Water regime-dependent effects of organic amendments on Cd/As accumulation and potential grain risk in rice.

Introduction: Cadmium (Cd) and arsenic (As) co-contamination in paddy soils poses a persistent challenge for rice safety due to their distinct responses to soil redox conditions. Although organic amendments have been widely used to regulate metal(loid) availability, their effects on Cd/As behavior in soil-rice systems under contrasting water regimes remain unclear. Methods: A pot experiment was conducted under flooding-moisture alternation (FM) and continuous flooding (F) to evaluate the effects of fulvic acid (FA), humic acid (HA), and woody peat (PT) on Cd/As availability, Cd/As in iron plaque (IP), and grain Cd/As accumulation. Results: HA reduced soil available Cd under both water regimes, whereas FA decreased it mainly under F. Available As changed little under FM but increased under F, particularly with FA and PT. Water regime also altered Fe Discussion: These findings demonstrate that amendment effects on grain Cd/As concentrations were water-regime dependent: PT simultaneously reduced both elements under FM, whereas under F, HA produced the greatest grain Cd reduction and FA was the only amendment that also reduced grain As. These concentration-based responses require further field validation together with biomass and yield assessment.