Microtopography-mediated hydrologic environment controls elemental migration and mineral weathering in subalpine surface soils of subtropical monsoonal China

Local topography and elevation gradients can exert important influences on soil formation processes such as elemental migration, mineral weathering, and soil organic matter (SOM) accumulation, yet these influences remain insufficiently investigated to date, particularly in surface soils of subtropical monsoonal regions. Here, we report on an investigation of a series of surface soils collected from four different topographic locations across the subalpine Dajiuhu Critical Zone Observatory (CZO), representing hillslope (planar), swale and river channel (convergent), and bulge (divergent) microtopographic sites. Evidence provided by rare-earth element (REE) patterns, immobile element ratios, clay-mineral compositions, and particle-size distributions suggests that these soils have rather uniform parent materials. X-ray diffraction (XRD) analysis revealed that secondary clay minerals in these soils are complex, being dominated by various interstratified clays. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy shows similar patterns among the swale, bulge, and river-channel soils that differ from those of the hillslope soils. Most soils at convergent sites with poor drainage contain more smectitic clays (interstratifled illite/smectite and chlorite/smectite) and less vermiculitic clays (hydroxyl-in-terlayered vermiculite and interstratifled illite/hydroxyl-interlayered vermiculite), and exhibit weaker chemical weathering and fewer elemental losses than those from non-convergent sites. The diversity of clay types can be ascribed to the complexity and heterogeneity, in particular of pH and hydrology, in these soil environments. Across the range of microtopographic sites investigated here, elemental migration and chemical weathering are generally coupled, with greater elemental losses associated with more intense chemical weathering. Soil organic carbon (SOC) content generally increases at higher elevations, which is attributable to lower temperatures and a consequent reduction of microbial remineralization, and under more reducing soil water conditions. Though influenced by eolian dust, variations in Fe/Mn, Ce anomaly, and C-org/P can reflect redox conditions of different soils. SOC retention is also closely associated with soil redox status, with more reducing conditions being more conducive to SOC preservation.