Arsenic (As) is an ubiquitous toxin in the environment and contamination of paddy soils with elavated levels of As presents a threat to food safety. The status of soil redox potential has a substantial impact on the speciation and the subsequent mobilization of As in paddy soils. The rice rhizospheric microenvironment is more oxidized than the bulk soil. How this rhizospheric effect affects the biogeochemical processes associated with the transformations of As remains poorly understood. In this study, we used a novel rhizobox with a field As-contaminated soil (101.2 mg As kg-1) to investigate the in situ transformation of As in the rhizosphere of two rice cultivars which differs in the radial oxygen loss (ROL). Results showed that during the rice seedling stage, dissolved concentrations of As in the rhizospheric porewater were 2.2-3.1 fold higher than those in the bulk soil, with the As concentrations decreasing with the distance from the root surface. The decreasing rate can be modelled through a reciprocal eqution [y= 378.3+ 283.8/(0.3743+ x), R2= 0.97]. Along the root profile, dissolved As concentrations decreased from the root tip to the mature zone, with the former being 32.4%-63.7% higher than the latter during different growth stages of two rice cultivars. Among the dissolved As species, rhizospheric concentartions of arsenate (As(V)) were 38.4%-64.3% lower in the cultivar Yangdao (with a higher ROL) than those in the cultivar Shenyou, despite a similar total dissolved As. This is likely attributed to a higher ROL in Yangdao which can promote the the formation of iron oxides (e.g. iron plaque) on the root surface, increasing As(V) adsorption. Bacterial 16S rRNA gene sequencing data showed that iron-reducing bacteria could play an important role in the coupling of As(V) and ferric (Fe(III)) reduction, including the geobacter and clostrium, which were increased by 3.1 and 12.4 times in the rhizosphere compared with those in the bulk soil, respectively. The As functional genes such as arsenite (As(III)) oxidase gene (aioA), As(V) reductase gene (arsC), As(V) respiratory reductase gene (arrA) and As(III) methyltransferase gene (arsM) were found 18.2, 13.8, 3.1 and 1.6 times higher in the rhizosphere than those in the bulk soil, respectively. These data show that the biogeochemical processes of As and Fe occurred in the rhozosphere can influence As mobilization in soil and subsequent availabiity to rice plants.