"Water Storage, Carbon Respiration and Mineral Weathering Below Soil: New Insights into the Deep Vadose Zone"

In forested uplands, rooting depths extend below thin soils into weathered bedrock. Precipitation infiltrating through these hillslopes is stored and partitioned into evapotranspiration demand and base flow to headwater streams. The ecologically regulated flowpaths and reactive potential of this water determine stream chemistry. How, where, and to what extent deeply rooted trees regulate these transport and chemical processes has yet to be systematically resolved. Here we use a novel Vadose Zone Monitoring System (VMS) installed in the Eel River Critical Zone Observatory (ERCZO) to demonstrate that the deep bedrock rhizosphere plays a pivotal role in the location and rates of chemical weathering reactions within the hillslope, thus dictating the solute composition of headwater streams. 

Through unique monitoring and simulation capabilities, we highlight the role of three key mechanisms by which vegetation alters fluid composition within the deep bedrock rhizosphere.  First, deep roots seasonally extract water many meters below soil and thus uniquely alter the storage volumes and transit times of fluids throughout the hillslope. Second, the dynamic hydrology of the bedrock rhizosphere controls the volume and connectivity of pore space open to gases, and consequently the fluxes of reactive O_2 and CO_2 across the bedrock vadose zone. Third, root-driven transport and respiration of organic carbon and the attendant fluxes of CO_2 and O_2 below soil impart reactivity to fluids migrating through the  bedrock vadose zone. 

In summary, effective simulation of the chemical evolution of water within a catchment requires a representation of the impacts of deep root carbon dynamics and water uptake on both fluid reactive potential and residence time.