Pedogenetic evolution of clay minerals and agricultural implications in three paddy soil chronosequences of south China derived from different parent materials

This study aims to understand how clay minerals change sequentially with paddy cultivation age and how parent materials (or original soils) affect the clay mineral behavior of paddy soils. Three paddy soil chronosequences in the hilly regions of South China, derived from purple sandy shale (PS), Quaternary red clays (RC), and red sandstone (RS), were selected to explore the dynamic changes in clay mineralogy, by comparing physical, chemical, and mineralogical properties of soil sequences. For RC and RS soils, both of which have a low K content, there was little change in the clay minerals. Long-term paddy cultivation can promote formation of illite-like minerals; however, this form of K storage was limited under present farming conditions. In PS soils, which are abundant in K-bearing minerals, the depotassication was strong, accompanied by marked transformation of clay minerals. Kaolinite-like minerals gradually decreased with paddy cultivation age; by contrast, derivative clay minerals such as secondary chlorite and halloysite gradually increased. Strong depotassication mainly occurred in the nonclay fractions. The rate of depotassication and the generation of clay fractions were much faster than in natural soils. The clay minerals of paddy soils mainly followed the feature of their original soils. Their evolutions could be distinguished based on their constituents, which are greatly affected by their parent materials. Moreover, paddy cultivation is able to modify clay mineralogy, according to the original mineralogy and paddy soil management.