PNNL

Abstract

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 water saturation drives oxic to anoxic shifts in soils with different characteristics and inundation regimes. Continuous redox monitoring of field sites unexpectedly revealed that flooding caused temporary spikes in subsurface dissolved oxygen followed by rapid consumption. We integrated field measurements, laboratory incubations, and model simulations to improve mechanistic understanding of oxygen consumption dynamics in coastal soils. This was coupled with laboratory incubations using surface and subsurface soils from a TAI gradient (defined here as upland forest to transitional forest to wetland) on the southern coast of Lake Erie to measure oxygen consumption rates in TAI soils during flood events. In our experiments, 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. While microbial activity may be a strong driver of oxygen consumption in TAI soils, the availability of dissolved carbon fueling microbial metabolism is also a key limiting factor in subsurface soils.