Time to Anoxia: Oxygen Consumption in Soils Varies Across a Coastal Gradient


Kaizad F. Patel1* (kaizad.patel@pnnl.gov), Kenton A. Rod1, Peter Regier2, Fausto Machado-Silva3, Jianqiu Zheng1, Cooper G. Norris1, Kennedy Doro3, Matthew Kaufman1, Nate McDowell1, J. Patrick Megonigal4, Teri O’Meara5, Peter Thornton5, Ken Kemner6, Nick Ward1, Michael N. Weintraub3, Vanessa L. Bailey1


1Pacific Northwest National Laboratory, Richland, WA; 2Marine and Coastal Research Laboratory, Pacific Northwest National Laboratory, Sequim, WA; 3University of Toledo, Toledo, OH; 4Smithsonian Environmental Research Center, Edgewater, MD; 5Oak Ridge National Laboratory, Oak Ridge, TN; 6Argonne National Laboratory, Lemont, IL



The coastal terrestrial-aquatic interface (TAI) is a highly dynamic system characterized by strong physical, chemical, and biological gradients. Shifting redox conditions due to variations in soil saturation and dissolved oxygen (O) concentrations strongly drive carbon (C) availability and transformations at the TAI. However, it remains challenging to predict across the TAI the rate at which saturation drives aerobic to anoxic shifts in soils with different characteristics and inundation regimes. The research team hypothesized that biological and chemical differences in soils would result in variable O consumption rates across the TAI gradient from uplands to wetlands. To test this hypothesis, researchers integrated field measurements, laboratory incubations, and model simulations to mechanistically understand O consumption dynamics in coastal soils.

To measure O consumption rates in TAI soils, researchers conducted a laboratory incubation using surface and subsurface soils from a coastal TAI gradient (upland forest to transitional forest to wetland) in the Western Lake Erie region. The cores were inundated with water and incubated for two weeks. Dissolved O concentration was measured continuously over the two-week incubation period using an O-sensitive optode spray.

Wetland soils turned anoxic the fastest, in under 10 hours, whereas upland soils turned anoxic in 18 hours. Upland B-horizon soils did not turn anoxic even after two weeks of saturation in the laboratory, and the O consumption patterns suggest C and/or nutrient limitation in these subsurface soils. These experimental results are consistent with in situ redox and O measurements in the field where wetland soils exhibited the highest O consumption rates along the coastal transect. O-D reaction scale simulations with AquaMEND accurately captured O drawdown dynamics in these systems. Sensitivity analysis indicated that microbial activity was a strong driver of O consumption in these soils, with the availability of dissolved C fueling microbial metabolism as the key limiting factor of O consumption.

This research, part of the Coastal Observations, Mechanisms, and Predictions Across Systems and Scales–Field, Measurements, and Experiments project, provides key context for the understanding of coupled hydrological and biogeochemical processes and contributes to an enhanced understanding of redox controls on C and nutrient cycling across the coastal TAI.