Sorption competition with natural organic matter as mechanism controlling silicon mobility in soil.

Growing evidence of silicon (Si) playing an important role in plant health and the global carbon cycle triggered research on its biogeochemistry. In terrestrial soil ecosystems, sorption of silicic acid (HSiO) to mineral surfaces is a main control on Si mobility. We examined the competitive sorption of Si, dissolved organic matter, and phosphorus in forest floor leachates (pH 4.1-4.7) to goethite, in order to assess its effects on Si mobility at weathering fronts in acidic topsoil, a decisive zone of nutrient turnover in soil. In batch sorption experiments, we varied the extent of competition between solutes by varying the amount of added goethite (α-FeOOH) and the Si pre-loading of the goethite surfaces. Results suggest weaker competitive strength of Si than of dissolved organic matter and ortho-phosphate. Under highly competitive conditions, hardly any dissolved Si (< 2%) but much of the dissolved organic carbon (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. We conclude sorption competition with dissolved organic matter and other strongly sorbing solutes can promote Si leaching in soil. Such effects should thus be considered in conceptual models on soil Si transport, distribution, and phytoavailability.