Interactive effect of salinity and cadmium toxicity on soil microbial properties and enzyme activities

Salinity has been proposed to increase the mobility and availability of heavy metals, with a potentially significant consequence for greater metal toxicity. However, the interactive effect of salinity and metal pollution on soil microbial properties and functions is still unknown. This investigation was performed to examine the response of several soil microbial properties and processes to the combined salinity and cadmium (Cd) toxicity in a clay loam soil amended with plant residue. The NaCl salt (0, 32.5 and 78.3 mM NaCl kg^-1 soil), Cd (0 and 30 mg kg^-1 soil) and alfalfa residue (0 and 1%) were added to the soil and the mixtures were incubated for 90 days under standard laboratory conditions (25 ± 1 °C and 70% of water holding capacity). Similar treatments without residue addition were also included in the experimental arrangement. Salinity increased soil Cd availability and toxicity, and subsequently decreased soil microbial respiration rate, microbial biomass and enzyme activity. The negative effect of increasing salinity on soil microbial and biochemical properties was stronger in Cd-polluted than unpolluted soils and at high than low salinity levels. The declines in soil microbial attributes and enzyme activity were linearly related to the concentration of soil available Cd. Nevertheless, the negative effect of salinity was reduced with addition of alfalfa residue in Cd-polluted soils. The interactive effect of Cd and NaCl was synergistic in residue-unamended soils, but antagonistic in residue-amended soils. It is concluded that (i) the multiple stresses induced by salinity and Cd pollution may synergistically affect soil microbial processes and attributes and (ii) application of organic residues has a high potential for lowering the synergistic effect of salinity in Cd-polluted environments and improving the important microbial indicators of soil quality.