Soil aggregate-specific heterogeneity in humic acid governs cadmium and lead fractionation and bioavailability

While humic acids (HAs) are known to modulate heavy metal behavior, their profound heterogeneity across soil aggregate fractions remains overlooked. Crucially, whether HA origin within distinct soil aggregates differentially governs heavy metal speciation and bioavailability is unknown-creating a critical knowledge gap for targeted soil remediation. This study deciphers, for the first time, how aggregate-specific HAs control cadmium (Cd) and lead (Pb) dynamics. HAs were extracted from four aggregate fractions: coarse particulate (>250 μm), microaggregate (53-250 μm), silt (2-53 μm), and clay (<2 μm). Comprehensive characterization revealed HAs from finer fractions (silt, clay) possessed higher aromaticity, molecular weight, and humification than coarser fractions. A soil incubation experiment with the European Community Bureau of Reference (BCR) sequential extraction demonstrated that finer-fraction HAs promoted Cd and Pb transformation into residual forms, significantly reducing acid-exchangeable Cd (up to 54 %) and Pb. In vitro bioaccessibility assays showed contrasting effects: coarser-fraction HAs increased Cd and Pb bioaccessibility by 40-54 %, while finer-fraction HAs decreased it by similar magnitudes. Statistical analysis identified the acid-exchangeable and reducible metal fractions as key bioavailability determinants. Essentially, we demonstrate that HA efficacy in metal immobilization is inherently aggregate-specific-fine-aggregate HAs achieve superior metal immobilization due to their condensed aromatic structures. This mandates a paradigm shift: soil remediation must target fine-aggregate-derived organic amendments to optimize heavy metal immobilization. Our work resolves long-standing unpredictability in HA-mediated remediation and provides a molecular blueprint for precision soil management.