From Inorganic Arsenic to Methylated and Thiolated Arsenic: Speciation Mechanisms, Management Implications, and Rice Safety in Paddy Systems

Rice is a globally important staple crop and a major dietary source of inorganic arsenic (As). Compared with upland crops, flooded rice cultivation profoundly alters soil redox conditions, making paddy soils one of the most active agricultural interfaces for As mobilization, transformation, and food-chain transfer. While previous research has primarily focused on total As and inorganic As [As(III)/As(V)], methylated and thiolated As species also carry critical agronomic and health implications. Dimethylarsinic acid (DMA) can accumulate in grain and induce straighthead disease, whereas dimethylmonothioarsenate (DMMTA) shows substantially higher toxicity and uptake potential; DMMTA root uptake can be approximately 10 times higher than DMA, and its straighthead-inducing potency can exceed DMA by more than fivefold. This review synthesizes the sources, biogeochemical transformations, plant uptake, grain accumulation, safety assessment, and management implications of As along the paddy soil–rice–grain continuum. Particular emphasis is placed on how water regimes, redox potential, Fe/Mn/Al oxides, sulfur cycling, dissolved organic matter (DOM), microbial functional genes, and crop genotypes regulate diverse As species. Quantitative evidence indicates that alternate wetting and drying (AWD) can reduce grain total As and inorganic As by medians of 32% and 22%, respectively, but may increase grain cadmium (Cd) by a median of 58%; meanwhile, DMA and DMMTA can account for approximately 10–90% and 1–21% of total grain As, respectively, emphasizing that grain-As risk cannot be evaluated using inorganic As alone. Future research should establish speciation-based monitoring systems for inorganic, methylated, and thiolated As; develop process models linking water regime, Fe/S cycling, microbial transformations, and plant transport; and translate these mechanisms into field decision tools that balance As–Cd risk reduction, crop yield, and rice safety under changing environmental conditions.