Redistribution of cadmium in soil aggregates under continuous carbon and nitrogen inputs: insights from sequential extraction and modeling

Continuous carbon (C) and nitrogen (N) inputs significantly affect cadmium (Cd) redistribution in soil aggregates, yet their impacts remain poorly understood. This study investigates Cd redistribution under labile C and two N sources (glucose + nitrate [CN], glucose + ammonium [CA], and glucose alone [CT]). CN and CA treatments increased Fe and Mn oxide-bound Cd (F3-Cd) by 99.7% and 38.7% in bulk soil, respectively, while CT reduced F3-Cd by 33.1%. Increased dissociative Fe oxides (DCB-Fe) and decreased Fe²⁺, coupled with NO₂^- consumption, confirmed enhanced NO₂^- reduction and Fe²⁺ oxidation in F3-Cd formation. Carbonate-bound Cd (F2-Cd) and organic matter-bound Cd (F4-Cd) also increased significantly (42.0-121.5%) across all treatments. Feature importance analysis highlighted dissolved organic carbon (DOC) as a key driver for F2-Cd, while DOC, amorphous Fe (oxalate-Fe), and soil organic carbon (SOC) influenced F4-Cd. Micro-aggregates (MAs) had higher F4-Cd levels compared to large macro-aggregates (LMAs) and small macro-aggregates (SMAs). Partial least squares path modeling showed that DOC influenced F2-Cd in LMAs, nitrate and ammonium cycling affected F3-Cd in SMAs, and genes related to Fe cycling and nitrification drove F4-Cd in MAs, potentially impacting mineral-associated SOC. Understanding C and N inputs' effects on Cd redistribution can improve remediation strategies for Cd pollution in agricultural soils.