A unique vadose zone monitoring system (VMS) has afforded the ability to sample water and gas for two consecutive years over an 18 m thick, variably saturated argillite weathering profile at the Eel River Critical Zone Observatory. Our data illustrates significant aerobic CO2 production many meters below the thin soils, which generates acidity and increased dissolved inorganic carbon. Radiocarbon characterization confirms that across dry and wet seasons, the CO2 produced at depth is modern, and thus sourced from the surface through rhizodeposition of the mature forest ecosystem. Critically, this CO2 production, which peaks 5 - 8 meters below the land surface, is necessary to accurately simulate the observed solute concentration depth profiles within a reactive transport framework. In the absence of this CO2 source, models calibrated from batch experiments using this regolith are unable to produce a reasonable representation of the increase in solute concentrations with depth through the weathering profile. Based on this collective platform of gas and solute characterization and modeling, we demonstrate that the carbon cycle associated with deep root functioning is necessary to the water-rock interactions which define the weathering profile of the hillslope. Using this calibrated simulation capability, we explore a series of hindcasting and forecasting scenarios. The former includes the development of the weathering profile in the absence of this carbonic acid source and the development of the weathering profile with rhizodeposition located at shallower depths. The latter include changes to solute composition and weathering signatures associated with a loss of rhizodeposition, a decline in water content, and changes to seasonal wet and dry conditions. In total, our simulation platform, which is constrained by direct observations of solutes in the bedrock vadose zone, affords a means to distinguish the coupling and dependence of solute signatures and weathering profile structure on the activity of rooting in bedrock.
Druhan, J.L., Tune, A. K., Wang, J. J., Rempe, D. M. (2019): A reactive transport framework for the co-evolution of rhizodeposition and mineral weathering below soil. American Geophysical Union 2019 Fall Meeting, San Francisco, CA, 9-13 December 2019 Abstract #B34B-05 (Invited).
This Paper/Book acknowledges NSF CZO grant support.