Iron fractionation and iron plaque-mediated effects of woody peat and its humate on cadmium and arsenic behavior in a ricesoil system.

Cadmium and arsenic (Cd/As), which are prevalent metal(loid) contaminants in paddy soils, pose dual threats to food safety and human health. The efficacy of woody peat and its alkali-extracted humate as heavy metal(loid) immobilizers is influenced by the presence of metal(hydro)oxides, yet their synergistic regulation of Cd/As mobility and accumulation in rice remain largely unexplored. Therefore, we conducted controlled incubation and pot experiments using Cd/As co-contaminated paddy soils to elucidate how woody peat and its humate affect Cd/As behavior in a ricesoil system. The results indicated that after 60 days, the CaCl2-Cd decreased by 14.40-95.75% across all the treatments, whereas the KH2PO4-As increased by 16.46-64.11%. The amendments facilitated the transformation of crystalline Fe oxides (Fed) into amorphous phases (Feo) under reducing conditions. Redistribution of Fe oxide fractions was closely associated with Cd/As availability, while the low redox potential (Eh) acted as a key regulatory factor during waterlogged periods. In both early and late rice, the grain Cd decreased by 40.85-69.41%, whereas the grain As increased by 13.10-37.20%. The Fe and Mn concentrations in the iron plaque (FeIP/MnIP) decreased, accompanied by reduced Cd retention but increased As sequestration in iron plaque. The distinct variations in the availability of Cd/As and their differential partitioning in iron plaque jointly contributed to the extent of Cd/As accumulation in the rice grains. This study provides novel insights into how woody peat and its humate regulate Cd/As dynamics in the ricesoil system through Fe fractionation and iron plaque regulation.