Localized intensification of arsenic release within the emergent rice rhizosphere

Trace element behavior in flooded/lowland rice soils, is controlled by root-zone iron oxidation. Insoluble iron species bind/capture toxic elements, i.e. arsenic. However, it was recently observed that within this territory of arsenic immobilization lies a zone of prolific iron release, accompanied by a significant flux of arsenic in close proximity to rice root apices. Questions still remain on how common this phenomenon is and whether the chemical imaging approach or soils/cultivars used influence this event. Here, three types of ultra-thin/high-resolution diffusive-gradients-in-thin-films (DGT) substrates, were integrated with oxygen-planar-optodes into a sandwich-sensor, providing a two-dimensional mapping of solute fluxes. The three DGT approaches revealed a consistent/overlapping spatial distribution with localized flux-maxima for arsenic, which occurred in all experiments, concomitant with iron mobilization. Soil/porewater micro-sampling within the rhizosphere, revealed no significant elevation in the solid phase’s total iron and arsenic concentration between aerobic and anaerobic zones. Contrary to arsenic, phosphorus bioavailability was shown to decrease in the arsenic/iron flux-maxima. Rice roots, in addition to their role in nutrient acquisition, also perform a key sensory function. Flux-maxima represent a significant departure from the chemical conditions of the bulk/field environment, but our observations of a complete rhizosphere reveal a mixed-mode of root-soil interactions.