Understanding reduced inorganic mercury accumulation in rice following selenium application: Selenium application routes, speciation and doses

- Soil application with Se could reduce inorganic mercury (IHg) accumulation in rice.
- Foliar application with Se could increase Se but not IHg accumulation in rice.
- IHg-Se antagonism in soil-rice systems depends on Se doses and Se application routes.
- Se speciation (selenite and selenate) may play a minor role in IHg-Se antagonism.
- IHg-Se interactions in soil could be important in explaining IHg-Se antagonism.

Selenium (Se) has recently been demonstrated to reduce inorganic mercury (IHg) accumulation in rice plants, while its mechanism is far from clear. Here, we aimed at exploring the potential effects of Se application routes (soil or foliar application with Se), speciation (selenite and selenate), and doses on IHg-Se antagonistic interactions in soil-rice systems. Results of our pot experiments indicated that soil application but not foliar application could evidently reduce tissue IHg concentrations (root: 0-48%, straw: 15-58%, and brown rice: 26-74%), although both application routes resulted in comparable Se accumulation in aboveground tissues. Meanwhile, IHg distribution in root generally increased with amended Se doses in soil, suggesting antagonistic interactions between IHg and Se in root. These results provided initial evidence that IHg-Se interactions in the rhizosphere (i.e., soil or rice root), instead of those in the aboveground tissues, could probably be more responsible for the reduced IHg bioaccumulation following Se application. Furthermore, Se dose rather than Se speciation was found to be more important in controlling IHg accumulation in rice. Our findings regarding the importance of IHg-Se interactions in the rhizosphere, together with the systematic investigation of key factors affecting IHg-Se antagonism and IHg bioaccumulation, advance our understanding of Hg dynamics in soil-rice systems.