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

The coastal terrestrial-aquatic interface (TAI) is a highly dynamic system characterized by strong physical, chemical, and biological gradients. In particular, shifting soil redox conditions, due in part to dynamic water conditions, is a strong driver of carbon availability and transformations across TAIs. However, one of the important unknowns across TAIs is how soils with different characteristics and inundation regimes respond quantitatively to water saturation and resulting shifts between oxic and anoxic subsurface conditions. We used field measurements, laboratory incubations, and model simulations to investigate oxygen consumption and redox transformations following short-lived oxygenation events under different environmental conditions. Soils were collected along a coastal gradient (upland to wetland) from the Western Lake Erie region (freshwater TAI) and the Chesapeake Bay (estuarine TAI) and incubated in microcosms for two weeks. When inundated in MilliQ water, the upland A horizon soils went anoxic in 24 hours, whereas the wetland and transitional soils went anoxic in 0.5 - 10 hours. In contrast, the upland B horizon soils did not go anoxic during the 2-week incubation. Model simulations suggested stronger abiotic controls of oxygen consumption in the wetlands vs. biotic controls in the upland soils. These simulations also suggested nutrient limitation in the subsurface soils. Subsequent incubations with glucose and acetate additions showed increased rates of oxygen consumption in the B horizon soils, suggesting that these soils were indeed carbon limited. These experiments provide insight on shifting redox conditions during flooding events, especially relevant in coastal systems that experience rapidly shifting hydrological conditions and are becoming increasingly vulnerable to sea level rise and episodic disturbances (e.g., storm surges, king tides).