Natural organic matter control on silicate interactions with iron oxides and silicon phytoavailability

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Over the last years, the importance of silicon (Si) for plant nutrition, especially for rice, has been increasingly recognized. Uptake of Si enhances biomass yields and stress tolerance against pathogens. Silicon is released by weathering of primary and secondary minerals. Pedogenic Fe oxides, likewise formed during weathering, are important sorbents of silicic acid (H4SiO4), and thus, potentially control plant-available Si. While the basic mechanisms of H4SiO4 sorption to Fe oxides are relatively well understood, possible effects of natural organic matter (OM), despite being abundant in all soils, did not receive much attention. The effects of sorption competition with dissolved OM or due organic coatings on mineral surfaces on the retention of H4SiO4 by Fe oxide phases are largely unknown. Consequently, the influence of OM on the phytoavailability of Si is poorly understood. Particularly in soils with strongly contrasting redox conditions, as in submerged rice cropping, little research has been conducted regarding the interactions among Fe oxides, Si and OM. Our project will, therefore, test for the first time the effects of natural OM on the Si retention by Fe oxides and the consequences for the phytoavailability of Si. Based on batch experiments under different environmental conditions, we will first study the adsorption and desorption of H4SiO4 in presence of OM and the impact of OM on the surface Si speciation (monomeric to polymeric Si) by relying on surface-sensitive X-ray photoelectron spectroscopy (XPS). We will study the consequences of the Si binding to Fe oxides in presence of OM for the Si uptake by rice plants in subsequent micro- and mesocosm experiments. Microcosm experiments will be conducted with defined Si-Fe oxide phases in order to test whether and to which extent Fe oxide-bound Si can be utilized by plants, with special consideration to the role of OM and the prevailing Si surface species. In mesocosms with natural soil, we will elucidate these processes under variable redox conditions by determining the release / immobilization of Si, the changes in Si surface species, and the plant uptake. The results of these experiments will promote our understanding of the impact of natural OM on the Si cycling in soils.