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

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 and its phytoavailability are largely unknown. Our project will therefore test the effects of natural OM on the Si retention by Fe oxides and the consequences for the phytoavailability of Si. For this, we combine a range of batch sorption experiments with 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 emphasis on 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 improve our understanding of the impact of natural OM on Si cycling in soils