Master Thesis

Trace element availability in a 2000-year-old paddy soil chronosequence

Asel Traut (10/2019-12/2020)

Support: Britta Planer-Friedrich, José Miguel León Ninin

Soil chronosequences are useful tools to investigate the changes in soil development processes. The cycling of carbon and iron are intertwined in paddy soils and are affected by their ageing. Accelerated organic carbon accumulation has been associated with paddy soil development, which in turn influenced the structure and content of iron (oxy)hydroxides. In this study the chronosequential approach was used to understand the binding and mobilization of Cu, Ni, Zn, As, Sb, W, and Mo in the series of paddy soils of 50, 100, 300, 700, 1000, and 2000 years old. Non-paddy soils of 50, 100, 300, and 700 years old were used as reference. After sequential extractions, it was determined that trace element’s binding in both chronosequences did not show preferences to any phases that could be assigned to the age of the soils. Meanwhile, total content of metal(loids) associated with both amorphous and crystalline iron, extracted with DCB, decreased as the age of the paddy soils increased. Trace elements in paddies were positively correlated to the amorphous iron, while in non-paddies they were found to be correlated to total Fe and S. Variations in total S contents in paddy soils were not linked to the age of soils in both chronosequences but affected the pattern of iron mobilization in incubation experiments through precipitation as sulfides. Mobilization of As, Mo, W, and Sb was closely governed by iron and sulfide-ion and decreased with soil ageing in the paddy chronosequence, while Cu, Zn, and Cd did not show correlation with age. Since As availability and binding depends on its speciation in soils, the species’ concentrations were determined in the liquid phase in paddy soil chronosequence. Their distribution was influenced by iron, sulfur chemistry, and bacteria involved in the arsenic biogeochemical cycle in paddy soils, which together created rather complicated pattern in the soil chronosequence. Nevertheless, the accumulation of dimethylated monothioarsenate was observed in the paddies with increasing soil age.