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Supporting resources for 'Deep roots supply reactivity and enhance silicate weathering in the bedrock vadose zone' Osorio-Leon, I.D., Rempe, D.M., Golla, J.K., Bouchez, and Druhan, J.L. (2025) 'Deep roots supply reactivity and enhance silicate weathering in the bedrock vadose zone', AGU Advances. Contents in this upload include: CrunchTope reactive transport model database, input and relevant output files Code to read in model results and build figures See the README.md for detailed instructions on how to run the code and reproduce results. 

Abstract Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures (Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and Li are negatively correlated in both events, but mean Li ratios and associated ranges of variation are distinct between them. We apply a previously developed reactive transport model (RTM) for the site to these event‐scale flow perturbations, but observed shifts in stream Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream Li signatures, we couple the RTM to a distribution of fluid transit times that evolve based on storm hydrographs. The approach guides appropriate flux‐weighting of fluid from the RTM over a range of flow path lengths, or equivalently fluid residence times. This flux‐weighted RTM approach accurately reproduces dynamic storm Li‐discharge patterns distinguished by the antecedent conditions of the watershed. 

This resource provides accompanying discharge and hydrologic data (silicon, sodium, silicon stable isotopes, and discharge and irrigation fluxes) presented in 'Stable Silicon Isotope Fractionation Reflects the Routing of Water Through a Mesoscale Hillslope', Earth and Planetary Letters, https://doi.org/10.1016/j.epsl.2024.119098 This dataset was collected at the Landscape Evolution Observatory (LEO) in Tucson, Arizona from 2022-2023. Discharge chemistry and flux are reported for each of the three LEO hillslopes. Files are provided in ".csv" format. 

Abstract Lithium isotope ratios (δ⁷Li) of rivers are increasingly serving as a diagnostic of the balance between chemical and physical weathering contributions to overall landscape denudation rates. Here, we show that intermediate weathering intensities and highly enriched stream δ⁷Li values typically associated with lowland floodplains can also describe small upland watersheds subject to cool, wet climates. This behavior is revealed by stream δ⁷Li between +22.4 and +23.5‰ within a Critical Zone observatory located in the Cévennes region of southern France, where dilute stream solute concentrations and significant atmospheric deposition otherwise mask evidence of incongruence. The water‐rock reaction pathways underlying this behavior are quantified through a multicomponent, isotope‐enabled reactive transport model. Using geochemical characterization of soil profiles, bedrock, and long‐term stream samples as constraints, we evolve the simulation from an initially unweathered granite to a steady state weathering profile which reflects the balance between (a) fluid infiltration and drainage and (b) bedrock uplift and soil erosion. Enriched stream δ⁷Li occurs because Li is strongly incorporated into actively precipitating secondary clay phases beyond what prior laboratory experiments have suggested. Chemical weathering incongruence is maintained despite relatively slow reaction rates and moderate clay accumulation due to a combination of two factors. First, reactive primary mineral phases persist across the weathering profile and effectively “shield” the secondary clays from resolubilization due to their greater solubility. Second, the clays accumulating in the near‐surface profile are relatively mature weathering byproducts. These factors promote characteristically low total dissolved solute export from the catchment despite significant input of exogenous dust. 

Bedrock mineral breakdown in mountainous landscapes sustains long-term atmospheric CO₂ drawdown and releases solutes that sustain ecosystems and set the chemistry of upland stream water. In the unsaturated Bedrock Vadose Zone (BVZ) of hillslopes, minerals, water, reactive gases, and deep roots interact to produce a hotspot of these weathering reactions. New evidence suggests that, in the BVZ, rock moisture sustains evapotranspiration, while deep roots drive deep CO₂ production. However, the contribution of the BVZ to catchment-scale chemical weathering fluxes, and particularly the role of deep roots as drivers of this reactivity, remains poorly constrained due to the difficulty of direct access and measurement of fluids in this compartment. Quantifying this BVZ chemical reactivity is crucial to advance our understanding of upland landscapes' influence on the global carbon cycle and river solute fluxes. Here, we leverage an innovative Vadose-zone Monitoring System (VMS) to investigate these processes and their biogeochemical implications. We present four years of in-situ monitoring of water and pore gas composition in a ~16-meter weathered bedrock profile at the Eel River Critical Zone Observatory (ERCZO) in California. Using this novel instrumentation, we show that the average geogenic chemical solute efflux from the BVZ is 80 ± 34 t km⁻² yr⁻¹, representing approximately 68% of the solute flux measured in the creek draining the hillslope. This highlights the substantial role of the BVZ in catchment-scale weathering. To further interpret these data, we develop a multi-component reactive transport model (RTM) calibrated with batch-reactor experiments using drilling cuttings from the hillslope. Results indicate that solute concentrations arise from a coupled set of primary mineral dissolution and secondary mineral precipitation reactions, which evolve with depth and align with the mineralogical composition gradient from soil to fresh bedrock. Our RTM demonstrates that water storage and drainage timescales in the vadose zone are sufficient to allow significant evolution of pore water due to chemical weathering. However, the model cannot reproduce observed solute concentrations unless elevated CO₂ from deep rhizosphere respiration is incorporated. We find that organic carbon respiration in the deep rhizosphere enhances chemical weathering fluxes at the ERCZO by a factor of ~1.8 ± 0.5, providing direct evidence of accelerated chemical weathering in the Critical Zone due to the presence and function of deeply rooted vegetation. A key characteristic of the linkage between deep rhizosphere respiration and weathering reactions is that, despite strong seasonal variations in water content and O₂ profiles, CO₂ concentrations in the BVZ remain stable, leading to steady weathering rates across seasons and years. Sensitivity analyses with our calibrated RTM reveal that this stability results from the interplay between labile organic carbon oxidation and seasonal hydrological conditions. During dry periods, O₂ is abundant throughout the BVZ due to opening of pore space, but DOC availability is constrained by low water content. Conversely, wet periods enhance availability of labile organic carbon, but O₂ becomes limiting. This balance maintains stable CO₂ profiles and, ultimately, steady weathering rates. We propose that this buffering mechanism allows watersheds to modulate seasonal variability in ecological, hydrological, and climatic inputs, contributing to the pervasive observation of stable concentrations of geogenic solutes in streams draining these landscapes despite their strong variations in discharge. These findings underscore the critical role of the BVZ in regulating solute fluxes from hillslopes, and suggest that deep vadose zone processes must be integrated into biogeochemical and hydrological models to accurately predict catchment responses to environmental change. 

Vegetation simultaneously drives transpiration, a significant component of the hydrological balance, and stimulates the breakdown of bedrock and formation of soil. Together these actions impact both the magnitude of streamflow and the chemical or solute load of the stream. Yet, it is still unknown under what conditions deeply rooted plants enhance or impede chemical weathering of rocks. An unestablished link at the heart of this gap in knowledge are the ways in which coupling between plant water demand, plant nutrient demand and recycling of these elements through the ecosystem manifest in the geochemical composition of watersheds and the rivers that drain them. Addressing this unknown is of utmost practical importance to water resource management, environmental stewardship, ecosystem resilience to disturbance (storms, fire, drought), and ultimately nutrient effluxes from watersheds. This presentation introduces two recent advancements in our collective capacity to deconvolve these vital linkages between ecology, hydrology and chemical weathering: novel observational tools and ecologically informed chemical weathering models. First, we present hydrologic and geochemical observations from within the deep root-zone gained from the successful deployment of a Vadose zone Monitoring System (VMS). The VMS allows for real-time moisture content monitoring as well as discrete sampling of water and reactive gases across partially saturated bedrock.^{This novel capability has now revealed that the mature, deeply rooted forest relies on water stored in bedrock above the water table during the extended dry season. The VMS has also shown CO₂concentrations and production rates in the deep root zone comparable to what is typically observed in shallow soils. This deep CO₂is radiocarbon modern and thus associated with recent photosynthetically fixed carbon. Water chemistry observations from these depths indicate that this CO₂production in the deep root-zone enhances chemical weathering by increasing carbonic acid formation. Thus, the extension of water and carbon fluxes to depths meters below soils leads to a hotspot of chemical weathering in the deep root zone where meteoric water, carbonic acid weathering potential, and primary minerals all intersect.} Second, we present the derivation and testing of a new ecologically informed reactive transport model (RTM) which directly simulates the uptake of both water and nutrients across the deep rhizosphere. A key aspect of our modeling framework is that, unlike transpired water, rock-derived nutrients taken up by plants are not lost to the atmosphere but rather recycled into litter and soils, creating a biological pool in the Critical Zone cycling of rock-derived nutrients. Our model leads to the hypothesis that the water and nutrient demands of ecosystems, coupled with the capacity to partially or fully recycle elements to the soil surface, regulate the observed rates and depth of chemical weathering reactions. These results reveal the capacity for plant water and nutrient demands to both enhance and impede mineral weathering reactions, to drive formation of secondary minerals, and to redistribute elements across the vertical weathering profile. Ultimately, our model allows us to demonstrate how plant water and nutrient requirements manifest in the export of water and solutes by streams. The development of this forward model in tandem with critical advancements in direct observation and sampling of the deep rhizosphere is now poised to provide a foundation upon which to improve our understanding of reactive transport processes in watersheds and Critical Zone systems, which in turn supports advancements in ecohydrology, global elemental budgets, watershed stewardship, and water quality resources.