Influence of salinity level on sediment denitrification in a Louisiana estuary receiving diverted Mississippi River water Dong Cheol Seo a,b , Kewei Yu a * and Ronald D. Delaune a a Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, USA; b Department of Enviro-Biotechnology, Gyeongsang National University, Jinju, Republic of Korea (Received 4 May 2007; final version received 13 September 2007) The Mississippi River water containing elevated nitrate is being diverted into Louisiana coastal estuaries to abate wetland deterioration attributed to lack of sediment and nutrients, rapid subsidence and accompanying salt water intrusion. In this study effect of salinity change on sediment denitrification at a Mississippi River freshwater diversion site (Davis Pond, Louisiana) was determined. Results show that the denitrification potential of the sediment was highest under fresh water condition (salinity close to 0%). Addition of sea water immediately inhibited the denitrification activity of the sediment. Further analysis, by separate treatment of NaCl and K 2 SO 4 addition, revealed that inhibition of the denitrification of the sediment by sea water was mainly caused by NaCl content in sea water. Denitrification activity of the sediment was not significantly affected by the sulfate content in sea water. Salinity increase seems a primary reason for the sediment denitrification rate decrease. A significant inverse relationship of denitrification rate and salinity was obtained [denitrification rate (mg N kg 71 day 71 ) ¼ 70.20 6 salinity(%) þ 10.41, R 2 ¼ 0.91]. Under highest sea water condition (salinity ¼ 36%), denitrification rate of the sediment would be 30.8% of its original activity (salinity ¼ 0%). Keywords: denitrification; salinity; nitrous oxide; wetland; sea level rise; Mississippi River; coast Introduction The Mississippi River accounts for approximately 90% of the freshwater input into the northern Gulf of Mexico (Mitsch et al. 2001). Extensive agricultural development and fertilizer use over the last century in the Mississippi River basin has substantially increased nutrient loadings into the rivers and northern Gulf of Mexico (Turner and Rabalais 1994). Large influx of nutrients from the Mississippi River discharge stimulates coastal eutrophication and development of bottom-water hypoxia (water O 2 5 2 mg l 71 ) on the continental shelf of the northern Gulf of Mexico (Turner and Rabalais 1994; Rabalais et al. 1996). The Mississippi River levee built for flood control has prevented major course changes, allowing nutrients found in river water to enter the Gulf of Mexico without any buffering effect or processing by coastal wetlands. In addition, levee construction also prevents Louisiana coastal wetlands from receiving sufficient fluvial sediment to counteract rapid regional subsidence and seal level rise (Swanson and Thurlow 1973), providing nutrients for wetland vegetation growth and freshwater to prevent saltwater *Corresponding author. Email: kyu1@lsu.edu Archives of Agronomy and Soil Science Vol. 54, No. 3, June 2008, 249–257 ISSN 0365-0340 print/ISSN 1476-3567 online Ó 2008 Taylor & Francis DOI: 10.1080/03650340701679075 http://www.informaworld.com