Abstract
Calcium (Ca) plays a crucial role for plant nutrition, soil aggregation, and soil organic matter (SOM) stabilization. Turnover and ecological functions of Ca in soils depend on soil Ca speciation. For the first time, we used synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the Ca K-edge (4038 eV) to investigate Ca speciation in soils. We present Ca K-edge XANES spectra of standard compounds with relevance in soils (e.g. calcite, dolomite, hydroxyapatite, anorthite, clay mineral-adsorbed Ca; Ca oxalate, formate, acetate, citrate, pectate, phytate). Calcium XANES spectra with good signal-to-noise ratios were acquired in fluorescence mode for Ca concentrations between 1 and 10 mg g−1. Most standard spectra differed markedly among each other, allowing the identification of different Ca species in soils and other environmental samples as well as Ca speciation by linear combination fitting. Calcium XANES spectra obtained for samples from different horizons of twelve temperate forest soils revealed a change from dominating lithogenic Ca to clay mineral-bound and/or organically bound Ca with advancing pedogenesis. O layer Ca was almost exclusively organically bound. With increasing SOM decomposition, shares of oxalate-bound Ca decreased. Oxalate-bound Ca was absent in calcareous, but not in silicate subsoil horizons, which can be explained by microbial decomposition in the former vs. stabilization by association to pedogenic minerals in the latter soils. Synchrotron-based Ca XANES spectroscopy is a promising novel tool to investigate the fate of Ca during pedogenesis and—when performed with high spatial resolution (µ-XANES), to study aggregation and SOM stabilization mechanisms produced by Ca.
Highlights
Calcium (Ca) in soils is an important nutrient element for plants and soil organisms (Marschner 1995; Briedis et al 2012; Paradelo et al 2015)
The shapes of the Ca K-edge X-ray Absorption Near-Edge Structure (XANES) spectra obtained in fluorescence mode for calcite that had been diluted with quartz to different concentrations from 1 up to 50 mg Ca g-1 were identical
Most published Ca K-edge XANES spectra of standard compounds with relevance in environmental samples refer to minerals (Neuville et al 2004; Sarret et al 2007; Brinza et al 2014; Blanchard et al 2016; Proffit et al 2016)
Summary
Calcium (Ca) in soils is an important nutrient element for plants and soil organisms (Marschner 1995; Briedis et al 2012; Paradelo et al 2015). Excess Ca is immobilized in plant foliage as Ca oxalate and other Ca carboxylates (e.g. polygalacturonate ‘‘pectate’’), and recycled to the topsoil with the foliage litter (Marschner 1995; Likens et al 1998; Franceschi and Nakata 2005; Clarholm and Skyllberg 2013) This mechanism decelerates or may even prevent topsoil acidification and base cation depletion (‘‘base pumping effect’’, Poszwa et al 2000; Clarholm and Skyllberg 2013). Whereas these processes generally are well known, little information exists about soil Ca speciation changes associated with Ca cycling through the soil–plant–microorganisms system
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