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The fine sand fraction (50–250 μm) of Mediterranean soils from southern Spain provides valuable information on soil genesis and the origin of their parent materials. This study considers the whole fine sand and heavy fine sand (ρ > 2.82 g cm−3) of Luvisols, Calcisols and Fluvisols, which form a chronosequence (ages from 600 to 0.3 ka) of the River Guadalquivir terrace system. Advanced techniques (X‐ray diffraction, inductively coupled plasma mass spectrometry, inductively coupled plasma atomic emission spectroscopy, variable pressure scanning electron microscope with an energy dispersive X‐ray spectroscopy system and Raman analysis) were employed. Inheritance is the principal pedogenic process. The whole fine sand consisted of carbonates (calcite and dolomite), tectosilicates (quartz, K‐feldspar and plagioclases), phyllosilicates (illite/moscovite, biotite, Na‐mica, chlorite, kaolinite, interstratified vermiculite‐chlorite, vermiculite‐illite and smectite‐illite) and iron oxides (goethite and haematite). The minor minerals (rutile, anatase, ilmenite, zircon, staurolite, monazite, barite, apatite, andalusite, garnet and titanite) are concentrated, also through inheritance, in the heavy fine sand. However, there is also substantiated evidence for neoformation of rutile in these soils, never reported previously. In addition, we report that (a) the geochemical indices calculated in fine sand (SiO2/CaO, Chemical Index of Weathering (CIW), Weathering Index of Parker (WIP), Weathering Index (WI), Base Depletion Index (BDI), Weathering Ratio (WR) and Sr/Zr) are closely related to soil age, allowing chronofunctions to be established, and (b) geochemical indices provide information on the origin of soils and permit the establishment of a "critical point" corresponding to "time zero;" that is, the start of pedogenic alteration of the parent material. Highlights: Mineralogy and geochemistry inform on the provenance of the fine sandRutile is neoformed in the fine sand and co‐exists with the polymorph anataseThe properties of the fine sand including geochemical indexes fit chronofunctionsGeochemical indices determine a "critical point" related to the start of weathering [ABSTRACT FROM AUTHOR]

Cropland soils are key systems in global carbon budgets due to their high carbon-sequestration potential. It is widely accepted that clays are one of the soil components that have a significant effect on the stabilisation of soil organic carbon (SOC), owing to its surface interactions with organic molecules. However, the identification of the direct effects of clays on SOC stabilization is complicated, mainly due to the difficulty of accurately characterizing the mineralogy of clays, especially phyllosilicates. In this study, the relationships between soil phyllosilicates and functional SOC pools in woodlands and comparable olive groves, under two contrasting management systems (bare soils versus soil under cover crops) and parent materials (calcareous and siliceous), were explored. The total mineralogy of soil and clay fractions and the soil-clay assemblages were analysed through the decomposition of X-ray diffraction patterns, and were then related to four SOC pools. Total and unprotected SOC was higher in olive groves under cover crops, and this was true independent of the parent material, proving the importance of herbaceous covers in SOC sequestration in woody crops. Some significant correlations between clay minerals and SOC fractions were found. Interestingly, mixed-layer content was correlated with the biochemically protected SOC fraction (r = 0.810, p < 0.05), and this was so even when the partial correlation coefficient was calculated (r = 0.761, p < 0.05). According to the partial correlation networks (PCN), four separated clusters of variables were obtained, which joined into only one at fdr < 0.25. The PCNs supported the direct correlation between mixed-layer content, especially those rich in smectite, and the biochemically protected SOC fraction, suggesting that smectite layers may stabilize organic molecules. Since potassium enrichment is higher in the rooted layers of woodland and soils under cover crops, and this increase is related to the collapse of swelling layers, these soils were poorer in smectite phases than the bare soils. This also would explain why the biochemically protected SOC was more abundant in the latter. [ABSTRACT FROM AUTHOR]