Spatial variation of arsenic transformation and functional microorganisms across the rice rhizosphere microzone with sulfur-based passivator

Arsenic (As) uptake by rice is critically dependent on As speciation, which is dynamically regulated by root-mediated biogeochemical processes in the rhizosphere microzones. However, the spatial heterogeneity of As transformation and associated microbial functional traits under immobilization amendments remains poorly understood. Using a rhizobox system, we revealed millimeter-scale gradients in As bioavailability and functional microbial communities across rice rhizosphere microzones following sulfur-based passivator application. Passivator addition significantly reduced total As, arsenite (As(III)) and arsenate (As(V)) in soil porewater and As accumulation in rice plants. The abundances of sulfate-reducing gene (dsrA) increased by 136-225% and the functional microorganisms (e.g., Clostridium and Bacillus) were also increased with passivator amendment. The spatial distribution of As across the rhizosphere microzones exhibited strong distance-dependent mobility patterns. Total As, As(III) and As(V) increased from root-proximal (0-2 mm) to far-rhizosphere (8-10 mm) zones by 36-83%, 36-110%, and 28-397%, respectively. The abundances Geobacter and dsrA gene peaked in the root-proximal zone (0-2 mm) with passivator addition, driving As sequestration. These findings highlight that sulfur-based passivator amplifies rhizosphere effects through sulfur-iron interactions, establishing a sulfate-iron reduction-dominated As immobilization pathway. Rhizosphere heterogeneity in As speciation and functional genes revealed mechanisms of rhizosphere-mediated As immobilization, advancing in situ As remediation strategies.