Does elevated CO enhance the arsenic uptake by rice? Yes or maybe: Evidences from FACE experiments.

Elevated CO (eCO) strongly affects rice yield and quality in arsenic (As) paddy soils. However, understanding of the As accumulation in rice under coupled stress of eCO and soil As is still limited while data are scarce. It greatly limits the prediction for future rice safety. This study investigated the As uptake by rice grown in different As paddy soils under two CO conditions (ambient and ambient +200 μmol mol) in the free-air CO enrichment (FACE) system. Results showed that eCO lowered soil Eh at the tillering stage and caused higher concentrations of dissolved As and Fe in soil pore water. Compared with the control, the increased As transfer abilities in rice straws under eCO contributed to the higher As accumulation in rice grains, and their total As concentrations were increased by 10.3-31.2%. Besides, the increased amounts of iron plaque (IP) under eCO failed to effectively inhibit the As uptake by rice due to the difference in critical stage between As immobilized by IP (mainly in maturing stage) and uptake by rice roots (about 50% contribution before filling stage). Risk assessments suggest that eCO enhanced the human health risks of As intake from rice grains produced in low-As paddy soils (<30 mg kg). In order to alleviate the As threats to rice under eCO, we consider that proper soil drainage before filling stage to improve soil Eh can serve as an effective way to reduce As uptake by rice. Pursuing appropriate rice varieties to reduce the As transfer ability may be the other positive strategy.