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1. Biogeochemical reactivity in the unsaturated zone revealed by in-situ measurements

   https://doi.org/10.3897/aca.8.e155241  

   Osorio-Leon, Ivan; Rempe, Daniella; Bouchez, Julien; Druhan, Jennifer (May 2025, ARPHA Conference Abstracts)

   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. 

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2. Signatures of the deep rhizosphere: Novel instrumentation and predictive models

   https://doi.org/10.3897/aca.8.e156623  

   Druhan, Jennifer; Osorio-Leon, Ivan; Benettin, Paolo; Rempe, Daniella; Bouchez, Julien (May 2025, ARPHA Conference Abstracts)

   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. 

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3. Enhanced Carbonate Weathering due to Woody Encroachment

   Sadayappan, Kayalvizhi; Keen, Rachel; Moreno, Victoria; Sullivan, Pamela; Nippert, Jesse; Li, Li (December 2021, American Geophysical Union Annual Conference)

   Woody encroachment is a widespread phenomenon in grassland ecosystems, driven by overgrazing, fire suppression, nitrogen deposition and climate change, among other environmental changes. The influence of woody encroachment on processes such as chemical weathering however is poorly understood. In particular, for fast reactions such as carbonate weathering, root traits associated with woody encroachment (e.g., coarser, deeper, and longer residence times) can potentially change fluxes of inorganic carbon into streams and back to the atmosphere, providing CO2-climate feedbacks. Here we examine the influence of deepening roots arising from woody encroachment on catchment water balance and carbonate weathering rates at Konza a tallgrass prairie within a carbonate terrain where woody encroachment is suspected to drive the groundwater alkalinity upwards. We use a watershed reactive transport model BioRT-Flux-PIHM to understand the ramifications of deepening roots. Stream discharge and evapotranspiration (ET) measurements were used to calibrate the hydrology model. The subsurface CO2 concentration, water quality data for groundwater, stream, soil water and precipitation were used to constrain the soil respiration and carbonate dissolution reaction rates. The hydrology model has a Nash-Sutcliffe Efficiency value of 0.88. Modelling results from numerical experiments indicate that woody encroachment results in overall lower stream flow due to higher ET, yet the groundwater recharge is higher due to deep macropore development from deepening roots. The deeper macropores enhance carbonate weathering rate as more acidic, CO2-rich water recharges the deeper calcite bedrock. Accounting for the change in inorganic carbon fluxes caused by such land use changes gives a better estimate of carbon fluxes in the biosphere. Such knowledge is essential for effective planning of climate change mitigation strategies. 

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4. Deepening roots can enhance carbonate weathering by amplifying CO₂-rich recharge

   https://doi.org/10.5194/bg-18-55-2021  

   Wen, Hang; Sullivan, Pamela L.; Macpherson, Gwendolyn L.; Billings, Sharon A.; Li, Li (January 2021, Biogeosciences)

   null (Ed.)

   Abstract. Carbonate weathering is essential in regulating atmosphericCO2 and carbon cycle at the century timescale. Plant roots accelerateweathering by elevating soil CO2 via respiration. It however remainspoorly understood how and how much rooting characteristics (e.g., depth anddensity distribution) modify flow paths and weathering. We address thisknowledge gap using field data from and reactive transport numericalexperiments at the Konza Prairie Biological Station (Konza), Kansas (USA), asite where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots can enhance weathering in two ways.First, deepening roots can control thermodynamic limits of carbonatedissolution by regulating how much CO2 transports vertical downward tothe deeper carbonate-rich zone. The base-case data and model from Konzareveal that concentrations of Ca and dissolved inorganic carbon (DIC) areregulated by soil pCO2 driven by the seasonal soil respiration. Thisrelationship can be encapsulated in equations derived in this workdescribing the dependence of Ca and DIC on temperature and soil CO2. The relationship can explain spring water Ca and DIC concentrations from multiple carbonate-dominated catchments. Second, numericalexperiments show that roots control weathering rates by regulating recharge(or vertical water fluxes) into the deeper carbonate zone and exportreaction products at dissolution equilibrium. The numerical experimentsexplored the potential effects of partitioning 40 % of infiltrated waterto depth in woodlands compared to 5 % in grasslands. Soil CO2 datasuggest relatively similar soil CO2distribution over depth, which in woodlands and grasslands leads only to 1 % to∼ 12 % difference inweathering rates if flow partitioning was kept the same between the two landcovers. In contrast, deepening roots can enhance weathering by ∼ 17 % to200 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/a. Weathering rates in these cases however are more than an order of magnitude higher than a case without roots atall, underscoring the essential role of roots in general. Numericalexperiments also indicate that weathering fronts in woodlands propagated> 2 times deeper compared to grasslands after 300 years at aninfiltration rate of 0.37 m/a. These differences in weathering fronts areultimately caused by the differences in the contact times of CO2-charged water with carbonate in the deep subsurface. Within the limitation of modeling exercises, these data and numerical experiments prompt the hypothesis that (1) deepening roots in woodlands can enhance carbonate weathering by promotingrecharge and CO2–carbonate contact in the deepsubsurface and (2) the hydrological impacts of rooting characteristics canbe more influential than those of soil CO2 distribution in modulatingweathering rates. We call for colocated characterizations of roots,subsurface structure, and soil CO2 levels, as well as their linkage to waterand water chemistry. These measurements will be essential to illuminatefeedback mechanisms of land cover changes, chemical weathering, globalcarbon cycle, and climate. 

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5. Linking Surface Processes, Solute Generation, and CO ₂ Budgets Across Lithological and Land Cover Gradients in Rocky Mountain Watersheds

   https://doi.org/10.1029/2023WR036850  

   Slosson, John R; Larsen, Isaac J; Winnick, Matthew J; Marmolejo‐Cossío, José M; Williams, Kenneth H (April 2025, Water Resources Research)

   Abstract Chemical weathering in mountain critical zones controls river chemistry and regulates long‐term climate. Mountain landscapes contain diverse landforms created by geomorphic processes, including landslides, glacial moraines, and rock glaciers. These landforms generate unique flowpaths and water‐rock interactions that modify water chemistry as precipitation is transformed to streamflow. Variations in lithology and vegetation also strongly control water chemistry. Prior work has shown that landslides generate increased dissolved solute concentrations in rapidly uplifting mountains. However, there is still uncertainty regarding the magnitude which different geomorphic processes and land cover variations influence solute chemistry across tectonic and climatic regimes. We measured ion concentrations in surface water from areas that drain a variety of landforms and across land cover gradients in the East River watershed, a tributary of the Colorado River. Our results show that landslides produce higher solute concentrations than background values measured in streams draining soil‐mantled hillslopes and that elevated concentrations persist centuries to millennia after landslide occurrence. Channels with active bedrock incision also generate high solute concentrations, whereas solute concentrations in waters draining moraines and rock glaciers are comparable to background values. Solute fluxes from landslides and areas of bedrock incision are 1.6–1.8 times greater than nearby soil‐mantled hillslopes. Carbonic acid weathering dominates surface water samples from watersheds with greater vegetation coverage. Geomorphically enhanced weathering generates hotspots for net CO₂release or sequestration, depending on lithology, that are 1.5–3.5 times greater than background values, which has implications for understanding links among surface processes, chemical weathering, and carbon cycle dynamics in alpine watersheds. 

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