Limestone dosage response of cadmium phytoavailability minimization in rice: A trade-off relationship between soil pH and amorphous manganese content

Limestone shows great potential to reduce the production of cadmium (Cd)-contaminated rice in acidic paddy soils, but has generated uncertainty effects. We conducted batch sorption and greenhouse experiments to investigate optimal conditions of pH and amorphous manganese content (Mn_ox) in limestone treated-soil for suppressing the Cd uptake by rice plants. The adsorption/desorption behavior of Cd in a soil/limestone mixture was dominated by the composition and density of sorption sites, followed by sorption conditions, which were mainly influenced by soil pH and exchangeable Ca²⁺. Interactions among soil factors were influenced both by the limestone effects and plant responses. The Cd uptake of rice plants did not matched to the doses of limestone applied. The increase in pH and decrease in Mn_ox following higher dosages of limestone treatment might produce contradictory effects on rice Cd uptake. We proposed a trade-off model to demonstrate how did the interactions of soil pH and Mn_ox affect the rice Cd uptake. To minimize the accumulation of Cd in rice grain harvested from acidic paddy soils, limestone was applied at 0.25 % to achieve an optimal pH of 6.5 and a Mn_ox of 95 mg kg^-1.