Silicon-Rich Amendments in Rice Paddies: Effects on Arsenic Uptake and Biogeochemistry.

Rice (Oryza sativa) is a staple food crop in many regions of the world and its contamination by arsenic constitutes a substantial health risk to many consumers. Silicon can mitigate rice uptake of arsenic as they share similar transport pathways in rice. Here we describe the use of silicon-rich amendments in rice paddy mesocosms and the associated effects on rice paddy biogeochemistry. Rice paddies were amended with calcium silicate fertilizer, rice husk, or rice husk ash. Several yield and biogeochemical parameters were monitored throughout the experiment, including weekly measurements of porewater chemistry and methane emissions. At harvest, elemental concentrations were measured in several plant parts to assess silicon’s role in affecting metal uptake and nutrient status. Overall, we found that silicon decreased rice uptake of arsenic, shifted grain arsenic speciation from inorganic arsenic to organic arsenic, and lowered translocation of manganese. However, there were subtle differences in performance of different amendments. Rice husk ash most strongly increased rice uptake of silicon while strongly decreasing plant uptake of arsenic. Only the husk material significantly increased methane emissions, although this increase was less than was predicted for a comparable incorporation of rice straw. Silicon-rich amendments also significantly affected the mineral composition of the root iron plaques, with silicon enhancing formation of ferrihydrite in the plaques. However, this change in root plaque mineral composition did not affect retention of arsenic, despite ferrihydrite’s higher affinity for arsenic. Overall, silicon-rich amendments provide a sustainable approach to limiting rice uptake of arsenic without adversely affecting yield or other biogeochemical phenomena.