Relationship between plant-available silicon and reducible iron in irrigated paddy soils

Abstract

ABSTRACTTo evaluate the influence of reducible iron (Fe) on the availability of silicon (Si) by rice (Oryza sativa L.), (1) relationships between the available Si content in soil (successive water elution method), the Si content in rice shoots and ears, and the levels of oxalate-extractable Si (Sio), Fe (Feo), and aluminum (Alo) in 13 paddy soils were analyzed; and (2) the Si concentration in soil solution following sequential flooding/drainage was traced in soil microcosm experiments. Both the available Si and Feo contents in soil were positively correlated with the Si content in rice shoots, while a similar correlation was not observed between the available Si, Sio and Feo. These results suggest that the Sio adsorbed to Feo is not necessary to determine the pool size of the Si available to plants. In microcosm experiments, intermittent drainages decreased Fe concentration in soil solutions by 83–96% compared to that in the continuous flooding treatment. The oxidation of Fe2+ resulted in the co-precipitation of Si with Fe. However, the decrease of Si in a soil solution following drainage was 22% or less, which were smaller than the values observed when solutions containing various ratios of metasilicic acid and Fe2+ were mixed under oxic conditions (24–97%). In addition, as the Si concentrations in soil solutions in the intermittent flooding treatment increased to the levels similar to those in the continuous flooding treatment after re-flooding before Fe was re-reduced, the effect of reduction/oxidation of Fe on the dissolved Si concentrations was small. Thus, the reducible Fe is not a major factor in rice-available Si in paddy soil.