Effects of Salinity and Plant Residue on Enzyme Activity and some Mineral Element Concentrations in a Cadmium-Polluted Soil under Laboratory Condition

Soil, as an important component of terrestrial ecosystems, plant growth media, and a habitat of diverse living organisms commonly encounters a variety of abiotic stresses. Soil microorganisms play an important role in maintaining soil quality and functioning, since they are responsible for the decomposition of dead organic material, nutrient cycling and degradation of hazardous organic pollutants. Metal toxicity and salinity are the major abiotic stresses affecting soil microbial activity and community structure in many areas of the world, in particular arid regions. Salinity may prompt the negative influences of soil pollution on soil microbial activity. On the other hands, application of organic amendments to saline soils or toxic metal-polluted soils may alleviate the negative consequences of these two abiotic stresses on soil microbial activity and population.

This study was conducted under controlled laboratory conditions at Shahrekord University. The main aim of this study was to investigate the effects of salinity and plant residue as an organic amendment on concentrations of soluble elements, enzyme activity and substrate-induced respirations (SIR) in a calcareous soil polluted with cadmium (Cd) over a three-month incubation experiment. A factorial experiment with two levels of cadmium (0 and 30 mg kg-1), three levels of salinity (1.35, 7.5 and 15 dS m-1) and plant residue treatments (with and without alfalfa residue) was conducted using a completely randomized design with four replications. Using cadmium chloride salt, the soil was contaminated, and subsequently amended with alfalfa residue (1%, w/w). After thorough mixing of soil and plant residue, salinity treatments were applied using NaCl salt. To reactivate the microbial population and for the aging effect, soil moisture was set at 70% of field capacity, and containers were pre-incubated at room temperature for 4 weeks. The samples were then incubated at 25±1 oC for 98 days.

Increasing salinity levels resulted in an increase in soluble Na concentration, a reduction in soil enzyme activity and SIR, and a decline in the concentrations of soluble Ca, Mg and K. The harmful impact of soil salinity on microbial properties was much greater in Cd-polluted soils than unpolluted soils.

Results indicated that addition of plant residue reduced the negative impacts of salinity and Cd pollution stresses on soil microbial and enzyme activity, and that resulted in increases in the concentrations of water soluble elements.The findings of the current study confirmed that application of adequate organic amendments can decrease Cd toxicity, enhance substrate availability and maintain soil microbial and enzyme activity in saline and polluted soils with substrate limitation.