Heavy metal pollution poses a serious hazard to the soil bacterial community. In this study, the 16 s rRNA high-throughput sequencing technology was used to analyze bacterial diversity and structure of dry field soil at different levels of heavy metal pollution. Further, the relationships between soil parameters and bacterial community were analyzed. Based on the study findings, we classified the levels of heavy metal pollution in soil samples from the study area could be divided into four grades: high risk (HR), considerable risk (CR), moderate risk (MR) and low risk (LR). In this study, heavy metal concentrations and pH showed significant effect on bacterial community structure. The distribution of bacterial community richness and diversity was MR > LR > CR > HR. Bacterial communities such as Acidobacteria, Chloroflexi and Gemmatimonadetes were highly resistant to the lower pH (pH < 6.5) and the high levels of heavy metal pollution compared with other bacterial community, which were abundant in HR samples. However, Proteobacteria, Actinobacteria, Bacteroidetes and Latescibacteria were more abundant in alkaline soils (pH > 7.5). Further, available Cd, Pb and Zn concentrations were lower in alkaline soils than acidic soils, which reduced the impact of heavy metals on bacterial community diversity and structure.

Excessive accumulation of cadmium (Cd) and arsenic (As) in rice (Oryza sativa L.) poses a potential health risk to populations. Cd and As exhibit opposite geochemical behavior in paddy soil, using appropriate remediation materials to reduce their migration and inhibit their uptake by rice is a great challenge. A pot culture experiment was conducted to investigate the effects of application of silkworm excrement biochar (BC) and ferrous sulfate (Fe(II)) on available Cd and As in paddy soils and their uptake by rice. Results showed that the application of BC + Fe significantly accelerated the tillering of rice plants, and the addition of BC alone to soil did not have a significant effect on the pH of soil, while applied 1% (w/w) BC and 1% (w/w) Fe(II) (1BC-1 Fe(II)) treatment could markedly reduce the soil pH. BC+Fe(II) could reduce the content of available Cd (reduced by 10%–23%) and As (reduced by 6%–33%) in soil. BC+Fe(II) has a distinct decreasing effect on the available As, thus inhibiting As uptake in rice tissues, and the effect was more obvious with an increasing mass ratio of Fe in BC+Fe(II) treatment. BC+Fe(II) decreased bioaccumulation factors (BF) of As compared to control and BC alone treatments. Compared with As, Cd was more readily transferred from the root to the shoot and accumulated in rice eventually. These findings provide a safe and reliable remediation strategy though application of BC+Fe(II) in Cd and As co-contaminated soil. However, the improvement effect of amendments should be paid a special attention on soil pH.