Natural organic matter control on silicate interactions with iron oxides and silicon phytoavailability
Final Report Abstract
Over the last years, the importance of silicon (Si) for plant nutrition has been increasingly recognized. Pedogenic Fe oxides are important sorbents of silicic acid and thus, potentially control plant-available Si. However, (1) the sorption behaviour of polymeric relative to monomeric Si, (2) their speciation on mineral surfaces and consequences for Si desorption, and (3) the effect of natural dissolved organic matter (DOM) on the sorption-desorption of monomeric versus polymeric Si have received little attention. In a first study, we therefore studied the adsorption of monomeric and polymeric silicic acid to goethite at variable Si-to-goethite ratios under acidic conditions (initial pH 4.5), simulating an active weathering environment. The Si-loaded goethites were subjected to desorption. Adsorption of Si to goethite at constant Si-to-Fe ratios was greater for solutions containing Si polymers than monomers. Surface accumulation of polymeric Si was only observed by X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy at low binding site availability in combination with small changes in solution pH, and in presence of high Si solution concentrations. Deviations from these conditions caused the accumulation of monomeric Si. In contrast to monomeric Si, desorption of polymeric Si was significantly lower. In a second study, we investigated the DOM-induced desorption of monomeric and polymeric Si from goethite surfaces at different initial Si-loadings and the net Si release during competitive sorption of monomeric and polymeric Si and DOM to Sipreloaded goethite. The DOM used was extracted from a forest-floor Oa layer under spruce vegetation. The results showed that monomeric Si was more efficiently desorbed by DOM (up to 100%) than polymeric Si (<20%). In line with this trend, we observed that monomeric Si sorbed less efficiently to goethite in presence of DOM. At low binding site availability, DOM cause a net release of monomeric Si. In contrast, DOM was hardly capable of impairing the sorption of polymeric Si to goethite. In a third study, we explored the competitive sorption of Si, DOM, and phosphorus using two different forest floor leachates (pH 4.1-4.7) to goethite. Under highly competitive conditions, hardly any dissolved Si (<2%) but much of the DOM (48- 80%) was sorbed. Pre-loading the goethite surfaces with monomeric Si hardly decreased the sorption of organic carbon and phosphate, whereas up to about 50% of the Si was released from surfaces into solutions, indicating competitive displacement from sorption sites. Overall, our experiments revealed that Si solution speciation, binding site availability, and pH are important interacting factors controlling Si speciation at Fe oxide surfaces and Si mobility in soils. Our results consistently point at a stronger persistence and lower phytoavailability of polymeric than monomeric Si on mineral surfaces. Monomeric Si is much stronger affected by competition with DOM and other strongly sorbing solutes and thus will be more prone to leaching and available to plants than polymeric Si.
Publications
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Competition of monomeric and polymeric silicic acid with natural organic matter for binding sites at goethite. Copernicus GmbH.
Dobritzsch, Jana; Klotzbücher, Anika; Klotzbücher, Thimo; Kaiser, Klaus; Mikutta, Christian & Mikutta, Robert
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Sorption competition with natural organic matter as mechanism controlling silicon mobility in soil. Scientific Reports, 10(1).
Klotzbücher, Thimo; Treptow, Christian; Kaiser, Klaus; Klotzbücher, Anika & Mikutta, Robert
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Effect of silicic acid polymerization on silicon accumulation at goethite surfaces. Jahrestagung der Deutschen Bodenkundlichen Gesellschaft. 2022
Dobritzsch, J.; Klotzbücher, A.; Klotzbücher, T.; Kaiser, K.; Mikutta, C. & Mikutta, R.
