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.

中文翻译：

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温带森林土壤中钙的命运：Ca K-edge XANES 研究

钙 (Ca) 对植物营养、土壤团聚和土壤有机质 (SOM) 稳定起着至关重要的作用。土壤中钙的周转和生态功能取决于土壤钙的形态。我们首次在 Ca K 边缘 (4038 eV) 使用基于同步加速器的 X 射线吸收近边缘结构 (XANES) 光谱来研究土壤中的 Ca 形态。我们展示了与土壤相关的标准化合物的 Ca K-edge XANES 光谱（例如方解石、白云石、羟基磷灰石、钙长石、粘土矿物吸附的 Ca；草酸钙、甲酸钙、乙酸盐、柠檬酸盐、果胶酸盐、植酸盐）。对于 1 到 10 mg g-1 之间的 Ca 浓度，在荧光模式下获得了具有良好信噪比的钙 XANES 光谱。大多数标准光谱彼此之间存在显着差异，允许通过线性组合拟合识别土壤和其他环境样品中的不同 Ca 种类以及 Ca 种类。从十二个温带森林土壤的不同层位获得的钙 XANES 光谱显示，随着成土作用的推进，从占主导地位的成岩 Ca 到粘土矿物结合和/或有机结合 Ca 的变化。O 层 Ca 几乎完全是有机结合的。随着 SOM 分解的增加，草酸盐结合的 Ca 的份额减少。钙质中不存在草酸盐结合的钙，但在硅酸盐底土层中不存在，这可以通过前者中的微生物分解与后者土壤中与成土矿物相关的稳定性来解释。