NOT SOIL NOR ROCK ALONE—PEDOGENESIS AND LITHOGENESIS IN UNSATURATED ZONES

The unsaturated zone (UZ) is a complex biogeochemical reactor that sustains modern life. Pedogenic reactions release vital elements including calcium, potassium, and magnesium that are otherwise locked up in minerals during geologically brief—but critically important—plant-accessible portions of the rock cycle. The UZ couples the atmosphere, lithosphere, and biosphere, serving as a highly non-linear regulator of water, nutrients, and carbon. UZ water acts as mobile solvent, delivering reactants and removing products from sites of weathering and mineral formation. UZ water partitions local pore space within a profoundly evolving geometry of larger-scale soil structure, thereby controlling the key large-scale connectedness (topology) of gas and liquid phases. The resulting dynamic mediates residence times, redox states, complexation, and chemical activities, along with fluid permeabilities and solute dispersivities in gas and liquid phases. Hysteresis in the relation between water content θ and matric potential ψ acts to contain fluxes of water and heat within the shallow UZ. This containment provides plant-root microcosms more opportunity for water and nutrient extraction and moderates weathering reactions by controlling equilibrium-versus-kinetic control of chemical processes. UZs may be classified according to water-table depth, overall water balance, and water-flux variations in time and space, the latter being strongly dependent on soil-structure development. Collectively these factors determine whether UZs act as (1) rapidly weathering profiles beneath oxygen-intensive life, (2) disequilibrium pass-through conduits with little opportunity for weathering or lithogenesis, or (3) quasi-equilibrated hyporheic reservoirs of water and solutes where weathering fluxes are low but lithogenesis is high. Field data and modeling from studies that transect a range of climate extremes illustrate how pedogenesis, lithogenesis, and nutrient dynamics are all tied to the same underlying unsaturated-zone processes.