Effect of the Combination of Phosphate-Solubilizing Bacteria with Orange Residue-Based Activator on the Phytoremediation of Cadmium by Ryegrass

Comprehensive bioremediation effect of phosphorus-mineralized bacterium Enterobacter sp. PMB-5 on cadmium contaminated soil-crop system

Phosphate-mineralizing bacteria (PMBs) have been widely studied by inducing phosphate heavy metal precipitation, but current researches neglect to study their effects on soil-microbe-crop systems on cadmium (Cd) contaminated. Based on this, a strain PMB, Enterobacter sp. PMB-5, was inoculated into Cd contaminated pots to detect soil characteristics, Cd occurrence forms, soil biological activities, plant physiological and biochemical indicators. The results showed that the inoculation of strain PMB-5 significantly increased the available phosphorus content (85.97%-138.64%), Cd-residual fraction (11.04%-29.73%), soil enzyme activities (31.94%-304.63%), plant biomass (6.10%-59.81%), while decreased the state of Cd-HOAc (11.50%-31.17%) and plant bioconcentration factor (23.76%-44.24%). These findings indicated that strain PMB-5 could perform the function of phosphorus solubilization to realize the immobilization of Cd in the complex soil environment. Moreover, SEM-EDS, FTIR, XPS, and XRD analysis revealed that strain PMB-5 does not significantly alter the soil morphology, structure, elemental distribution, and chemical composition, which suggested that remediation of Cd contamination using strain PMB-5 would not further burden the soil. This research implies that PMB-5 could be a safe and effective bioinoculant for remediating Cd-contaminated soils, contributing to the sustainable management of soil health in contaminated environments.

Effect of various phosphorus levels on the extraction of Cd, the transformation of P, and phosphorus-related gene during the phytoremediation of Cd contaminated soil

Phytoremediation has emerged as a common technique for remediating Cd pollution in farmland soil. Moreover, phosphorus, an essential element for plants, can alter the pectin content of plant cell walls and facilitate the accumulation of Cd in plant tissues, thereby enhancing phytoremediation efficiency. Therefore, pot experiments were conducted in order to investigate the effect of phosphorus levels on Cd extraction, phosphorus transformation and phosphorus-related genes during phytoremediation. The results revealed that an optimal application of suitable phosphate fertilizers elevated the soil's pH and electrical conductivity (EC), facilitated the conversion of soil from insoluble phosphorus into available forms, augmented the release of pertinent enzyme activity, and induced the expression of phosphorus cycling-related genes. These enhancements in soil conditions significantly promoted the growth of ryegrass. When applying phosphorus at a rate of 600 mg/kg, ryegrass exhibited plant height, dry weight, and chlorophyll relative content that were 1.27, 1.26, and 1.18 times higher than those in the control group (P0), while the Cd content was 1.12 times greater than that of P0. The potentially toxic elements decline ratio and bioconcentration factor were 42.86% and 1.17 times higher than those of P0, respectively. Consequently, ryegrass demonstrated the highest Cd removal efficiency under these conditions. Results from redundancy analysis (RDA) revealed a significant correlation among pH, total phosphorus, heavy metal content, phosphorus forms, soil enzyme activity, and phosphorus-related genes. In conclusion, this study suggests applying an optimal amount of suitable phosphate fertilizers can enhance restoration efficiency, leading to a reduction in soil Cd content and ultimately improving the safety of crop production in farmlands.

Effects of Priestia aryabhattai on Phosphorus Fraction and Implications for Ecoremediating Cd-Contaminated Farmland with Plant-Microbe Technology

The application of phosphate-solubilizing bacteria has been widely studied in remediating Cd-contaminated soil, but only a few studies have reported on the interaction of P and Cd as well as the microbiological mechanisms with phosphate-solubilizing bacteria in the soil because the activity of phosphate-solubilizing bacteria is easily inhibited by the toxicity of Cd. This paper investigates the phosphorus solubilization ability of Priestia aryabhattai domesticated under the stress of Cd, which was conducted in a soil experiment with the addition of Cd at different concentrations. The results show that the content of Ca2-P increased by 5.12-19.84%, and the content of labile organic phosphorus (LOP) increased by 3.03-8.42% after the addition of Priestia aryabhattai to the unsterilized soil. The content of available Cd decreased by 3.82% in the soil with heavy Cd contamination. Priestia aryabhattai has a certain resistance to Cd, and its relative abundance increased with the increased Cd concentration. The contents of Ca2-P and LOP in the soil had a strong positive correlation with the content of Olsen-P (p < 0.01), while the content of available Cd was negatively correlated with the contents of Olsen-P, Ca2-P, and LOP (p < 0.05). Priestia aryabhattai inhibits the transport of Cd, facilitates the conversion of low-activity P and insoluble P to Ca2-P and LOP in the soil, and increases the bioavailability and seasonal utilization of P in the soil, showing great potential in ecoremediating Cd-contaminated farmland soil with plant-microbe-combined technology.