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 IP , Cd IP , and As IP relationships. Under FM, As IP was positively associated with Fe IP , whereas Cd IP showed no significant relationship with Fe IP . Under F, both Cd IP and As IP were positively associated with Fe IP . The changes in soil availability and IP-associated retention did not directly translate into grain Cd/As accumulation. PT produced the largest simultaneous decreases in grain Cd and As concentrations under FM, by 47.20% and 23.96%, respectively, and resulted in the lowest screening-level grain risk index (GRI). Under F, all amendments reduced grain Cd, with HA showing the greatest effect, whereas only FA also reduced grain As. The GRI decreased across all amendment treatments under F. Grain Cd under FM and grain As under F were mainly linked to root uptake and shoot-to-grain transfer, whereas grain Cd under F and grain As under FM were more related to soil mobility and IP-mediated retention. 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.