NEAR SURFACE STRUCTURE GOVERNS DEEP WEATHERING FLUXES (Invited Presentation)

The physical structure of Earth’s living skin is often thought to be static over short time periods. However, changing climatic and land use/cover conditions can rapidly alter how biota physically structure the subsurface, govern moisture distribution and fluxes, and throttle the rate of respiration and organic carbon delivery to soils. Using multiple examples from across biomes and lithologies we use observations and reactive transport modeling (RTM) approaches to explore how near surface structure governs deep weathering fluxes. Specifically, we focus on ecosystems in transition (woody encroachment into grasslands, regenerating prairie and forest from row crop agriculture) and watersheds where ecotone boundaries may be sensitive to changing climatic conditions (e.g., snow dominated montane systems and temperate forest boundaries). For example, shifts in the physical pore structure (e.g., density and effective pore area) are observed as ecosystems undergo biotic transitions, which control the occurrence of preferential flow and fluxes of soil gasses (e.g., CO₂ and O₂). Pedon RTM simulations in areas undergoing woody encroachment show that deepening roots can control carbonate dissolution by regulating how much CO₂ is transported downward to deeper, more reactive zones. Hillslope RTM simulations demonstrate that changes in soil structure that alter the partitioning of vertical compared to lateral fluxes have profound impacts on the distribution of dissolved organic carbon into the subsurface and generation of weathering solutes. In high elevation mountain watersheds, decreasing snow fractions may lead to longer droughts and deepening of roots which tap deeper groundwater sources and enhance the proportion of flow penetrating to depth. We articulate an emerging story depicting how changes in near surface structure are altering the trajectory of deep weathering processes.