BayCEER Workshop 2019

Can Sulfate Help Safe Rice Production?

Rice represents the primary dietary source of inorganic arsenic (As), a non-threshold class-1 carcinogen. Sulfur has a high affinity for As both in soil minerals and plant chelators. Therefore, sulfate fertilization has long been suggested to decrease As bioavailability, uptake and translocation. However, previous studies reported contradictory results of either decreased or no effects on grain As accumulation. Moreover, sparse data exists about how sulfate fertilization influences grain As speciation variations, despite the fact that food guideline thresholds only address inorganic As, but not weakly carcinogenic methylated As. Here we re-examined sulfate fertilization via mesocosm-scaled rice cultivation experiments, with special focus on white (polished) rice. Sulfate fertilization decreased grain inorganic As accumulation in two studied soils over two cropping seasons, independent of seeding practice (water seeded versus dry seeded) and irrigation manner (continuous flooding versus alternate wet/dry irrigation). The effects of sulfate fertilization on grain methylated As accumulation were highly soil-specific and management-related, and no general pattern was found. Surprisingly, over two cropping seasons, the ratio for grain methylated As to inorganic As was significantly increased upon sulfate fertilization in both soils and under all treatments, strongly indicating sulfate as an important but previously overlooked environmental factor in influencing grain As speciation variations. In summary, our study evidenced sulfate fertilization as an effective way to help produce rice with lower carcinogenic inorganic As, but proportionally more methylated As.