Wednesday, June 6, 2007 - 3:45 PM
320

Nitrogen cycling in the riparian zone of a tidal freshwater river

Scott H. Ensign, Curriculum in Ecology, University of North Carolina at Chapel Hill, 3431 Arendell St, Institute of Marine Science, Morehead City, NC 28557, Michael F. Piehler, Ph.D., Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell St, Institute of Marine Science, Morehead City, NC 28557, and Martin Doyle, PhD, Department of Geography, University of North Carolina-Chapel Hill, CB #3220, Chapel Hill, NC 27599-3220.

Propagation of tides up coastal rivers results in periodic inundation of the riparian zone, subsequently affecting benthic redox condition and microbial nitrogen cycling.  To examine nitrate uptake in relation to tidal hydrology, monthly measurements of denitrification were made in three intertidal habitats of a blackwater river in southeastern North Carolina in 2006 –2007.  Sediment cores were incubated in the laboratory at in-situ temperature and changes in N2 concentration within the overlying water were made with a membrane inlet mass spectrometer.  Denitrification rates were generally highest in a hardwood forest riparian zone (46-208 umoles N m-2 hr-1), followed by an emergent marsh (38-124 umoles N m-2 hr-1), and a non-vegetated mud flat (0-150 umoles N m-2 hr-1).  Redox and soil moisture sensors were installed in intertidal sediments to determine the time scale of redox cycling under the tidal flooding regime.  During winter, redox of a mud flat (316-340 mV Eh) responded inversely to water level and soil moisture.  Hardwood forest riparian soils were more reduced (242-268 mV Eh), indicating conditions more conducive to denitrification.  These data coupling tidal hydrology and riparian biogeochemistry are integral to improved understanding of tidal freshwater rivers and the effects of sea level rise on coastal ecosystems.