Arsenic mobilization and speciation during iron plaque decomposition in a paddy soil

Purpose: Little information is available concerning the mobilization and speciation of arsenic (As) in paddy soils during iron plaque decomposition. It is important to investigate these processes since they affect As bioavailability and contaminate surface and ground water systems. Materials and methods: A microcosm experiment was conducted to investigate the reductive dissolution of iron plaque and subsequent As mobilization under NaN₃ sterilized (abiotic treatments) and non-sterilized (biotic treatments) paddy soil conditions. Results and discussion: In the biotic treatment, As and iron (Fe) were quickly released into the soil solution, with more than 76. 1% of total arsenic (T_As) on the roots lost in 27 days. In the abiotic treatment, both iron plaque decomposition and As release were significantly slower, with only 39. 4% of T_As on the roots lost in 85 days. A part of arsenate reduction reaction occurred before and may also occur after release from roots in both abiotic and biotic treatments. Bacterial abundance, quantified by real-time PCR, varied significantly between treatments. Terminal restriction fragment length polymorphism combined with principal component analysis (PCA) revealed that microbial community structures were also different between treatments. Conclusions: The changes in microbial factors (bacterial abundance and microbial diversity and activities) significantly affected iron plaque decomposition, As mobilization, and speciation processes. Iron plaque reductive dissolution was likely the major factor leading to As release. Most As released was trapped in the solid phase during the incubation period.