Water-extractable metals as indicators of wheat metal accumulation: Insights from Cd, Pb, Mn, Cu, and Zn

There is a long-standing debate over the effectiveness of chemical extraction methods in assessing soil metal phytoavailability. This study addresses the limitations of widely-used chemical extraction methods and presents the water-extractable pool as a more reliable indicator based on wheat pot experiments using homogenized agricultural soil amended with lime materials, phosphate, and biochar. Over 120 days' pot experiments, Cd accumulation in whole wheat plants and tissues exhibited positive relationships with water-extractable Cd concentrations at heading and maturity stage (Spearman's rho: 0.521-0.851; P < 0.05), revealing that the water-extractable pool instead of other pools better indicates wheat metal accumulation. Water-extractable metal concentrations are effective in assessing phytoavailability of metals primarily in ionic forms in soil solution (e.g, Zn, Cd), but less reliable for metals strongly complexed with dissolved organic matter (DOM) or sensitive to redox conditions. It demonstrated that water-extractable metal concentrations and chemical forms are key factors, fundamentally determined by metal properties and impacted by environmental factors. This study clarifies a more direct link between chemical extraction and plant metal uptake mechanisms. Given the extensive application of chemical extraction methods over several decades, this study will help advance soil metal risk assessment and remediation practices.