B079-0006
Spatiotemporal redox dynamics regulate mineral and metabolic constraints on carbon export from floodplain soils
Monday, 14 December 2020
Poster
Carolyn Anderson1, Christian Dewey2, Malak M Tfaily3, Ravi K Kukkadapu4, Kew William5, Peter S Nico6, Patricia M Fox6, Scott E Fendorf2 and Marco Keiluweit1, (1)University of Massachusetts Amherst, Amherst, MA, United States, (2)Stanford University, Department of Earth System Science, Stanford, CA, United States, (3)University of Arizona, Tucson, AZ, United States, (4)Pacific Northwest National Lab, Richland, WA, United States, (5)Pacific Northwest National Laboratory, Environmental Molecular Sciences Laboratory, Richland, United States, (6)Lawrence Berkeley National Lab, Berkeley, CA, United States
Abstract:
Floodplain soils are large and dynamic reservoirs of carbon (C), where seasonal flooding regulates both C storage and export downstream. Timing and frequency of flooding events are altered by climate change, increasingly subjecting floodplain soils to extreme flooding or droughts. These shifts have profound implications on greenhouse gas emissions and dissolved organic carbon (DOC) export. Yet, the underlying (hydro)biogeochemical controls on C retention and export in floodplain soils are poorly constrained, limiting our ability to predict responses to climate change. Here we aimed to determine how seasonal flooding, and associated spatiotemporal variations in redox conditions, impact the dominant controls on microbial soil C cycling. Using in-field monitoring and advanced analytical and molecular tools, we examined how changes in mineral interactions and microbial metabolism during flooding and subsequent drainage affected C export from floodplain soils of the mountainous East River watershed (Gothic, Colorado).
Our results show the abundance of reactive iron (Fe) mineral phases varies with soil depth and is a significant control on C concentrations across floodplain soils, and that these mineral associations are sensitive to seasonal redox dynamics. Specifically, we find that reducing conditions during flooded periods caused reductive dissolution of Fe (hydr)oxides, leading to redox-driven mobilization of mineral-associated organic matter and enhancing DOC export. At the same time, flooding decreased CO2 production and selectively preserved chemically-reduced organic matter, likely due to metabolic constraints on microbial respiration. Upon drainage and re-oxygenation of floodplain soils, CO2 production increased, partly due to the oxidation of reduced organic compounds, but was limited by the concurrent entrapment of DOC by newly precipitated Fe (hydr)oxides.
Combined, our results reveal that spatiotemporal redox variations during seasonal flooding shift the relative and interactive effects of mineral and metabolic constraints on CO2 and DOC export from floodplain soils. These findings provide a mechanistic framework for understanding how changes in the intensity and timing of flooding may alter predominant pathways, rates, and controls of C export from floodplain soils.
