Arsenic bioavailability to rice plant in paddy soil: influence of microbial sulfate reduction

Abstract

High arsenic (As) mobility in anaerobic paddy soil due to microbial Fe- and As-reducing processes results in As accumulation into rice plants. Sulfur (S) also undergoes microbial reducing processes in the anaerobic paddy soil, while this process interacts with the Fe- and As-reducing processes, forms secondary minerals, and thus influences As mobility in paddy soil. This work was carried out to investigate the role of sulfur and sulfate-reducing bacteria (SRB) in As redox transformation and bioavailability to rice plant in an anaerobic paddy soil. Anaerobic incubation experiments with or without sulfate (SO4 2−) and SRB were carried out to monitor S and iron (Fe) dynamics and their relations to As speciation and release to soil solution. Rice cultivation in pot experiment was then carried out to check the SO4 2− amendment in bioavailability and uptake into rice plants. The inoculation of an enriched SRB community into the sterilized paddy soil increased reduction and release of As to the soil solution after flooding, showing its ability in arsenate (As(V)) as well as SO4 2− reduction to arsenite (As(III)) and sulfide (S2−), respectively. Sulfate addition (2 and 100 mM) into the anaerobic paddy soil increased the reduction of SO4 2− and ferric iron (Fe3+) and resulted to lower dissolved As in the soil solution, which limited As mobility in the paddy soil. Application of SO4 2− (100 mg kg−1) into paddy soil finally decreased the concentration of dissolved As in the soil solution by 23.5 % and also decreased As content in rice roots and iron plaque by 22.6 and 30.5 %, respectively. The results revealed the important role of sulfur and the SRB activity in As speciation and mobility in anaerobic paddy soil, implying a lower As bioavailability to rice plants under sulfur-enriched anaerobic paddy soil, and an efficient way to reduce As accumulation in rice plants by sulfur fertilization in paddy soils.