Physicochemical properties of three typical purple soils with different parent materials and land uses in Sichuan Basin, China

Mainly located in the Sichuan Basin, known as the 'Land of Plenty', purple soils (developed from purple sandstone and shale) have been long noted as highly productive in spite of their rocky texture. However, information on the physicochemical properties (including mineral properties) of purple soils remained obscure for the scientific community. For example, it is not clear what causes the purple color of the soils. In this study, eight soil profiles of the Jiufeng (JF, acidic), Shibao (SB, neutral), and Yanting (YT, calcareous) areas, representing the three most widely distributed purple soils in Sichuan Basin, were individually sampled under both cropland and forestland, and then analyzed using common chemical analysis and conventional X-ray diffraction to clarify the effects of clay mineral and anthropogenic influences on the typical soils. The results show that considerable variability of soil pH and cation-exchange capacity (CEC) exists between soil types (p < 0.05). Organic matter, total nitrogen, and CEC of forestland are higher than those of farmland, while farmland has a higher content of Olsen-P, indicating significant farming effects on nutrient status. Acidic and neutral purple soils had higher total P contents than calcareous soils while calcareous soils had higher TN, clay, and CaCO3 contents. OM, TN, TP, and Olsen-P decreased with soil depth both in farmland and forestland soil profiles. Vertical distribution of organic-bound Fe-oxides (Fep) and clay particles also shows the anthropogenic interference on farmland. Illite, chlorite, kaolinite, goethite, and trace amounts of an illite–chlorite mixing layer in clay fractions were identified for typical purple soils. Hematite was observed in the coarser fractions of the calcareous pedons, confirming that the purple soil color was mainly derived from the parent rocks. Understanding of the physicochemical properties and mineral species of soil profiles can shed light on the sustainable uses of typical purple soils in subtropical area.