Ferrihydrite level in paddy soil affects inorganic arsenic species in rice grains

Rice is consumed by ∼50% of the global population, grown primarily in flooded paddy fields, and is susceptible to arsenic accumulation. Inorganic arsenic, particularly in reduced form (As(III)), is considered the most toxic and is more likely to accumulate in rice grains under flooded systems. We postulate that increased levels of highly reactive iron minerals, such as ferrihydrite, in paddy soils can regulate the bioavailability of arsenic and reduce its uptake by priming iron plaque formation. To clarify, two rice varieties, Norin and Sabharaj, differing in arsenic uptake rate, were grown in paddy soil under flooded conditions with arsenate (As(V)) spiked-irrigation water. 2-line ferrihydrite was added at 0.00% (control), 0.05%, and 0.10% w/w and served as the highly reactive iron species. Irrespective of rice varieties, total inorganic arsenic (As(III) + As(V)) in grains in ferrihydrite systems decreased by 85 to 93% compared to the control. These results support ferrihydrite's intrinsic role in controlling paddy soils' rhizosphere chemistry. Our findings indicate that fresh reactive iron minerals are critical in the early formation of root iron plaque, which enhances the defense mechanism against arsenic. The findings may have implications for reducing toxic inorganic arsenic accumulation in lowland rice.