General Background. Rice is the major staple food worldwide and increasingly popular also in Europe, e.g. as baby food or in alternative diets. However, rice also accumulates arsenic, a human carcinogen which is why in 2016 an arsenic limit for rice grains was introduced in the European Union. The high arsenic accumulation is related to high arsenic mobility under anoxic conditions which form because rice is typically grown under flooded conditions in organic-rich soil. In the wake of investigating arsenic mobility in paddy soils and its uptake in the rice plant in cooperation with colleagues from major rice-growing areas in Italy and France we discovered novel arsenic species in the rice field soils. These species are arsenic-sulfur species, so called thioarsenates. Both inorganic and methylated thioarsenates exist. In our group we have been investigating these species, e.g. in hydrothermal areas, for years, but no one has looked for them in the context of rice, yet.
Given the global threat of chronic poisoning by consumption of As-contaminated rice as staple food and the ubiquitous natural occurrence of reduced S in paddy soils, there is an urgent need to know under which conditions and to which extent thioarsenates form and how they affect As mobility in the rhizosphere, uptake and translocation in the plant as well as accumulation in the rice grain. Both aspects are part of 2 current PhD projects in our group.
Clarifying the role of thioarsenates is all the more important as they are expected to form in significant quantities especially in degraded paddy soils low in Fe, in poorly drained organic soils, in acid sulfate soils or upon sulfate fertilization which has recently been advertised to reduce grain As concentrations. If intensification of land use proceeds as it has in the past, increasing soil degradation and compaction, over-fertilization, and reclamation of new land (e.g. mangrove mud rich in sulfate) will increase the share of such soils used for rice cultivation. Furthermore, it is important to understand how water management affects thiolation versus methylation because both to save water consumption and reduce greenhouse gas emissions, more and more rice cultivation in the up-coming years will rely on alternate wetting-drying strategies instead of growing rice under continuously flooded conditions.
Specific background. A huge problem when sampling for thioarsenates in paddy soil is the heterogeneity of the paddy soils including the rhizosphere with rice roots releasing organic acids and oxygen into the soil. Since thioarsenates are very sensitive to transformation at low pH, the rice roots themselves might lead to speciation changes. So, depending on where you sample (close to the top of the soil with influence of atmospheric oxygen, deep down in the soil with no atmospheric oxygen and no release of exudates from plant roots, or close to plant roots) you might find thioarsenates in significant shares or not at all. If rhizosphere influences are unclear it will never be possible to make predictions for thioarsenate occurrence and importance on a more global scale. In the field, there is virtually no chance of accounting for this heterogeneity because we do not really see the rhizosphere extension and cannot sample at dedicated spots.
Task. We will therefore use transparent rhizonboxes filled with soil (and with sulfate fertilizer added to maximize thioarsenate formation) containing one plant and do dedicated sampling at spots in the bulk soil and with increasing vicinity to the plant roots. The sampling will be done for pH, redox potential, sulfide, iron species, As total and As species by IC-ICP-MS as well as potentially organic acids and take place over several times during rice plants growth (about 5 months). To better see the effect of root radial oxygen loss (ROL) we will use two different rice species, one with high and one with low ROL (the ROL characteristics will be determined in a pre-experiment).
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