Si cycling and isotope fractionation: Implications on weathering and soil formation processes in a typical subtropical area

Abstract

The translocation of silicon (Si) is closely related to soil evolution. However, how to trace the transformation and migration paths of Si and further understand its effects on soil formation and evolution remains as a challenge in geochemistry and soil science. Here we studied the Si isotope (δ30Si) values and physical, chemical and mineralogical properties of rock, soil, plant and water in representative small watersheds in the south of Anhui province, subtropical China. The aims are to illustrate weathering and soil formation processes by tracing the fractionation of Si isotope among those ecosystem components as well as soil components and plant organs. Results show that the δ30Si values of bulk soil and clay are significantly related to many soil development indicators, such as total, free and amorphous Al oxides, as well as active Fe oxides, clay, silt and sand contents, Al/Si molar ratio and chemical index of alteration (CIA). These indicators evidence the relationships between Si isotope changes and mineral weathering and soil development degree. Biological resilication (accumulation of Si) by plant does not lower soil δ30Si because further fractionation occurs among plant organs, and soil phytoliths have significantly higher δ30Si than soil clay and silt particles. However, soil desilication (loss of Si) lowers soil δ30Si because much 30Si is transported to streams in runoff. The consistently positive δ30Si values of water and their relationships with Si concentrations and discharge suggest that dissolved Si (DSi) in stream water mainly comes from weathering of primary minerals; the contribution of dissolution of secondary minerals, quartz and phytolith is minor. Differential δ30Si values in clay, silt, sand, rock and water are the result of soil formation processes which directly evidence a dominate neoformation pathway of secondary clay minerals in this area. This study can help to further understand mechanisms and processes of Si translocation during soil evolution and, extend the application of Si translocation in soil genesis studies.