Neoformed aluminosilicate and phytogenic silica are competitive sinks in the silicon soil-plant cycle

In soils, mineral weathering and phytolith dissolution release aqueous monosilicic acid that can be taken up by plant, adsorbed on mineral surfaces, entrapped in neoformed clay minerals, or exported to watersheds. The balance between biotic and abiotic processes determines the fluxes of bioavailable silicon (Si), impacting plant growth and health, and diatoms biomass, hence the oceanic capacity to fix carbon dioxide. Here we quantified this balance in an experimental system using rice and an albic soil material (quartz grains) containing no weatherable silicate minerals. Materials representing a soil weathering gradient were prepared by adding variable amounts of silicate minerals within fresh powdered tephrite (< 83 mu m) as a source of weatherable minerals whereas phytogenic Si was supplied within phytolith-rich biochar. We quantified the distribution of Si in leachate, soil and plant reservoirs. Weatherable minerals rapidly released Si that flowed into two main sinks: allophanic substances neoformed in soil from released Si and aluminum (Al), and phytoliths formed by precipitation of absorbed silica in plant tissues. The leaching of Si was negligible. Allophane rapidly formed at a maximum rate of 0.85 g allophane kg(-1) soil day(-1). Of the Si stock of weatherable minerals, 7-14% contributed to allophanic Si and less than 1% to phytogenic Si. The contribution of supplied phytoliths to plant Si accumulation markedly increased from 26 to 80% with the decrease in weatherable mineral reserve from 405 to 5 cmol(c) kg(-1). Weatherable minerals primarily controlled bioavailable Si while phytoliths increasingly contributed to Si plant uptake with increasing mineral depletion. Returning phytoliths to soil thus boosts the biological recycling of Si in proportion to the increase in soil weathering stage.