Collaborative Research: How do hydrology and biogeochemistry control carbon flux from headwater streams to the atmosphere?

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It is known that streams collect, process, and release much of a basin's water. It is less known that streams also collect, process, and release much of a basin's carbon. Carbon is collected by streams in various forms, some of that carbon is processed by microbes in the streambed into carbon dioxide (CO2), and much of that CO2 is vented to the atmosphere during downstream flow. These processes are particularly important in small, headwater streams. Since headwater basins drain significant land surface, these coupled water-carbon processes may be globally significant. This project will measure, model, and understand this coupled water-carbon cycle via intensive study of the stream carbon cycle within one headwater stream draining a 96-hectare basin in western Oregon. By using the Watershed 1 of the HJ Andrews Experimental Forest, this study will leverage a history of detailed data collection and a large number of existing measurements. The project team will partner with the Audubon Society to teach 1000 school children stream hydrology and ecology in the field.This project has two main goals. The first is to develop an integrated modeling framework to simulate carbon cycling in streams that is widely transferable to model subsurface and surface processes of streams in earth system models. The second is to test and present an integrated theory of stream and groundwater hydrology, carbon biogeochemistry, and CO2 production and efflux in a stream. These goals will be accomplished by developing an integrated carbon model for headwater streams that simulates hydrologic processes (e.g., transport, hyporheic exchange) and biogeochemical processes (e.g., transformation between particulate and dissolved organic carbon, aerobic respiration rates of these carbon "pools"). Modeling will be coupled with direct measurements of groundwater delivery of soil-respired CO2, biological and physical processes controlling carbon cycling, measurements of dissolved oxygen, hydrology, and CO2 efflux from the stream to subcanopy respiration measured at a flux tower above the stream. In situ hyporheic mesocosms will be used to manipulate and monitor reactive transport along flow paths to parameterize biogeochemical rate expressions for use in the integrated model. Finally, the project will upscale the reach-scale processes to watershed scale to explain carbon fluxes from river networks.