Clays are not created equal: How clay mineral type affects soil parameterization

Abstract:

Clay minerals dominate the soil colloidal fraction and its specific surface area, thus affecting many aspects of soil behavior and fluxes across the land surface. Consequently, soil clay fraction is an integral component in estimating soil hydraulic and mechanical properties (SHMP) in most pedotransfer functions (PTFs), which are simplified empirical relations used to parameterize land surface models. At present, most PTFs rely on soil texture, but ignore differences among clay mineral types that dominate soil in different climatic regions. However, the clay mineralogy impacts the specific surface area, and the dominant mineralogy varies systematically across the globe. In particular, the large specific surface area of montmorillonite in temperate regions compared to kaolinite, which dominates tropical regions (16% of the land surface), systematically reduces both the soil hydraulic conductivity and its macroscopic friction. We propose new clay-type informed PTFs for determining SHMP based on grain-size distribution that also use soil specific surface area to represent, in spatially-resolved manner, the additional influence of clay mineralogy on soil saturated hydraulic conductivity and mechanical strength for hydrologic and natural hazard modeling. These new PTFs, and the resulting spatially distributed and clay mineral corrected SHMPs, exhibit strong climatic and spatial segregation of active and inactive clays. Our new approach results in large differences from previous maps of tropical regions with implications for quantifying many Earth surface processes.