Using rice as a remediating plant to deplete bioavailable arsenic from paddy soils

Rice plant is efficient in arsenic (As) accumulation due to enhanced soil As release under flooded condition and its effective As uptake. Therefore, rice plant can be used to remove bioavailable As from paddy soil. In this study, the depleting dynamics of soil As with rice growth was mapped two-dimensionally with the zirconium-oxide diffusive gradients in thin films (DGT) technique. Further, the key biochemistry promoting soil As bioavailability was studied to better understand the underlying processes. Results revealed that the average DGT-As encompassing the root zone decreased steeply from 331 in the seedling stage to 136 in the heading & flowering stage and further to 118 μg l^-1 at harvest, which was 26% lower than that of the control at 160 μg l^-1. During this process, rhizosphere porewater As developed a dynamic profile similar to Fe and dissolved organic carbon, with the diversity of arrA gene peaking at heading & flowering stage. The data support soil As release from microbial reduction of Fe hydroxides fueled by root exudation as carbon source. Arsenic was mainly accumulated in the roots, accounting for up to 95% of total As in rice plants. Removal of rice roots resulted in ~19% lower DGT-As in post-harvest soil compared to without removing the roots. As a result, a sharp decline in As accumulation in rice plants was obtained in the second planting after removing one crop of rice roots. The results highlight that rice, as a paddy-adapted plant, is effective in As uptake in the roots, and thereby removing rice roots efficiently depletes bioavailable As from paddy soils.