Salinization Depresses Soil Enzyme Activity in Metal-polluted Soils Through Increases in Metal Toxicity

Salinity may increase metal mobilization and toxicity with a potentially significant consequence for soil enzymatic activity and nutrient cycling. The goal of this study was to investigate changes in soil enzyme activity in response to salinization of a clay loam soil artificially polluted with cadmium (Cd) and lead (Pb) during an incubation experiment. Soil samples were polluted with Cd, Pb, and a combination of Cd and Pb, pre-incubated for aging, and then salinized with three levels of NaCl solution, and were finally incubated for 120 days. NaCl salinity consistently increased the mobilization of Cd and Pb with greater increases at high than low salinity levels. While the increased Cd mobilization was greater in co-polluted than Cd-polluted soils, the increase of Pb mobilization was lower in co-polluted than Pb-polluted soils at high salinity level. The salinity-induced increases in metal mobilization and toxicity significantly depressed soil microbial respiration, microbial biomass content and enzymatic activities. The increased soil electrical conductivity, Cd mobilization and pH after salinization were the most important factors governing microbial activity and biomass in metal-polluted soils. Changes in microbial biomass and mobile metal pool with increasing salinity had the major effects on enzyme activities, particularly under the combined metals. Secondary salinization of metal polluted soils would impose an additional toxicity stress on enzymatic activities as biochemical indicators of soil quality, and therefore should be avoided for the maintenance of soil microbial and biochemical functions, especially in arid regions. In metal-polluted soils, the observed responses of enzymes to salinity can be used to advance our knowledge of microbial processes when modelling the carbon and nutrient cycling.