Effect of Short-Term Changes In Salinity On Nitrogen Cycling In a Louisiana Coastal Marsh.

With rising sea level and increased freshwater interception for human use, coastal systems are experiencing increasing salinity over the long-term.  However, there are events which can trigger nearly instantaneously salinity shifts to coastal wetlands with effects lasting weeks to several months.  Increasing salinity of fresh and brackish marshes occurs when storm surge from tropical storms make landfall along low relief coastlines, while decreasing salinity of brackish and salt marsh occurs during large freshwater release triggered by flood events.  Coastal Louisiana has experienced both sets of circumstances recently.  Therefore, we investigated the effects of salinity shifts on nitrogen cycling in coastal marsh soils of Breton Sound, LA which receives annually, high nitrate Mississippi River water during the spring flood as well as hurricane storm surges.  Potential denitrification (PD) and potential N-mineralization (PMN) rates were determined on fresh, brackish and salt marsh soils and sediments at 0, 9 and 35 ppt salinity.  The PD and PMN rates ranged from 4 – 10 times higher in the marsh soils compared to the adjacent bayou sediments, correlated to soil C and microbial biomass C.  PMN increased with increasing salinity of fresh marsh soil, while salinity had no significant effect on PMN for the other soils or salinity treatments suggesting the presence of a salinity-tolerant microbial consortium.  Denitrification of salt marsh soil was an order of magnitude lower under fresh condition than 35 ppt demonstrating a salinity effect on denitrification.  If this salinity effect continues over the long-term, it may increase nitrate loading to the coastal waters with decreased denitrification activity which could potentially add to problems of algal blooms and coastal hypoxia.