The influence of freeze-thaw cycles on Se migration and soil microorganisms in northeast paddy soil

• FTCs promoted Se downward migration and increased the Se content in paddy soil. • FTCs caused toxic effects to ecosystems and impacted soil microbial communities. • FTCs and Se enhanced amino acid and carbohydrate metabolism of microorganisms. Freeze-thaw cycles (FTCs) are a typical climatic feature in northeast of China. Due to global climate change, the frequency of freezing and thawing during the autumn and winter seasons is increasing, which may affect soil physicochemical properties and biological characteristics in freeze-thaw regions. In northeast of China, selenium (Se) is used to spray paddy fields to increase Se content in rice. However, the impact of FTCs on the environmental behavior of Se in paddy soils in northeast of China is not well understood, especially under the background of increasing frequency of FTCs. In this study, indoor simulated FTCs experiment was conducted to investigate the influence of FTCs frequency, soil water content, size of soil aggregate on the Se migration and soil microorganisms in paddy soil column. The results showed that FTCs increased the proportion of microaggregates and soil organic matter content while decreased soil pH. After 60 days, the Se concentration in microaggregates and macroaggregates at 6–10 cm depth increased from 0.148 mg/kg to 0.601 mg/kg and from 0.154 mg/kg to 0.630 mg/kg, respectively; the increment were more than those of the UNFT-Se treatment group (from 0.148 mg/kg to 0.309 mg/kg and from 0.159 mg/kg to 0.318 mg/kg). Se concentration in microaggregates and macroaggregates at 16–20 cm depths increased from 0.144 mg/kg to 0.367 mg/kg and from 0.152 mg/kg to 0.378 mg/kg, respectively; the increment were more than those of the UNFT-Se treatment group (from 0.144 mg/kg to 0.196 mg/kg and from 0.168 mg/kg to 0.207 mg/kg), indicating that FTCs promoted the downward migration of Se in the soil column. Correlation analysis indicated there was a positive correlation ( r = 0.458, p < 0.01) between Se concentration and organic matter as well as a negative correlation ( r = −0.406, p < 0.01) between Se concentration and soil pH, respectively. Additionally, high-throughput sequencing results showed that FTCs induced changes in the soil microbial community. These findings have important implications for geochemical studies of exogenous Se in soils of the seasonally freeze-thaw aeras.