Review of Current Techniques for Tracking as Behavior Across Micro-Interfaces in Paddy

Arsenic (As) in rice poses health risks, and the soil micro-interface is the key zone controlling As transformation and migration in the soil-water-rice system. Sharp redox transitions along the soil-water interface and rhizosphere cause simultaneous millimeter-scale changes in redox-sensitive elements like Fe and As, influenced by disturbances such as overlying water changes. These factors make it challenging to track As behavior across soil micro-interfaces. Sampling techniques like soil slicing and the Rhizon sampler provide limited information on As due to their low-resolution (cm level) and high soil disturbance (particularly for soil slicing). Micro-electrode and optode sensors are useful to measure soil parameters (e.g., Eh, pH, and dissolved O₂) that are closely associated with As transformation in micro-interfaces, yet cannot detect its elemental concentration. By integrating those methods with high resolution porewater samplers (e.g., In situ equilibrium dialysis samplers (Peeper), diffusive equilibration in thin films (DET), diffusive gradients in thin films (DGT), and the Integrated Porewater Injection (IPI) sampler), the understanding of As behavior has been largely improved. This review synthesizes approaches for tracking As across paddy micro-interfaces and offers insights for redox-sensitive elemental biogeochemistry in other saturated soils or sediments.