EFFECTS OF SALINITY ON BIOLOGICAL ACTIVITIES IN SOILS (PHOSPHATASE, UREASE, SULFATASE)

A fast and sensitive method to determine urease activity in soils is described. It involves extraction of NH(,4)-N with 1 N KCl and measurement of NH(,4) concentration by electroconductivity using a steady state flow system across a membrane.;Evidence is presented that free soil phosphatase is bonded to humic substances in the form of electron donor-acceptor complexes, the humic substances being the acceptor.;A laboratory study determined the effects of salinity on nitrogen mineralization and microbial biomass in soils, and on free soil enzyme activities, including urease, sulfatase and phosphatase. Salts applied were NaCl and CaCl(,2). Application was by column leaching, field irrigation with high Na water, direct addition, and addition with pre-incubation of up to 77d. Maximum salt concentrations used were 26.7 meq/100 g for the column application, 5.7 meq/100 g for the field application, 1800 mM NaCl or 1047 mM CaCl(,2) for the direct application, and 34.2 meq/100 g for the pre-incubation experiments.;Both enhancement and inhibition of biological activities by NaCl and CaCl(,2) were observed. Column application produced enhancement if the leaching was followed by a 2 weeks drainage period. Direct application showed inhibition. A connection between microbial biomass and phosphatase activity could not be observed. On soil-free preparations of phosphatase, NaCl had only insigificant effects.;A substance with phosphatase activity was isolated from a field soil. The isolation procedure involves two steps. First ammonium humates are formed by intermittent shaking of soil in 1.0 N ammonium acetate (pH 7.0) for 20 hours. They are extracted by repeated rinsing with distilled water, after the ammonium acetate has been removed by filtration. The extract is dialyzed and freeze-dried. Then the phosphatase-active part is isolated from the humic substances by de-complexation with 17.0 mM tetramethyl-p-phenylenediamine (TMePDA).;A model is proposed, according to which salt concentration influences enzyme activities indirectly. The mechanism proposed involves pH variations in the diffuse double layer due to changes in ionic strength, and control of enzyme activity by the pH in the double layer and the bulk solution.