Temporal and spatial variations of arsenic species in the rhizosphere of paddy soils
Eva Voggenreiter (07/2019-11/2020)
Support: Britta Planer-Friedrich, Alan Nicol
Arsenic speciation in the rhizosphere of paddy soils is controlled by the striking changes in biogeochemical conditions compared to the anaerobic bulk soil, for instance due to root radial oxygen loss, iron-plaque formation and organic acid exudation. In contrast to inorganic and methylated oxyarsenic species, where rhizospheric influences are well studied, less research has been dedicated so far to inorganic and organic thioarsenates. Experiments utilizing planted rhizonboxes and unplanted controls showed a significant enrichment of total arsenic in the pore water of the rhizosphere compared to the bulk soil at 42 days (0.41 μM versus 0.11 μM) and 95 days (0.42 μM versus 0.21 μM) after transplantation (DAT). The reason likely is an increased dissolution of iron-(hydr)oxides, indicated by the high rhizosphere concentrations of dissolved iron (up to 750 μM), which were spatially resolved using diffusive equilibrium in thin film methods. Many arsenic species showed lower concentrations in the rhizosphere compared to the bulk soil at 42 DAT due to oxidation by root radial oxygen loss and adsorption to iron-plaque. Arsenite was likely oxidized to arsenate in the rhizosphere, which subsequently adsorbed to iron-plaque. Similarly, monomethylated thioarsenates were probably oxidized step-wise to monomethylarsenate which then partially bound to iron-plaque. Concentrations of higher-order thiolated species (mono-, dimethylated dithioarsenates and trithioarsenate) in the rhizosphere at 95 DAT were even lower compared to 42 DAT, likely due to an increased oxygenation by extensive root growth in the rhizonbox. In contrast, no species transformations were observed for dimethylated monothioarsenate from 42 DAT to 95 DAT. Its concentrations in the rhizosphere at 42 DAT were even similar to those of inorganic oxyarsenic species, making it the most relevant thioarsenate in pore water. Overall, organic thioarsenates made up a higher share of total arsenic with an average of 27%, while inorganic thioarsenates accounted for 1.2%. Formation of organic over inorganic thioarsenates was not significantly favored by pH changes due to organic acid exudation in the rhizosphere, as we initially hypothesized. To conclude, these results prove that thiolated arsenic species, especially dimethylated monothioarsenate, have to be included when further researching arsenic in paddy soils.