Impacts of Increased Soil Si on Fe Mineral Composition and As Cycling in Rice Paddies.

Arsenic (As) is a ubiquitous element in soils worldwide and has the potential to negatively impact human and ecosystem health under certain biogeochemical conditions.  One such strategy that has been proposed to decrease arsenic uptake by rice plants is via an increase in dissolved silicon (Si) in paddy soil solution (pore-water). Dissolved Si and arsenite, the dominant form of arsenic in most paddy soil solutions, are chemical analogs at circumneutral pH and share an uptake pathway. However, several soil processes that influence As cycling may be affected by Si including arsenic desorption from bulk soil, formation and mineralogy of iron(III) oxide plaque on and near rice roots, and adsorption/desorption onto/from Fe plaque; the effect of silicon on these soil processes will ultimately dictate the effectiveness of increased dissolved Si in decreasing As uptake by rice.  However, Si and As also compete for sorption sites onto soil solids, and alteration of dissolved Si may thus affect As mobility.  Alteration of Si can also affect the mineralogy of newly formed iron (Fe) oxide plaque, which can sequester As.  Here, we incorporate different forms of Si into paddy soil and investigate the fate of As and impacts on Fe plaque using spectroscopic and conventional tools.  We show that Si alteration can affect the quantity and mineralogy of Fe oxide plaque, associated As, and can improve rice yields depending on the choice of amendment.  These processes occurring at the soil-plant interface will be discussed in the larger context of global food security.