Spatial variation of arsenic transformation and functional microorganisms across the rice rhizosphere microzone with...

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.