Phosphate-solubilizing bacteria facilitate rhizospheric processes of Bidens pilosa L. in the phytoremediation of cadmium-contaminated soil: Link between phosphorus availability and cadmium accumulation

Abstract

Although cadmium (Cd) hyperaccumulators have been widely used in phytoremediation of Cd-contaminated soils, the relationship between soil phosphorus (P) uptake and Cd accumulation during phytoremediation remains unclear. In this study, a phosphate-solubilizing bacterium (PSB), Enterobacter sp., and the Cd hyperaccumulator B. pilosa L. were selected to address this knowledge gap. Our results show that Enterobacter sp. inoculation enhances P cycling processes in the rhizosphere of B. pilosa L., resulting in an increase in soil available phosphorus (AP), by 16.2% to 84.3% in low-contaminated soil and by 17.6% to 64.8% in high-contaminated soil. Inorganic P solubilization was the primary process driving the increase in AP content, contributing the most to soil P cycling. Moreover, Enterobacter sp. inoculation significantly promoted the growth of B. pilosa L., boosting total phosphorus, phospholipids, primary metabolic phosphorus, and Cd concentrations in plant tissues. Notably, a strong positive correlation was observed between soil AP and Cd concentrations in plant tissues. P-functional microbes in the rhizosphere, encoding genes such as gcd, ppa, and ppx-gppA, predominantly enhance P bioavailability in soils. Furthermore, in P-deficient and heavily contaminated soils, Proteobacteria replaced Actinobacteria as the predominant hosts for key genes involved in soil P cycling. This study provides valuable insights into the critical link between P availability and Cd accumulation, emphasizing the role of P cycling in enhancing Cd accumulation during phytoremediation mediated by PSB.