Biogeochemistry and Function Across the Terrestrial-Aquatic Interface: Transition Zones Present Unique and Non-Conservative Behavior

Authors

Stephanie J. Wilson¹* (wilsonsj@si.edu), Patrick Megonigal¹, Fausto Machado-Silva², Roberta Bittencourt Peixoto², Matthew Kovach², Leticia Sandoval², Roy Rich¹, Alice Stearns¹, Evan Phillips¹, Peter Regier³, Allison Myers-Pigg³, Stephanie Pennington³, Anya Hopple¹, Kendalynn A. Morris³, Nate McDowell³, Ben Bond-Lamberty³, Mike Weintraub², Nick Ward³, Ken Kemner⁴, Vanessa Bailey³

Institutions

¹Smithsonian Environmental Research Center, Edgewater, MD; ²University of Toledo, Toledo, OH; ³Pacific Northwest National Laboratory, Richland, WA; ⁴Argonne National Laboratory, Lemont, IL

URLs

https://compass.pnnl.gov/FME/COMPASSFME

Abstract

Coastal ecosystems are dynamic transition zones between land and water, spanning shoreline, wetland, and upland ecosystems that compress and expand in response to tides, lake-level changes, weather, and climate. Changes to inundation regimes significantly alter carbon cycling and soil biogeochemistry of coastal terrestrial aquatic interfaces (TAIs).

Understanding these shifts and the major drivers of ecosystem function is critical to representing and predicting change in coastal TAI biogeochemistry. As a part of the Coastal Observations, Mechanisms, and Predictions Across Systems and Scales-Field, Measurements, and Experiments (COMPASS-FME) project, researchers installed in situ sensor networks and conducted high-resolution monitoring of the interactions among water, soil, and plants to determine how biogeochemical cycling of carbon and nutrients shift across the TAIs in response to shifting inundation regimes. Seven field sites were established with transects spanning upland forests, stressed forests transitioning to wetland, and herbaceous wetland to capture temporally dynamic behaviors across diverse coastal systems in the Chesapeake Bay and Great Lakes regions. At each site, monthly measurements of soil methane fluxes, redox profiles, conductivity, and porewater constituents were made. In the Chesapeake Bay, soil redox potential decreased along gradients from upland forest to wetland, and soil conductivity and porewater sulfate concentrations increased. In both regions, wetland zones had the highest methane (CH₄) emissions, with diminished CH₄ uptake in transitional forests prior to tree mortality suggesting that: (1) soils respond more rapidly than vegetation to changing hydrological regimes; and (2) flooding exerts stronger control on CH₄ than terminal electron acceptor abundance (e.g., SO₄^2-). These results suggest that transition zone soils don’t simply fall between wetland and upland endmembers; they have unique properties. This has important implications for understanding the impacts of changing inundation on greenhouse gas emissions and carbon storage along the terrestrial-aquatic interface at the ecosystem scale.