Interactive Effects of Salinity and Cadmium Pollution on Microbial Respiration and Biomass in a Calcareous Soil Treated with Plant Residue

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. Anthropogenic activities have increased the concentration of heavy metals and soluble salts in soil, resulting in a major constrain for soil microbial performance and functions. Furthermore, soil microbial activity and biochemical processes are often limited by substrate availability in arid areas due to the low organic inputs. Although the individual effects of salinity and metal toxicity stresses on soil biological activity are generally well-known, their combined effects on microbial growth, population and functions are largely uncertain. The main aim of this study was to investigate the interactive effects of salinity and cadmium (Cd) Pollution on microbial respiration and biomass in a calcareous soil treated with plant residue. It was hypothesized that salinity would increase mobility and availability of Cd with subsequent reductions in microbial activity and biomass, and that addition of plant residue would modify these salinity effects.
This study was conducted under controlled laboratory conditions at Shahrekord University. A factorial experiment with two levels of cadmium (0 and 30 mg kg-1), three levels of salinity (1.35, 7.5 and 10 dS m-1) and two levels of plant residue (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. Next, the samples were incubated at 25±1 oC for 98 days. Soil C mineralization (microbial respiration) was measured weekly, and available Cd and microbial biomass carbon were measured at monthly intervals. In this experiment, the Bliss independence model was used to determine the type and nature of the interaction between salinity and pollution (synergistic and antagonistic).
The results showed that NaCl salinity increased the concentration of soil available Cd in both polluted and unpolluted soils over the experimental period, and the increases were greater at high than low salinity levels. This effect of salinity was less pronounced in residue-amended and unamended soils. In general, a strong synergistic effect of both stresses was observed on Cd availability in residue-unamended soils while this effect was mostly antagonistic in residue-amended soils. This indicates addition of plant residue to enhance soil organic matter may indirectly repress or lower salinity effect on Cd toxicity. Soil salinity decreased microbial biomass carbon and respiration with subsequent increases in specific respiratory quotient due to the increases in Cd solubility and availability. However, the changes in microbial properties were much lower in residue-amended and unamended soils. Addition of plant residue decreased the negative effects of both the individual and combined salinity and Cd pollution on microbial biomass and respiration. The interactive effect of these two stresses was mainly synergistic in residue-treated soils while it was antagonistic in residue-untreated soils. Overall, a strong synergistic effect occurred when both stresses were combined in the absence of plant residue while this effect was antagonistic in the presence of plant residue.
This study provided evidence that salinity could synergistically increase the mobility, bio-availability, and toxicity of soil Cd in Cd-polluted soils with carbon limitation. This was reflected by synergistic reductions in soil microbial biomass and respiration. However, addition of plant residue to increase soil organic matter lowered this effect of salinity, resulting in the antagonistic effects of salinity and pollution on soil microbial biomass and respiration. The reason for increase in the microbial activity in soils treated with plant residue was the increase of available substrate. As a result, using the plant residue increased the stimulatory effect of microbial activity. These findings point to the importance of providing adequate organic residues to enhance soil microbial performance and agricultural sustainability in polluted soils under salinity stress. However, further information on responses of microbial indicators to the joint effect of salinity and Cd toxicity is required.