Interactions between carbon cycling and bedrock weathering in a forest of the Northern California Coast Ranges

Date

2021-12-06

Authors

Tune, Alison Kristine

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Abstract

The overarching objective of this dissertation is to better understand Earth's carbon cycle by identifying linkages between plant mediated carbon cycling and bedrock weathering processes. Such processes have been extensively studied in soils to identify the magnitudes and dynamics of carbon sources and sinks. However, recent studies reveal the prevalence of root networks that extend beneath soils into bedrock that may also participate in these dynamics. The contribution of roots in bedrock to carbon cycling in the subsurface is presently poorly understood. To identify the processes operating in the deep root zone and their role in mineral weathering and carbon cycling, this dissertation uses specialized instrumentation installed within an actively weathering forested hillslope in the Northern California Coast Ranges. This dissertation documents carbon and water fluxes at high spatial and temporal resolution throughout a 16 m weathering profile underlying a mixed hardwood-conifer ecosystem. These measurements demonstrate that substantial rhizosphere-related respiration occurs in the deeper, bedrock root zone such that shallow, soil respiration dynamics are likely insufficient to capture subsurface carbon processes in forests characterized by rooting into bedrock. Carbon dioxide emission from the ground surface (also termed soil efflux) is typically thought to be controlled by microbial and plant activity in shallow soils, however, this dissertation reveals that the deeper bedrock contributes to these emissions and can account for up to 100% of the efflux during seasonal drought when soils are dry but roots continue to pull water from the deeper bedrock layers. The carbon dioxide produced in the deeper root zone dissolves in water and this water transits downward, acidifying deeper parts of the weathering profile and driving weathering reactions. The flux of reactivity delivered to the deeper weathering profile scales with the flux of water through the weathered bedrock, such that very little dissolved carbon dioxide is transported during drought years and instead remains in place in the root zone. Monitoring through an extreme drought period also revealed shifts in gas dynamics that reflect changes in the carbon source for respiration, indicating that plant water stress has implications for bedrock weathering. Carbon isotope measurements and laboratory incubation experiments reveal that modern, root-related processes are responsible for the observed carbon dynamics. The organic matter contained within the bedrock (termed petrogenic organic carbon) was shown to be oxidized within the root zone and lower unsaturated zone, with rates comparable to that in riverine systems. Rates of petrogenic organic carbon oxidation in the unsaturated zone may be important to climate over long time scales. The results of this dissertation research reveal that roots that penetrate bedrock fractures beneath soils play an important and sometimes dominant role in forest water and carbon cycling with implications for landscape evolution, water quality, and climate feedbacks

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