Silicon cycle in rice paddy fields: insights provided by relations between silicon forms in topsoils and plant silicon uptake

Abstract

Silicon (Si) enhances the stress resistance of rice plants. Silicon cycling in paddy fields is, however, still poorly studied. We examined relationships between Si forms in topsoil and plant Si uptake for four Vietnamese and three Philippine regions (ten fields per region). Mean rice straw Si concentrations within regions ranged from 3.0 to 8.4 %. For most of the Vietnamese fields they were lower than the critical value of 5.0 %, suggesting Si limitation of plant growth. For fields with low Si availability, straw Si concentrations were positively related to acetate-extractable Si in topsoil (i.e., dissolved and adsorbed Si). Such a relationship was not found for fields with high Si availability, presumably due to a maximum Si uptake capacity of rice plants. Mean annual Si uptake by rice within regions ranged from 0.31 to 1.40 Mg Si ha−1 year−1. They are determined by the continuous supply of plant-available Si during the cropping season and by aboveground biomass production. Weatherable silicate minerals mainly determine spatial differences in supply of plant-available Si. Concentrations of alkaline carbonate-extractable Si in topsoil (an estimate of amorphous Si) largely differed between regions; (regional means of 2.2–16.7 g Si kg−1). The differences in concentrations and amounts in topsoil are not related to phytolith (i.e., amorphous Si in straw) input, presumably due to other yet uncertain factors on carbonate-extractable Si in soil (e.g., differences in phytolith solubility or contribution of non-phytolith sources to Si in the extracts).