Paddycrusts: interfacial bioregulators of heavy metal transport and speciation

Heavy metal contamination in paddy soils poses persistent challenges to food safety and sustainable agriculture. Paddy crusts (PCs) are dynamic interfacial biocomplexes that function as biogeochemical regulators of metal transport and speciation. This review aims to (i) summarize PC formation and community assembly, (ii) synthesize mechanistic processes governing metal interception, immobilization, and transformation, and (iii) evaluate evidence for PC-based remediation from laboratory systems to field trials. This review presents an integrated mechanistic framework built upon three interconnected processes: physical barrier and interception at the interface; adsorption and immobilization primarily mediated by extracellular polymeric substances (EPS) and microbial surfaces, alongside biomineralization; and redox-regulated biotransformation driving speciation shifts. We then summarize remediation performance and limitations across experimental scales, from controlled systems to field trials. The evidence underscores a generally favorable role of PCs in cadmium (Cd) immobilization and rice risk reduction. In contrast, outcomes for arsenic (As) are more variable and context-dependent. Under carbon-rich and reducing microconditions, PCs may promote Fe(III) reduction together with As(V) reduction and/or methylation, increasing As(III) and/or methylated species (e.g., DMA) in pore and surface waters. Finally, we outline future research directions, emphasizing cross-scale mechanistic understanding, synthetic community design, lifecycle risk management, and climate-smart agronomic integration. We conclude that predictable PC-based remediation requires element-specific risk screening, lifecycle management of crust maturity and senescence, and agronomic integration guided by functional indicators (EPS traits, Fe/Mn dynamics, and gene markers). These advancements are crucial to developing predictable and scalable PC-based remediation strategies for paddy ecosystems.