Fate of silicon in tropical agricultural soil clays using XANES spectroscopy

• Silicon (Si) is an abundant element in terrestrial ecosystems and makes essential contributions to agricultural crops, environments, and industries; however, current knowledge of Si speciation in soil systems is based primarily on crystalline mineral types with limited information about the effect of Si-adsorbed species on the most chemically reactive clay fraction. Herein, we investigated Si speciation in the clay-sized fraction of tropical soils. Silicon K-edge X-ray absorption near edge structure (XANES) spectra of clay samples, together with a wide range of reference compounds varying in clay and Fe/Al oxide mineralogy, showed the predominance of Si in a beidellite structure and of Si adsorbed to montmorillonite and to kaolinite. Unique features, including a pre-whiteline feature, white-line position and intensity, and post-edge numbers and positions of Si reference spectra, could differentiate most Si reference standard spectra. Silicon adsorbed to goethite, Si-ferrihydrite-natural organic matter complexes, and Si in the structure of biotite, illite-smectite, chlorite, quartz, and Si nanoparticle, variously occurred in different samples. The CaCl2 extraction data revealed that extracted Si was below the limits of Si deficiency in one-half of the total soil samples and accounted for only 0.02% of total soil Si. Soil extractable Si had a positive correlation with pH, organic matter, and clay. Our results present a novel application of Si K-edge XANES as a tool for identifying Si speciation in natural clays that are the most chemically reactive phase of Si retention and release in terrestrial and aquatic systems.

Ämnesord

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Markvetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Soil Science (hsv//eng)

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