JWSS - Journal of Water and Soil Science

The use of plants and soil microorganisms is a promising technique for the phytoremediation of heavy metal-contaminated soils. This study was carried out in order to evaluate the soil microbial potential with four Cd concentration levels (0, 10, 30 and 100 mg kg-1); the study also addressed the inoculation of arbuscular mycorrhizal fungi (AMF) species (a mixture of Glomus species including G. intraradices, G. mosseae and G. fasciculatum) as well as plant growth promoting rhizobacteria (PGPR) (a mixture of Pseudomonas species including P. putida, P. fluorescens, and P. aeruginosa) with the Centaurea cyanus plant. The soil sample was spiked uniformly with Cd nitrate salt to create different Cd concentrations. The contaminated soils were then sterilized and subsequently inoculated with AMF and PGPR. The results indicated that with increasing the soil Cd concentration, colonization percent, abundance of rhizobateria, shoot biomass, and shoot relative biomass were significantly decreased, while the  proline content and the shoot Cd concentration were significantly increased (P≤0.05). The mean of Cd extracted in AMF and PGPR treatments was 1.8 and 2.8 and the translocation factor was 1.2 and 1.5 times higher, as compared to the corresponding control treatments, respectively. It could be concluded that microbial inoculation, in addition to improving plant growth, plays an important role in the Cd phytoremediation efficiency by plant.

Cadmium (Cd) is of special importance among heavy metals because its toxicity to the plant is 20 times higher than other heavy metals. The present study was conducted to evaluate the trend of available soil Cd changes over time and its relationship with soil properties. Treatments consisted of 13 soil samples and two Cd rates (12.5 and 25 mg kg-1) as a factorial in a complete randomized design with two replications. DTPA extractable Cd was measured upon 5, 10, 20, 30, 60 and 90 days after adding Cd rates to the soils. The results showed that DTPA extractable Cd was increased as Cd application rates was raised in all soils. DTPA extractable Cd was decreased over time; however, at the end of the experiment, much of the added cadmium to the soil remained in use. Among the soil properties, calcium carbonate showed a significant negative correlation with DTPA extractable Cd in most of the incubation times in both Cd rates. DTPA extractable Cd also showed a significant negative correlation with pH and soil sand and a significant positive correlation with OC. Also, the results of the fitting of cadmium adsorption data with the kinetic equations showed that the exponential function equation was the most suitable kinetics descriptive equation for variations in cadmium adsorption in the studied soils.

Biochar as an efficient strategy for the improvement of soil properties and organic waste management may reduce the potential effects of abiotic stresses and increase soil fertility. However, the effects of this organic amendment on soil microbial indicators under combined salinity and pollution have not been studied yet. Therefore, the objective of this study was to evaluate the influence of sugarcane bagasse biochar on some soil bioindicators in a Cd-polluted soil under saline and non-saline conditions. A factorial experiment was carried out with two factors, including NaCl salinity (control, 20 and 40 mM NaCl) and sugarcane bagasse biochar (soils unamended with biochar, amended with uncharred bagasse, 400 ^oC biochar, and 600 ^oC) at 1% (w/w) using a completely randomized design. Results showed that salinity increased the mobility of Cd (12-17%), and subsequently augmented its toxicity to soil microorganisms as indicated by significant decreases in the abundance and activities of the soil microbial community. Conversely, sugarcane bagasse biochar application reduced the concentration of soil available Cd (14-18%), increased the contents of soil organic carbon (89-127%), and dissolved organic carbon (4-70%), and consequently alleviated the effect of both abiotic stresses on soil microbial community and enzyme activity. In conclusion, this experiment demonstrated that the application of sugarcane bagasse biochar could reduce the salinity-induced increases in available Cd and mitigate the interaction between salinity and Cd pollution on the measured soil bioindicators.

In addition to the distribution of elements in the soil solid phase, element species in the solution are also very important due to their importance of providing elements for root uptake. For a deeper study of the chemical cycle of elements in saline soils treated with biochar, the study of speciation is very useful and provides a method to reduce or transform the toxicity caused by toxic elements in saline soils. Therefore, to investigate the effect of biochar on Cd speciation in two saline calcareous soils, 15 mg kg^-1 Cd as cadmium chloride was added to the soil sample (200 g), and the soils were incubated for three weeks at 25±2 °C at 80% field capacity. After the incubation period, salinity levels of 20 and 40 mmol kg^-1 as sodium chloride (equal to 3.65 and 7.30 dS m^-1) were added to the soils. Then, the 1% (w/w) of the sugarcane bagasse and biochars produced at 400 and 600 °C were added to the soils, and then incubated for three months at 25±2 °C at 80% field capacity. At the end of the incubation period, for the speciation of Cd in the soil solution (in a 1 to 2 ratio), the concentration of dissolved cations and anions in the soil samples was measured. The results showed that the interaction between salinity, biochar, and soil on Cd²⁺, CdCl⁺, CdCl₂⁰, and Cd(SO₄)₂^2- was significant. The application of biochar in sandy soil reduced (p <0.05) the concentration of CdCl⁺, CdCl₂⁰, CdSO₄⁰, and CdOH⁺ species compared to the control soil, while it did not affect clay soil. Also, salinity caused by sodium chloride in sandy soil increased the concentration of CdCl⁺ and CdCl₂⁰ species and decreased CdSO₄⁰ and CdOH⁺ species compared to the control soil (p <0.05). The results showed that biochar in saline sandy soil was more effective than clay soil in reducing Cd toxicity.