Contents lists available at ScienceDirect Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv Interactive effect of salinity and cadmium toxicity on soil microbial properties and enzyme activities Fayez Raiesi ⁎ , Ellham Sadeghi Department of Soil Science and Engineering, Faculty of Agriculture, Shahrekord University, P.O. Box 115, Shahrekord, Iran ARTICLE INFO Keywords: Multiple stresses Microbial functions Cd toxicity Salinity Organic amendments ABSTRACT Salinity has been proposed to increase the mobility and availability of heavy metals, with a potentially sig- nificant 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 re- sponse 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. 1. Introduction Soil as a natural and heterogeneous ecosystem is often simulta- neously exposed to multiple man-made or environmental stressors (Schimel et al., 2007; Holmstrupa et al., 2010; Suzuki et al., 2014; Schaeffer et al., 2016). Conceptually, biotic and abiotic stressors may operate independently or interactively in nature, and potential inter- actions between multiple stressors fall into three categories: synergistic, antagonistic or additive under natural conditions (Crain et al., 2008; Suzuki et al., 2014). The effects of abiotic stressors are often worse or synergistic when these stressors are combined (Crain et al., 2008; Holmstrupa et al., 2010; Suzuki et al., 2014). The way multiple stressors affect natural ecosystems (e.g., soil) is important for their management and protection (Schaeffer et al., 2016). Therefore, it is necessary to understand and account for the interactive effects of multiple stressors on microbial community and functions in soil ecological studies (Schimel et al., 2007; Schaeffer et al., 2016). Soil microbial community and enzyme activities, as key indicators of soil quality and health, are directly involved in important biogeochemical processes and ecosystem functions (Brookes, 1995; Raiesi, 2006; Schimel et al., 2007; García- Ruiz et al., 2008). Both salinity and toxic metal (TM) pollution are the two most major environmental stresses affecting negatively the func- tions and activities of soil microbial community (Kızılkaya et al., 2004; Tripathi et al., 2006; Yuan et al., 2007). These environmental stresses are serious threats and factors of soil quality degradation worldwide, and are predicted to increase in the future, particularly in arid en- vironments (Metternicht and Zinck, 2003). It has been showed that microbial properties and functions are well-established soil quality in- dicators to monitor the impact of environmental stresses induced by metal pollution in most ecosystems (Brookes, 1995; Kızılkaya et al., 2004; Renella et al., 2005) and salinity stress in dry-land ecosystems (Tripathi et al., 2006; Yuan et al., 2007; Boyrahmadi and Raiesi, 2018). Although plant responses to the combined effects of TM and salinity have been studied extensively (Usman et al., 2005; Ghallab and Usman, https://doi.org/10.1016/j.ecoenv.2018.10.079 Received 22 June 2018; Received in revised form 18 October 2018; Accepted 22 October 2018 ⁎ Corresponding author. E-mail addresses: raiesi@sku.ac.ir, f_raiesi@yahoo.com (F. Raiesi). Ecotoxicology and Environmental Safety 168 (2019) 221–229 0147-6513/ © 2018 Elsevier Inc. All rights reserved. T