Exploring Key Soil Parameters Relevant to Arsenic and Cadmium Accumulation in Rice Grain in Southern China

Simultaneously decreasing arsenic and cadmium in rice by soil sulfate and limestone amendment under intermittent flooding

Water management in paddy soils can effectively reduce the soil-to-rice grain transfer of either As or Cd, but not of both elements simultaneously due to the higher mobility of As under reducing and Cd under oxidizing soil conditions. Limestone amendment, the common form of liming, is well known for decreasing Cd accumulation in rice grown on acidic soils. Sulfate amendment was suggested to effectively decrease As accumulation in rice, especially under intermittent soil flooding. To study the unknown effects of combined sulfate and limestone amendment under intermittent flooding for simultaneously decreasing As and Cd in rice, we performed a pot experiment using an acidic sandy loam paddy soil. We also included a clay loam paddy soil to study the role of soil texture in low-As rice production under intermittent flooding. We found that liming not only decreased rice Cd concentrations but also greatly decreased dimethylarsenate (DMA) accumulation in rice. We hypothesize that this is due to suppressed sulfate reduction, As methylation, and As thiolation by liming in the sulfate-amended soil and a higher share of deprotonated DMA at higher pH which is taken up less readily than protonated DMA. Decreased gene abundance of potential soil sulfate-reducers by liming further supported our hypothesis. Combined sulfate and limestone amendment to the acidic sandy loam soil produced rice with 43% lower inorganic As, 72% lower DMA, and 68% lower Cd compared to the control soil without amendment. A tradeoff between soil aeration and water availability was observed for the clay loam soil, suggesting difficulties to decrease As in rice while avoiding plant water stress under intermittent flooding in fine-textured soils. Our results suggest that combining sulfate amendment, liming, and intermittent flooding can help to secure rice safety when the presence of both As and Cd in coarse-textured soils is of concern.

Decreasing arsenic in rice: Interactions of soil sulfate amendment and water management

Accumulation of inorganic arsenic (iAs) and dimethylarsenate (DMA) in rice threatens human health and rice yield, respectively. We studied the yet unclear interactions of soil sulfate amendment and water management for decreasing As accumulation in rice grain in a pot experiment. We show that soil sulfate amendment (+200 mg S/kg soil) decreased grain iAs by 44% without clearly increasing grain DMA under intermittent flooding from booting stage to maturation. Under continuous flooding during this period, sulfate amendment decreased grain iAs only by 25% but increased grain DMA by 68%. The mechanisms of sulfate amendment effects on grain iAs were not explained by porewater composition or in-planta As sequestration but were allocated to the rhizosphere. Grain iAs closely correlated with As in the root iron-plaque (r = 0.92) which was effectively decreased by sulfate amendment and may have acted as an iAs source for rice uptake. Although both sulfate amendment and intermittent flooding substantially increased porewater DMA concentrations, it was the continuous flooding, irrespective of sulfate amendment, that resulted in rice straighthead disease with 47-55% less yield and 258-320% more DMA in grains than intermittent flooding. This study suggests that combining soil sulfate amendment and intermittent flooding can help to secure the quantity and quality of rice produced in As-affected areas. Our results also imply the key role of rhizosphere processes in controlling both iAs and DMA accumulation in rice which should be elucidated in the future.