Influence of Salinity on Soil Enzyme Activities

Abstract

AbstractEfforts 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 (ECe) ranging up to 22 mmho/cm. The range of ECe 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 ECe; 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 ECe 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.