Impact of phosphorus amendments on cadmium uptake by sunflower inoculated with Rhizophagus irregularis mycorrhizae in a contaminated soil

Immobilization of heavy metals (HMs) by phosphorus compounds is an efficient and cheap technique in decreasing their phytoavailability in soil, depending highly on the type and rate of HMs. Greenhouse research was performed to evaluate the impact of various amendments on cadmium (Cd) absorbed by sunflower plants in HM-contaminated soil collected from a mining area. The experiment was performed as a randomized complete block design with two factors, namely mycorrhizal fungi (Rhizophagus irregularis) and amendments (di-ammonium phosphate, humic acid, bone meal, and humic acid +bone meal), in three replications. The results showed that applying all the amendments reduced the amount of soluble Cd and Cd2+ species in the soil. The highest decreases occurred with the di-ammonium phosphate treatment (51% for soluble Cd), probably through decreasing pH and increasing phosphorus solubility, as a consequence, forming insoluble Cd phosphates. Mycorrhizal inoculation significantly decreased (p≤0.05) the Cd in the soil solution and the plant shoot but increased it in the plant root, probably through Cd accumulation in the fungal hyphae as Cd phosphates. The use of phosphate compounds is strongly recommended in order to immobilize Cd in highly contaminated sites.