Master Thesis

Effect of root radial oxygen loss on arsenic speciation in the rice rhizosphere

Lena Brüggenwirth (12/2017-01/2019)

Support: Britta Planer-Friedrich, Jiajia Wang, Carolin Kerl

For more than half of the world´s population rice is the most important staple food. Rice is well known for its very efficient accumulation of the toxic metalloid arsenic (As). Arsenic speciation in paddy soils is dominated by inorganic As and methylated As species. However, recent studies also revealed the occurrence of (methylated) thiolated As species. Their formation and transformation processes in the rice rhizosphere, which represents a special microenvironment due to root radial oxygen loss (ROL) from the rice plant roots, have not been studied so far and are assumed to greatly differ from the bulk soil.

In a hydroponic study, a highly controlled system, the direct effect of ROL on the selected species monomethylmonothioarsenate (MMMTA), dimethylmonothioarsenate (DMMTA) and monothioarsenate (MTA) was examined by exposing them to three rice varieties that differ in their ROL rates. Furthermore, a more natural system was assessed by means of a rhizonbox study which investigated the formation and transformation of methylated thiolated As species over different rice growing stages in the rhizosphere.

The determined ROL rates of the three rice varieties resulted in the following order: Nongken 57 (NK) > Yangdao 6 (YD) > Arelate (AR). MMMTA was unstable towards ROL and was converted to monomethylarsenate (MMA), whereas DMMTA and MTA remained stable. In the rhizonbox study, speciation changes could be observed over the different growing stages caused by different rates of ROL. More oxidizing conditions prevailed in the tillering stage (20-30 DAT), followed by more reducing conditions towards the end in the ripening stage (100 DAT). The species distribution was mostly dominated by arsenite, arsenate, dimethylarsenate (DMA) and, surprisingly, by DMMTA, which made up an average of 15 % of total arsenic. MMMTA was only present at the end of the growth phase, when the root activity was low and almost no oxygen was released. In contrast, MTA concentrations were close to detection limit throughout the growth period, despite its stability towards ROL.

This thesis gives insights into the formation and transformation processes of methylated thiolated As species in the rice rhizosphere. It was demonstrated that these species can occur in substantial amounts in the rice rhizosphere and are determined by ROL of the rice roots. The high share of the most toxic species, DMMTA, throughout the entire growth period demonstrates the importance of understanding the biogeochemical processes, which determine its formation and transformation in the rhizosphere. This is the foundation for avoiding As accumulation in rice grains and ensuring food safety.