Tillage-regulated impacts of engineered Fe/Zn-humic complexes on lead toxicity and soil biochemical health

The application of engineered humic Fe/Zn complexes in remediating lead (Pb) biotoxicity, as well as their impact on CO₂ efflux, soil carbon distribution, and spinach growth, remains unexplored. This study revealed the impact of engineered Fe- and Zn-enriched humate complexes on Pb immobilization, geochemical fractionation, and translocation in a spinach crop, and the effects on soil biochemical health and CO₂-C efflux from tillage and no-tillage Pb-contaminated soils in closed chambers. Advanced statistical models like PLS-PM were employed to determine the direct and total effects of the applied amendments under different tillage regimes. Results demonstrated that Zn- and Fe-humate applications decreased Pb contents by 48% and 72% in tillage soil, while under no-tillage soil Pb concentration was reduced by 35%; 8.7 mg kg^-1 (Zn-humate) vs 33%; 5.9 mg kg^-1 (Fe-humate) compared to respective controls. Soil C distribution showed dramatically varying trends in tilled and no-tilled soils. Soil extracellular enzyme activity was enhanced under both tillage and no-tillage operations with Zn- and Fe-humate complexes. Human-associated health risk was reduced by 3-fold by the application of Fe-humate, and it was 2-fold with Zn-humate. Overall, findings revealed that the tillage-driven application of Fe/Zn humate complexes significantly enhanced spinach growth, reducing Pb bioavailability, highlighting their potential for sustainable soil management in contaminated farmlands.