Localized intensification of arsenic release within the emergent rice rhizosphere

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Environmental Science & Technology, © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://pubs.acs.org/doi/full/10.1021/acs.est.9b04819
Behavior of trace elements 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 approaches or soils/cultivars used influence this event. Here, three types of ultrathin/high-resolution diffusive gradient in thin films (DGT) substrates were integrated with oxygen planar optodes in a multilayer system, providing 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 microsampling within the rhizosphere revealed no significant elevation in the solid phase’s total iron and arsenic concentrations 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
This work was funded by the National Natural Science Foundation of China (No. 41771271), the Fundamental Research Funds for the Central Universities (021114380126), the National Natural Science Foundation of China (No. 41807023), and the Jiangsu Province Natural Science Foundation (BK20180344). The study also received support from the Newton Fund/Royal Society and NFSC (R1504GFS and 21511130063)