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

Abstract

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 (H₄SiO₄) 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.

Figure 1

Relationship between equilibrium Si concentrations in solution and amounts of Si sorbed at the goethite surfaces for different experiments (Si sorption test and Si pre-loading of goethite for the experiments with forest floor solutions). Error bars (X- and Y-axis), denoting the standard deviation of replicates (n = 3), are smaller than the symbols. Data were plotted using SigmaPlot 11.0 from Systat Software, San Jose, CA.

Figure 2

Examples of XPS spectra of the Si 2s region for goethite with variable Si pre-loadings prior to the sorption experiments with forest floor solutions. The vertical red line at 152.0 eV highlights that there were no differences in peak position between pre-loading levels, thus indicating the prevalence of monomeric Si. Data were plotted using SigmaPlot 11.0 from Systat Software, San Jose, CA.

Figure 3

Changes in composition of the two forest floor solutions upon different additions of goethite with and without Si pre-loadings. The error bars indicate the standard deviation of experimental replicates; stars indicate significant differences between treatments (p < 0.05; ANOVA on ranks), i.e. at least two treatments differ significantly from each other. SUVA₂₈₀ indicates the specific UV absorbance measured at 280 nm and normalized to DOC. Data were plotted using SigmaPlot 11.0 from Systat Software, San Jose, CA.

Figure 4

Amount of organic C and Si at goethite surfaces after interaction with the two forest floor solutions upon different additions of goethite with and without Si pre-loadings. The error bars indicate the standard deviation for experimental replicates; stars indicate significant differences between treatments (p < 0.05; ANOVA on ranks), i.e. at least two treatments differ significantly from each other. Data were plotted using SigmaPlot 11.0 from Systat Software, San Jose, CA.

Figure 5

Percentage of Si desorbed from the goethite surfaces with high Si pre-loading due to interactions with the two forest floor solutions as a function of the goethite amounts added (data for other Si-preloading levels are not shown because less pronounced or no desorption occurred); the error bars indicate the standard deviation for experimental replicates. Data were plotted using SigmaPlot 11.0 from Systat Software, San Jose, CA.

Figure 6

Nature and implications of sorption competition between Si (blue symbols), organic matter (dark grey symbols), and phosphorus (green symbols) at the variable-charge surfaces of pedogenic minerals.