HOW SENSITIVE IS THE RATE OF BEDROCK WEATHERING TO NEAR-SURFACE CHANGES IN CRITICAL ZONE ARCHITECTURE?

We often surmise that regolith structure and the resultant hydraulic and biogeochemical properties are directly controlled by the structure of the underlying bedrock. Yet, studies continue to show we are in a new epoch on Earth, the Anthropocene, a period of time in which human activity plays a controlling role in the cycling of water, carbon, and nutrients in the Earth system. One question that emerges as we continue to move into the Anthropocene, is the degree to which near-surface changes in critical zone architecture such as altered pore networks and connectivity due to changes in root distributions and architecture can govern the rate of bedrock weathering? We explore this question at two watersheds in Kansas (USA), 1) the Konza prairie which has experienced woody encroachment into grasslands for over 40 years, and 2) the temperate Fitch Forest which is mantled by soil with a strong shrink-swell capacity and where the timing and duration of precipitation are projected to change in the coming decades. At Konza hydro-bio-geochemical evidence suggests that woody encroachment increases weathering rates, while model experiments suggest the woody encroachment compared to grasslands could push weathering fronts two times deeper and increase porosity and permeability by an order of magnitude over a 300-year period. At the Fitch Forest, soil structural and hydrologic data demonstrate the sensitivity of hydraulic conductivity, and thus the depth of infiltrating water, to changing meteoric conditions, while modeling results reveal that the depth of permeability contrasts at the hillslope scale has a negligible impact on weathering rates, though it does alter the lateral partitioning of water (via shallow and deep groundwater flow paths). At first glance the results from Konza and the Fitch Forest may seem contradictory, but they illuminate that the sensitivity of bedrock weathering rates to surficial changes are not the same everywhere, and, in this case, the difference may be attributed to the degree to which the depth distribution of biotic processes change in the subsurface.