Subsurface structure controls water transit time, carbon transformation, and carbonate weathering at the hillslope scale

Abstract:

It is generally known that changes in subsurface structure associated with climate and land cover can alter flow paths and change biogeochemical transformations and transport. However, causal and quantitative connections between subsurface structures, hydrological connectivity, and biogeochemical reaction rates remain elusive. In this work, we asked the question: how and to what degree does subsurface structure control water transit time, soil organic carbon (SOC) transformation, and carbonate weathering at the hillslope scale? To answer this question, we first developed a reactive transport model using soil moisture and water chemistry data from a forested hillslope at the University of Kansas Fitch Natural History Reservation in Kansas, United States, which is co-located with a National Ecological Observatory Network site. Numerical experiments were carried out with different permeability depth distributions under a range of water infiltration rates. Simulation results demonstrate that a sharper decrease in the permeability depth distribution (i.e., higher constant between shallow soil and deep bedrock layer) reduces the vertical hydrological connectivity (VC), quantified as the proportion of water exiting the hillslope from below a 2-meter depth. Lower VC partitions yielded a greater abundance of relatively young water (i.e., short transit time) flowing through the soil layer into the stream. Shorter transit times promotes contact between young water and SOC in the shallow layer and, therefore, promotes the transformation of SOC into dissolved organic carbon (DOC) and CO₂. Under the conditions of higher VC, however, less soil CO₂ was produced at the shallow soil and transported into the deep carbonate bedrock, yielding lower calcite weathering rates. The influence of subsurface structures appears more significant under high flow conditions. Further analyses of the transit time-reaction rate relationships indicate that SOC transformation rates into DOC and soil CO₂ depend more on the transit time through the soil (i.e., younger water), while calcite weathering rates rely heavily on groundwater transit time (i.e., older water). This work suggests that subsurface structure plays a critical role in determining carbon transformation pathways.