Influence of Salinity on Soil Enzyme Activities

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 CaCl 2 , NaCl, and Na 2 SO 4 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, α‐glucosidase, α‐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 > CaCl 2 > Na 2 SO 4 . 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.