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Influence of Salinity on Soil Enzyme Activities¹

ABSTRACT

Efforts were made to assess the levels of soil enzyme activities that have a specific role in the N, C, P, and S cycles of saline soils. Fieldmoist soil samples were treated with four rates of CaCl₂, NaCl, and Na₂SO₄ solutions applied to produce electrical conductivity readings of saturation extracts (EC_e) ranging up to 22 mmho/cm. The range of EC_e values included threshold salinity levels associated with reduced yields of agronomic crops. After 7 d of equilibration, the following soil enzymes were assayed: amidase, urease, acid phosphatase, alkaline phosphatase, phosphodiesterase, inorganic pyrophosphatase, arylsulfatase, rhodanese, -glucosidase, -galactosidase, dehydrogenase, and catalase.

Soil enzyme activities decreased with increasing EC_e; however, the degree of inhibition varied among the enzymes assayed and the nature and amounts of salts added. Dehydrogenase activity was severely inhibited by salinity, whereas, the hydrolases showed a much lesser degree of inhibition. Generally, the inhibition of soil enzyme activities by the salt solutions decreased in the following order when compared at the same EC_e level: NaCl > CaCl₂ > Na₂SO₄. Reduced enzyme activities in saline soils may be due to the osmotic desiccation of microbial cells releasing intracellular enzymes which become vulnerable to attack by soil proteases, a "salting-out" effect modifying the ionic conformation of the active site of the enzyme-protein, and specific ion toxicities causing nutritional imbalances for microbial growth and subsequent enzyme synthesis.

¹ Contribution from the Dep. of Soil and Environmental Sci., Univ. of California, Riverside CA 92521. Presented before Div. S-3, Soil Sci. Soc. Am., Atlanta, Ga., 2 Dec. 1981.

² Assistant Professor and Professor, respectively, Univ. of California, Riverside.

Received for publication November 16, 1981. Accepted for publication July 30, 1982.

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