In rhizospheric soil, arsenic can be activated by both biological and abiotic reactions with plant exudates or phosphates, but little is known about the relative contributions of these two pathways. The effects of microorganisms, low-molecular-weight organic acid salts (LMWOASs), and phosphates on the migration of As in unrestored and nano zero-valent iron (nZVI)-restored soil were studied in batch experiments. The results show that As released by microbial action accounted for 17.73%, 7.04%, 92.40%, 92.55%, and 96.68% of the total As released in unrestored soil with citrate, phytate, malate, lactate, and acetate, respectively. It was only suppressed in unrestored soil with oxalate. In restored soil, As was still released in the presence of oxalate, citrate, and phytate, but the magnitude of As release was inhibited by microorganisms. The application of excess nZVI can completely inhibited As release processes induced by phosphate in the presence of microorganisms. Microbial iron reduction is a possible mechanism of arsenic release induced by microorganisms. Microorganisms and most environmental factors promoted As release in unrestored soil, but the phenomenon was suppressed in restored soil. This study helps to provide an effective strategy for reducing the secondary release of As from soils due to replanting after restoration.

In estuarine sediment, the nitrite oxidation process mediated by bacteria is significant in the nitrification process. Nitrospira is the most widely distributed in diverse group of nitrite-oxidizing bacteria (NOB), and can adapt to various environment. In this study, the Nitrospira-specific primer nxrB 169F-638R was used to analyze the microbial communities in the sediments of low-, middle-, and high-level zones of the Luan River estuary. The structure of microbial community and its response to environmental factors were discussed. The abundance and diversity of Nitrospira were the highest in the low-level zone and the lowest in the high-level zone. Lineage II (43.58%) and lineage IV (32.09%) were the dominant Nitrospira. It was found that the distribution pattern of Nitrospira was also affected by complex environmental factors such as the concentration of NH4+, Fe and Cu cations. Therefore, this study is helpful in understanding the niche differentiation and adaptation strategies of Nitrospira in an estuarine sediment environment and in providing references for single-step nitrification.