Roots accelerate oxidation and dissolution processes of galena and shift microbial structure under phytostabilization

Phytostabilization is a promising natural-based remediation technique that immobilizes heavy metals in the rhizosphere, preventing their leaching into soil and groundwater. This study aims to investigate how phytostabilization plant roots impact the geochemical alteration and mineralogical transformations of galena, as well as the response of microbial communities. Microspectroscopic analysis and sequential extraction indicated that root activity accelerates the oxidation and dissolution of galena. These processes can be achieved by root-induced macropores and oxygen secretion. Meanwhile, organic acids secreted by roots can attack the crystal structure of galena or release Fe(III) from clay minerals through acidification. Galena acts as an electron donor and transfers electrons to Fe(III), accelerating its dissolution. The dissolved Pb(II) is immobilized by carbonates produced by plant roots and amorphous Fe/Al (oxyhydr)oxides resulting from the root-induced dissolution of bearing-Fe/Al minerals, thus not increasing the bioavailability of heavy metals. As plants colonization, the environmental factors affecting microbial community structure shift from pH and Pb fractions (CaCl2Pb, NaClOPb, NaOACPb, and KClO3Pb) to cation exchange capacity (CEC). This shift may occur because the increase in CEC associated with plant colonization, which reduces the toxicity of heavy metals to bacteria by adsorbing them. Additionally, alfalfa accumulates twice as much Pb as ryegrass. These findings provide a comprehensive perspective for developing more effective phytostabilization techniques and bacterial community assembly processes for soils contaminated with metal sulfide minerals.