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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.
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REWTCrunch: A Modeling Framework for Vegetation Induced Reactive Zone Processes in the Critical Zone
Abstract Vegetation optimizes its geochemical environment for resource management via root exudation. We refer to the soil zone where biogeochemical behavior is significantly influenced, directly or indirectly, by root processes as the vegetation induced reactive zone (VIRZ). Root exudates react with VIRZ soil substrates creating temporally variable chemical environments through depth that extend below the rooting zone, impacting weathering, and releasing solutes and gases. We present a new framework, REWTCrunch, to capture VIRZ dynamics by integrating three modeling advances: the multicomponent reactive transport model CrunchFlow, the root exudation model REWT, and the multilayer canopy‐root ecohydrologic model MLCan. REWTCrunch's high‐resolution, process‐based simulation of root exudation, and the transport and transformation of carbon (C) and nutrients according to mass‐balanced and charge‐balanced reaction networks gives new insight into vertically resolved root‐soil‐microbe‐water interactions and their influence on solute fluxes at a daily timescale. We benchmark REWT and CrunchFlow, illustrate coupling mechanisms, and present REWTCrunch simulations for an agricultural site in the US Midwest. Results demonstrate root‐sourced reactive C can augment or reduce solute concentrations in the soil by several orders of magnitude. Silicate weathering products illustrate after‐harvest effects of plant C inputs in leaching patterns. Calcium simulations reveal the development of a stable weathering front. Aluminum concentrations are particularly responsive to root‐sourced reactivity, and analysis of leaching concentration versus leaching flux indicates hysteresis behavior. REWTCrunch significant improves our ability to simulate the link between root processes and soil biogeochemistry, thereby filling an important gap in the numerical simulation of root processes, weathering, and long‐term soil health.
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- PAR ID:
- 10363350
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 127
- Issue:
- 2
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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