Understanding Drivers of Oxygen Consumption in Flooded Coastal Soils

The Science

Oxygen is an important driver of biogeochemical processes in soils. Coastal systems experience frequent flooding due to tidal cycles, variable rainfall, and storm surge events. These events can result in rapid consumption of oxygen, but the time scale of these processes is unknown. This study investigated oxygen dynamics in flooded soils, specifically testing how quickly oxygen was consumed in different coastal soils (upland vs. wetland, surface vs. subsurface soils) after a flood. By comparing field measurements with laboratory incubations and model simulations, researchers were able to identify different mechanisms controlling oxygen dynamics in wetland vs. forest soils.

The Impact

Oxygen consumption in soils is a complex process that typically occurs due to a combination of chemical and biological mechanisms. Even within a site, this study found that these mechanisms varied by landscape position; there was a stronger influence of biological (microbial) mechanisms in the upland forest soils, whereas chemical processes were more important in the wetland soils. By conducting laboratory incubations and model simulations, the team was able to test these systems in a controlled setting and isolate crucial processes and mechanisms. This work highlights the effectiveness of integrating field measurements, laboratory incubations, and model simulations to develop a stronger understanding of soil biogeochemical processes in coastal systems. This is especially relevant in coastal systems that experience flooding and are becoming increasingly vulnerable to events like sea level rise.

Summary

The coastal terrestrial–aquatic interface (TAI; defined here as a landscape that spans upland forest through a transitional forest to wetland) is a highly dynamic system characterized by strong physical, chemical, and biological gradients. Changing water levels cause regular changes in soil redox conditions. The consequent consumption of terminal electron acceptors in turn strongly influences carbon availability and transformations across TAIs. However, while redox dynamics are well described, there is limited ability to quantitatively forecast the rates at which the redox conditions change across a TAI within which soils have different characteristics and inundation regimes. This study integrated field measurements, laboratory incubations, and model simulations to improve mechanistic understanding of oxygen consumption dynamics in coastal soils.

Continuous in situ monitoring unexpectedly revealed that flooding conditions resulted in temporary spikes of subsurface dissolved oxygen, followed by its rapid consumption in the wetlands. To further investigate the mechanisms of oxygen consumption in a controlled setting, laboratory incubations were performed using surface and subsurface soils collected from a TAI gradient in Western Lake Erie. Oxygen consumption rates were measured during lab-simulated flood events in these TAI soils. Results showed that wetland soils reached anoxia the fastest, in ~9 hours on average, whereas upland soils turned anoxic in ~18 hours. Subsurface upland soils did not turn anoxic even after two weeks of saturation in the lab, and their oxygen consumption patterns suggested carbon and/or nutrient limitation. These results are consistent with in situ groundwater redox and oxygen measurements in the field, where wetland soils exhibited the highest rates of oxygen consumption along the TAI. Model simulations of oxygen consumption suggested that oxygen consumption had stronger abiotic controls in wetland soils but stronger biotic controls in upland soils, providing a useful framework for future incubation experiments. This work also determined that microbial activity is a strong driver of oxygen consumption in TAI soils, although it is constrained by the availability of dissolved carbon in subsurface soils.

Contacts

Funding

This research is based on work supported by COMPASS-FME, a multi-institutional project supported by the Department of Energy, Office of Science, Biological and Environmental Research program as part of the Environmental System Science Program. This project is led by Pacific Northwest National Laboratory, which is operated for the Department of Energy by Battelle Memorial Institute.