26, 1980.