Asian Journal of Soil Science and Plant Nutrition

Cadmium (Cd) is a non-essential toxic metal that threatens soil quality and food safety in intensively managed rice–wheat systems. A two years field study (2020–21 and 2021-22) was conducted to assess the effects of organic and conventional farming practices on the distribution and chemical fractionation of Cd across soil depths and aggregate size classes. Available Cd concentrations in surface soil (0–15 cm) ranged from 0.024 mg kg⁻¹ (conventional treatments) to 0.046 mg kg⁻¹ (organic treatments) after rice, and from 0.0165 mg kg⁻¹ (conventional) to 0.0818 mg kg⁻¹ (organic) after wheat, showing marked enrichment at the surface compared with sub-surface layers (15–30 cm: 0.031–0.040 mg kg⁻¹; 30–60 cm: 0.027–0.032 mg kg⁻¹). Aggregate-associated Cd was distributed among macro- (≈30–33%), micro- (≈32–35%), and silt+clay fractions (≈32–36%), with slightly higher accumulation in finer fractions under organic inputs. Sequential extraction revealed striking differences in chemical speciation: in conventional systems, Fe–Mn oxide–bound Cd dominated (57–68%), whereas in organic systems carbonate-bound Cd increased to 44–52%, and organic matter–bound Cd rose to 17–21%. The exchangeable fraction remained low (<3%) except under high organic inputs (6–7%), while the residual fraction contributed 4–13%. Although total Cd concentrations were below phytotoxic thresholds, the shift from stable oxide-bound forms toward more labile carbonate and organic-bound pools under organic inputs indicates potential for long-term accumulation. The findings demonstrate that while organic management enhances soil aggregation and fertility, it can also alter Cd partitioning, underscoring the need to monitor Cd content in organic amendments to sustain soil health without compromising environmental safety.