Oxygen input controls the spatial and temporal dynamics of arsenic at the surface of a flooded paddy soil and in the rhizosphere of lowland rice (Oryza sativa L.) : a microcosm study

The impact of oxygen (O2) input at the soil surface and in the rhizosphere of rice (Oryza sativa L.) on the spatial and temporal dynamics of arsenic (As) was investigated in a flooded paddy soil. A soil microcosm and root-mat technique were designed to mimic submerged conditions of paddy fields. Water-filled containers with (planted) or without (unplanted) 27-day-old rice seedlings were fitted for 20 days on top of microcosms containing an As-affected soil (Bangladesh). After the initial establishment of strongly reduced conditions (-230 mV) in both planted and unplanted soils, the redox potential gradually increased until the day 8 to reach+50 mV at 2 mm from the surface of unplanted soils only. This oxidation was associated with an accumulation of NH4-oxalate extractable As (25.7 mg kg-1) in the 0.5-mm top layer, i.e. at levels above the initial total content of As in the soil (14 mg kg-1) and a subsequent depletion of As in soil solution at 2 mm from soil surface. Root O2-leakage induced the formation of an iron (Fe) plaque in root apoplast, with no evidence of outer rhizosphere oxidation. Arsenic content reached 173 mg kg-1 in the Fe plaque. This accumulation induced a depletion of As in soil solution over several millimetres in the rhizosphere. Arsenic contents in root symplast and shoots (112 and 2.3 mg kg-1, respectively) were significantly lower than in Fe plaque. Despite a large As concentration in soil solution, Fe plaque appeared highly efficient to sequester As and to restrict As acquisition by rice. The oxidation-mediated accumulation of As in the Fe plaque and in the oxidised layer at the top of the soil mobilised 21 and 3% of the initial amount of As in the planted and unplanted soils, respectively. Soil solution As concentration steadily decreased during the last 16 days of the soil stage, likely indicating a decrease in the ability of the soil to re-supply As from the solid-phase to the solution. The driving force of As dynamic in soil was therefore attributed to the As diffusion from reduced to oxidised soil layers. These results suggest a large mobility of As in the soil during the flooded period, controlled by the setting of oxic/anoxic interfaces at the surface of soil in contact with flooding water and in the rhizosphere of rice. (Résumé d'auteur