The Effects of Salinity on Nitrogen Cycling in Wetland Soils and Sediments of the Breton Sound Estuary, LA

• Main Author: Marks, Brett Whitfield
• Other Authors: Benfield, Mark C.
• Format: Others
• Language: en
• Published: LSU 2010
• Subjects: Oceanography & Coastal Sciences
• Online Access: http://etd.lsu.edu/docs/available/etd-04262010-140636/

Wetlands in the coastal zone are slowly becoming more saline under rising sea level over the long-term. However, there are a number of events in the coastal environment which lead to quick and temporary changes in the salinity of coastal marshes. Seawater driven inland from storm surge can significantly increase salinity in oligohaline wetlands over the short-term (weeks). Recent large-scale efforts to restore coastal wetlands in Louisiana have utilized Mississippi River surface water diversions to re-introduce freshwater into coastal marshes, decreasing the salinity of coastal marshes. We examined the effect of salinity changes on two important nitrogen cycling processes, potential denitrification and N-mineralization, in fresh and salt marsh soils/sediments in the Breton Sound estuary, LA. All soils/sediments were subjected to freshwater and saline treatments (0-35 ppt) simulating conditions within the soil that are caused by instantaneous flux of seawater due to storm surge events or high rates of freshwater flow directed by a surface water diversion. At 0 ppt potential denitrification in fresh and salt marsh soils reached 373 ± 22.2 and 9.18 ± 3.27 mg N2O-N kg-1 d-1, respectively. At 35 ppt, the rates were 615 ± 182 in salt marsh and 99.7 ± 21.1 mg N2O-N kg-1 d-1 in fresh marsh soils. Potentially mineralizable N rates in fresh marsh soils at 0 and 35 ppt averaged 28.6 ± 3.71 and 38.2 ± 4.31 mg-N kg-1 d-1, respectively. In salt marsh soils at 0 and 35 ppt, PMN rates were 12.3 ± 0.4 and 8.70 ± 0.32 mg-N kg-1 d-1, respectively. The effects of changing salinity on N-mineralization and potential denitrification will allow us to begin to discern the mechanisms of salinity-driven influences on overall nitrogen cycling and marsh biogeochemical function. Significance of these findings are applicable to large surface water diversion projects in the coastal Florida Everglades and Mississippi River Delta, where more saline sediments are exposed to freshwater and nitrogen pulses as well as impacts of increased salinity driven into the fresh-brackish marsh from hurricanes. Sudden fluxes in salinity had short-term effects on N mineralization, while denitrification showed significant effects with sudden salinity changes in wetlands soils.