Precipitation versus dissolution at the phosphorus and Solanum nigrum L. root exudate interface: Implications for lead rebound in rhizosphere soils

Lead (Pb) rebound in the root-zone of plants is a primary risk associated with rhizosphere effect in phosphorus (P)-amended vegetated soils. Herein, we investigated the differential effects of root exudates (RE) from Solanum nigrum L. on Pb availability under conditions with variable P mineral forms. Batch precipitation-dissolution tests verified the addition of RE to the soils boosted Pb release and lowered the utilization efficiency of P as a result of favoring Pb retention in soil solid phases. Typically, oxalate, selected as a representative RE composition, significantly enhanced Pb release after being immobilized with highly soluble phosphate (NaH₂PO₄), but no detectable Pb for the treatments with low P-solubility hydroxypyromorphite (Pb₅(PO₄)₃OH). Sequential extraction results revealed that a portion of released Pb was re-adsorbed onto the soil surfaces after adding P, while the presence of oxalate increased the dissolution risk associated with residual Pb and organic P due to the soil pH-induced effects. Further morphological analyses revealed that the solid-to-liquid partitioning of Pb was largely dependent upon the P-induced precipitation and RE-induced dissolution reactions. Both P and RE reduced the negative effect of Pb on soil microbial activity by recruiting more plant-growth-promoting and reactive oxygen species-generating genera, e.g., Firmicutes and Actinobacteria, thus facilitated microbial resistance to Pb in near-rhizosphere soil. These findings highlight the role of P-based treatments in altering the rhizosphere-mediated Pb rebound with biotic and abiotic interaction for remediation of Pb-contaminated soils.