Arsenic and Silicon Dynamics in Flooded Rice Fields.

While second to drinking water in terms of human exposure, arsenic entry into rice plants and ultimate storage in grains represents an important route of As exposure particularly on a global level.  Rice plants are more susceptible than other grains to accumulate arsenic because they are typically grown under flooded conditions where arsenic is mobilized as iron (hydr)oxides reductively dissolve, and both As(V) and As(III) are absorbed by rice roots due to their similarities with plant nutrients (phosphate and silicic acid).  As(III) is the dominant As species present in flooded rice fields and it may compete with dissolved Si for both adsorption onto iron (hydr)oxides and for uptake into rice roots.  Thus, the fate of As in rice grains is strongly influenced by dissolved Si levels, which vary widely due to differences in chemical weathering rates, mineralogy, and agricultural practices (e.g. straw incorporation).  Here, we compare plant-available Si and As pools with total Si and As concentrations in rice grain, husk and straw from Cambodia.  Soil and plant samples were obtained from Kandal, Prey Veng, Battambang, Banteay Meanchey, Siem Reap and Kampong Thom provinces, which vary in their groundwater As concentrations, irrigation sources, and agricultural practices.  In addition to illustrating the country-wide exposure risk of As, our data show that agricultural practices strongly influence grain As levels.  Modifying agricultural practices may help to decrease grain As levels and thus decrease As exposure to humans worldwide.