Understanding the spatio-temporal dynamics of denitrification in a tidal marsh

Jessica Moon, Biology Department, Murray State University, Murray, KY, Theodore H. DeWitt, NHEERL Western Ecology Division, U.S. EPA, Newport, OR, Amanda M. Nahlik, Office of Research and Development, US Environmental Protection Agency, Corvallis, OR and Kusum Naithani, University of Arkansas

Background/Question/Methods
Estuarine food webs are subsidized through material and nutrient fluxes originating from upslope terrestrial ecosystems. Within these active coastal zones, wetlands play an important role in trapping sediment and processing nutrients. Wetlands are known to affect oxygen minimum zones because they intercept nutrients, such as nitrogen, from upland and tidal sources and can remove it from the biosphere through the process of denitrification. However, their ability to perform this function in the future may be dependent on climate change. Wetlands within coastal temperate rainforests, such as those along the western continental shelves, may experience changes in the delivery of these nutrients through an increase in upwelling events and shifts in the forest community composition. Our understanding of these complexities and our ability to predict thresholds of functionality given perturbations such as climate change, is dependent on building models that can capture both the spatial and temporal dynamics of the process at the appropriate scales. Unfortunately, modeling the complexity of denitrification in coastal systems remains particularly challenging, lagging behind models made in terrestrial and submerged systems. To develop a tidal denitrification model for these systems, we assessed the spatial and temporal variability of soil nitrate levels and O2 availability, two primary drivers of denitrification in surface soils of Winant Salt Marsh located in Yaquina Estuary, OR.
Results/Conclusions
We found low temporal variability in soil nitrate concentrations across years, tide series, and tide cycles, but high spatial variability linked to elevation gradients (i.e., habitat types). Spatial variability within the high marsh habitat (0 - 68 μg N g-1 dry soil) was linked to connectivity to upslope N-fixing red alder. Denitrification rates, measured using static core incubations, followed a similar pattern, with hotspots along the upland-wetland interface below sub-watersheds that contained a high percent cover of red alder. We also found drawdown rates of O2 to be spatially variable for denitrification, ranging from suboptimal (> 80% O2 saturation) across an entire tide series (i.e., across days) to optimal (i.e., ~ 0 % O2 saturation) within one overtopping tide event (i.e., within hours). Temporally, O2 drawdown appears to lag behind water level drawdown after an overtopping tidal event. We have used these findings to develop the first version of a denitrification tidal simulation model with the goal of exploring hydro-biogeochemical processes in hydric tidal soils that have a potential to be affected by a complex assemblage of climate change stressors.