Soil aggregates as functional units for cadmium sequestration: Differential regulation by nitrogen enrichment and labile carbon inputs.

While cadmium (Cd) speciation in soil is known to control its environmental risk, how nitrogen (N) enrichment and labile organic carbon (LOC) inputs redistribute Cd fractions within soil aggregates remains unclear. This study examined how ammonium enrichment (AT), nitrate enrichment (NT), and glucose input (CT) altered carbonate-bound Cd (CB-Cd) and organic matter-bound Cd (OM-Cd) within soil aggregates. Both N enrichment and glucose input enhanced CB-Cd formation, with CT increasing CB-Cd by 39.19% via stimulated microbial activity and carbonate precipitation. Different sources of enriched N regulated OM-Cd, with AT decreasing OM-Cd by 15.55%, and NT increasing OM-Cd by 24.61%. This was attributed to competitive adsorption and suppressed microbial decomposition of recalcitrant organic matter. Aggregate hierarchy was also crucial in determining Cd speciation, where macroaggregates, with higher LOC and genes involved in carbonate precipitation, favored CB-Cd partitioning, whereas microaggregates, with greater surface area and enriched alkyl/aromatic C, served as the major OM-Cd sink. Microbial community analysis revealed that glucose reshaped communities, enriching r-strategists like Amycolatopsis and Trichoderma, which were positively correlated with CB-Cd and OM-Cd. Metagenomic data indicated that glucose stimulated genes for labile C degradation, reinforcing CB-Cd formation, while N addition suppressed C-degradation genes. Random forest and PLS path models identified alkyl C, O-alkyl C, and polysaccharide derivatives as primary SOC components regulating CB-Cd, while alkyl C, phenolic, and aromatic compounds regulating OM-Cd. These findings reveal a mechanism for stabilizing Cd in less bioavailable fractions via SOC and N management, leveraging soil aggregates' role in long-term metal sequestration.