Citation:

Moon, J., J. Stecher, Ted DeWitt, A. Nahlik, M. Fennessy, L. Michael, R. Regutti, Bob Mckane, AND K. Naithani. Understanding the Spatio-Temporal Dynamics of Denitrification in an Oregon Salt Marsh. AGU 2016 Fall Meeting, San Francisco, CA, December 12 - 16, 2016.

Impact/Purpose:

The highest uncertainties in net nitrogen (N) fluxes between the atmosphere and biologically active pools are predominately due to denitrification (DeN). This diminishes confidence in our assessment of wetland N removal at transition zones between upland and aquatic systems. This is relevant given the exponential spread of dead zones in coastal oceans since the 1960s and expected future shifts in climate. Salt marshes are highly susceptible to a range of climate change effects (e.g., sea-level rise, salinity change, vegetation change, storm severity), all of which may affect the spatial and temporal dynamics of DeN. To address this, we have built a probabilistic model of DeN in a salt marsh on the Oregon coast. Salt marshes in this region will be impacted by sea level rise and upslope shifts in red alder, a N-fixing plant. Our model accounts for spatial variability in soil nitrate and oxygen availability as a function of landscape position metrics (e.g., elevation, distance to channel, and upslope red alder). Initial simulations suggest that as sea-level rises, DeN will increase non-linearly; “hot spots” near the marsh-upland toe-of-slope will become more active as inundation periods lengthen. However, the model also revealed that only a small fraction of the dissolved N coming into the watershed from N-fixing red alder is denitrified in the marsh during inundation periods. With optimal soil moisture conditions for DeN maintained during non-inundating neap tides, the next step is to include those periods in the model. We will also include terms to account for soil accretion, changes in NO3- loading dynamics due to changes in the distribution of upslope red alder, and marsh-area loss to open water habitat. This abstract contributes to SHC 2.61.3.

Description:

Salt marshes are highly susceptible to a range of climate change effects (e.g., sea-level rise, salinity changes, storm severity, shifts in vegetation across watershed). It is unclear how these effects will alter the spatial and temporal dynamics of denitrification, a potential pathway of nitrogen interception and removal from adjacent estuaries. Our overall objective is to determine whether salt marshes in the Pacific Northwest act as sources or sinks of nitrogen to estuaries, and to be able to predict changes in these dynamics under future climate scenarios. We have built a probabilistic denitrification model based on observations from a salt marsh in the Yaquina Estuary (Newport, Oregon). We observed a non-linear relationship between denitrification rates and distance to the marsh-upland interface and soil nitrate concentrations, which are indicators of nitrate delivery flow paths from upslope red alder. We also modeled spatial variability in oxygen availability as a function of elevation, which affects inundation period, and distance to channel, which affects the saturation period through the dewatering rate. Simulations suggest denitrification “hot spots” occur in mid-marsh locations, where both nitrate availability and inundation periods are maximized. Once marsh accretion is outpaced, sea level rise will likely reduce salt marsh area due to steep adjacent uplands that limit marsh retreat, and increase inundation duration near the marsh-upland interface. Expansion of red alder cover is concurrently expected to increase nitrate availability to downslope ecosystems. Taking these effects together, our future scenario simulations suggest a movement of “hot-spots” towards the marsh-upland boundary.

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