Bachelor Thesis

The influence of phosphate on arsenic uptake in red rice of the Camargue

Johannes Stemper (05/2016-02/2017)

Support: Britta Planer-Friedrich, Jörg Schaller

Red rice from Camargue is known for very high arsenic concentrations in literature. With phosphate being an analogue of arsenate regarding chemical properties, and competing for the same sorption sites, enhanced phosphate fertilization may reduce arsenic uptake. To quantify this effect, a red and a brown rice variety from France was cultivated under three different phosphate fertilization levels. After maturity, grains, (flag-) leaves and pore waters were analyzed regarding total arsenic concentration and arsenic speciation (grains and pore waters only). There were no significant differences in total arsenic and speciation of grain, plant and pore water samples between the phosphate fertilization levels. This may be explained by the high initial availability of phosphorus in the soil. Whole grains (0.47 mg kg-1) as well as polished grains (0.28 mg kg-1) of the red variety showed significantly higher total arsenic concentrations than those of the brown variety (whole grain: 0.36 mg kg-1, polished grain: 0.21 mg kg-1). Flag leaves of the brown variety, however, showed significantly higher total arsenic concentrations compared to those of the red variety. Flag leaves of the brown variety therefore represent a stronger sink for arsenic compared to those of the red variety. This may be a reason for the higher total arsenic concentrations in red grains. In whole grains and polished grains of both varieties, arsenite was the predominant species, followed by DMA and arsenate. In both varieties, the percentage of arsenate was higher in whole grains than in polished grains, and in red whole grains higher than in brown whole grains. No correlations could be found between the temporally variable arsenic speciation of pore water and the speciation of grains. The concentration of inorganic arsenic in the red whole grains exceeded with 0.27 mg kg-1 the EU threshold of 0.25 mg kg-1 for all phosphorus fertilization levels.