Influence of freeze-thaw process on As migration and microorganisms in aggregates of paddy soil.

A natural phenomenon known as the seasonal freeze-thaw (FT) cycle happens in cold temperature zone such as high latitude and high altitude regions where the soil frequently freezes and thaws in response to temperature variations. Global warming would increase the number of FT cycles in FT regions. However, the influence of FT process on arsenic (As) migration in paddy soil is seldom investigated. Herein, indoor simulation experiment was conducted to investigate the influence of FT process (60 cycles) on As migration from surface to deep soil and microorganisms in paddy soil column. Compared to non FT treatment groups, the concentrations of As in microaggregates of 8-10 cm depth and 18-20 cm depth in soil column of FT treatment group increased by 3.69 mg/kg and 4.16 mg/kg, respectively; the concentrations of As in macroaggregates of 8-10 cm depth and 18-20 cm depth in soil column of FT treatment group increased by 3.34 mg/kg and 3.94 mg/kg, respectively, indicating that FT process accelerated the As migration from surface to deep soil. FT process affected the microbial community structure by changing the physicochemical properties of the soil, which decreased the diversity and uniformity of bacterial community distribution in the soil. The relative abundance of two As-resistant bacteria, e.g., Sphingomonas and Lysobacter, increased by 8.2% and 11.35% after 60 cycles, respectively; moreover, total As in the soil was significantly (p < 0.05) negatively correlated with the alpha index of the soil microorganisms. This study would provide basic data for future study on determining environmental behavior and risk of metals in farm soils in seasonal FT aeras.