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Understanding the Spatio-Temporal Dynamics of Denitrification in a Tidal Marsh
Citation:
Moon, J., Ted DeWitt, A. Nahlik, AND K. Naithani. Understanding the Spatio-Temporal Dynamics of Denitrification in a Tidal Marsh. 2021 Ecological Society of America (ESA), NA, Virtual, August 02 - 06, 2021.
Impact/Purpose:
Within coastal zones, wetlands play an important role in trapping sediment and removing nutrients that would otherwise end up in the estuaries and potentially degrade water quality. One mechanism for this is microbial denitrification of nitrate in soil porewater. Climate change may affect denitrification in coastal wetlands by altering the delivery of nitrate to the marsh (e.g., changes in precipitation can alter transport rates and changes in temperature and precipitation may change upslope abundance of nitrogen fixing plants) or marsh soil biogeochemical conditions (e.g., changes in sea level can alter porewater oxygen and salinity due to changes in inundation frequency). Additionally, climate change is expected to increase upwelling events that bring nutrient rich waters into coastal wetlands during tidal events. To understand the implications of these complex climate change stressors and predict how wetlands will respond to these and other changes, we need to build new computational models that incorporate these processes. Presently, there are few models of denitrification for tidal wetlands. To develop one for tidal wetlands in the Pacific Northwest, we investigated how denitrification rates, nitrate concentrations, and oxygen levels varied in space and time within the surface soils of Winant Slough located in the Yaquina Estuary, OR. We included nitrate and oxygen levels in our study because they are the primary drivers of denitrification rates in these systems. We found that nitrate concentrations do not vary much across years, tide series, and tide cycles, but they do vary across space, such as across wetland habitat types (e.g., channels, low marsh, high marsh). Within the high marsh habitat, nitrate concentrations and denitrification rates were higher along the upland-wetland interface where there was a high percent cover of upslope nitrogen fixing red alder trees. We also found that the drawdown of oxygen, which determines the rate of denitrification, varied across the marsh, ranging from suboptimal (i.e., greater than 80% oxygen saturation) across an entire tide series (i.e., across days) to optimal (i.e., ~ 0 % oxygen saturation) within one overtopping tide event (i.e., within hours). Oxygen drawdown also lags behind water level drawdown after an overtopping tidal event. We have used these findings to develop the first version of a denitrification tidal model, with the goal of exploring how the functions of these wetlands, like denitrification, will change as climate change stressors advance.
Description:
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 in coastal waters 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 west coast of the United States, 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 oxygen (O2) availability, two primary drivers of denitrification in surface soils of a 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 trees (Alnus rubra). 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.