The Effect of Rising Salinity and Forest Mortality On Soil Nitrogen and Phosphorus Mineralization In Tidal Freshwater Forested Wetlands.

Tidal freshwater forested wetlands are likely sensitive to climate change and sea level rise through increased salinity that eventually leads to state change to oligohaline marsh.  We are investigating the biogeochemical, geomorphic, and vegetative consequences of sea level rise on tidal freshwater forested wetlands along longitudinal landscape salinity gradients (three tidal freshwater forest sites, with increasing salinity and tree mortality, and one oligohaline marsh, from upstream to downstream) along the Waccamaw and Savannah Rivers.  Here we report on patterns of soil net nitrogen and phosphorus mineralization fluxes measured quarterly over a year using in situ incubations of modified resin cores.  Soil nitrogen mineralization peaked at the downstream forested sites that experience chronic salinity incursions and have had substantial tree mortality (mean: 1674 umol N m^-2 d^-1) compared to upstream freshwater sites (1306 umol N m^-2 d^-1) and oligohaline marsh sites (1014 umol N m^-2 d^-1).  Soil phosphorus mineralization increased downstream on the Savannah River and peaked in the oligohaline marsh on both rivers.  The increases in both nitrogen and phosphorus mineralization at salt-impacted sites was due to greater rates of turnover of soil nutrient pools instead of differences in nutrient pool size.  The wetland soils of the alluvial Savannah River had greater phosphorus mineralization rates (271 umol P m^-2 d^-1), a lower ratio of N:P mineralized (17 mol-N:mol-P), and more mineral soils with greater clay content compared to the blackwater Waccamaw River (88 umol P m^-2 d^-1; 73 mol-N:mol-P).  Hummocks, present only at the freshwater upper sites of both rivers, had greater soil nitrification rates than adjacent hollows.  In summary, tidal freshwater forested wetlands experience changes in biogeochemical fluxes and vegetation in response to salinity incursion. Specifically, we found greater rates of both soil nitrogen and phosphorus mineralization likely due to enhanced microbial activity following tree mortality (and not due to sulfate inputs) caused by salinity increases.  Substantial changes in nutrient stoichiometry also were associated with state change to oligohaline marsh.