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Faculty for Biology, Chemistry, and Earth Sciences

Environmental Geochemistry Group - Prof. Dr. Britta Planer-Friedrich

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Master Thesis

Influence of sulfate fertilization and rice straw addition on total concentrations and speciation of arsenic in paddy soil pore water - Results from Satellite and Mesocosm experiments

Milagros Galarza Nuñez (04/2022-10/2022)

Support: Britta Planer-Friedrich, Carolin Kerl

Flooded conditions in paddy soils drive the release of arsenic (As), its uptake and translocation in rice plants and accumulation in the grain. Local agricultural practices also affect As concentrations in paddy soils. Sulfate fertilization is considered a strategy to decrease As concentrations by precipitation with FeS mineral phases and comply with food regulations. In contrast, rice straw incorporation for nutrient return increases the organic carbon pool, reducing soil redox potential and enhancing the release of As to the pore water. However, previous studies often reported contradictory results on the effect of sulfate fertilization and rice straw return on total concentrations and speciation of As among experimental setups at different scales. In the present study, laboratory satellite experiments and larger-scale mesocosms confirmed a decrease in aqueous As concentrations upon fertilization with ammonium sulfate in the bulk soil. The added ammonia slowed down redox reactions, decreasing the release of As from iron (oxy) hydroxides; and hindering the reduction of sulfate, which caused low to non-detectable thioarsenate concentrations in the pore water. The application of rice straw showed an initial increase of aqueous As concentrations, which later decreased due to its sequestration into natural organic matter. Rice straw return increased methylation due to increased microbial activity, which enhanced the formation of methylated thioarsenates. The combined application of ammonium sulfate and rice straw produced the highest total concentration of As and thioarsenate shares (21.2% on average). Among all thioarsenates, higher-order thiolated species predominated in the bulk soil pore water. The temporal changes in As concentration and formation of thioarsenates in the satellites and mesocosms differed from the previous N2 purged anoxic incubation experiments, where lower Eh values (below 0 mV) likely accelerated the mobility and thiolation of As. Differences were also observed between the bulk soil and the rhizosphere in the mesocosm experiments. Root exudates increased As concentrations, whereas root oxygen release favored the formation of monothiolated species, such as dimethylated monothioarsenate. Experiments with bulk soil likely underestimate health and food safety risks, as the rhizosphere favors the presence of highly carcinogenic DMMTA, which can be taken up by rice plants and accumulate in the rice grain.

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