Clay mineral composition of upland soils and its implication for pedogenesis and soil taxonomy in subtropical China

Clay minerals are intermediate products generated during soil development, and their neoformation and transformation are closely related to pedogenesis. Here we aimed at identifying the difference in the clay mineral composition of upland soils derived from different parent materials and different soil-forming environments and exploring the importance of clay mineral composition in pedogenesis and soil taxonomy. We sampled 60 soil B horizons in Hunan Province of subtropical China by digging soils derived from granite (GR), slate and shale (SS), Quaternary red clay (QRC), limestone (LS), and sandstone (SDS). The clay mineral composition and its correlation with parent materials, elevation, micro-topography, and pedogenic processes were investigated using X-ray diffraction and Pearson’s correlation analysis. The clay mineral was dominated by kaolinite, followed by 2:1-type minerals (illite and vermiculite), and a small fraction of mixed-layer minerals. The composition of soil clay minerals varied with parent materials. Kaolinite was predominant in soils derived from GR and LS; mixed-layer minerals prevailed in QRC, whereas illite and vermiculite were prevalent in SDS. In addition, elevation and micro-climate could also explain the variations in clay mineral composition. Increase in elevation was associated with decreased 1:1 clay mineral content and increased 2:1 clay mineral content, especially in soils developed from LS. The composition and content of clay minerals indicated that Ferrosols, Ultisols, and Acrisols had undergone intense weathering; Primosols, Entisols, and Leptosols were characterised by weak weathering, and Plinthic Ali-Udic Cambosols, Plinthudults, and Plinthosols were characterised by strong redox status. This study suggests that clay mineral composition is related to the parent material, climate, and micro-topography, and that it can serve as an indicator of pedogenesis and soil type in subtropical China.