The Biogeochemical Consequences of Saltwater Intrusion to Freshwater Wetland Sediment

Abstract

Saltwater intrusion driven by water extraction, coastal modifications, and climate change may alter the biogeochemical cycling of freshwater coastal wetlands. Anaerobic respiration in freshwater wetlands is typically dominated by methanogenesis, leading to a high methane (CH4) flux, but the availability of sulfate (SO42—) in seawater may shift the dominant pathway to SO42— reduction, decreasing CH4 flux. Seawater may also impact nitrogen cycling by releasing ammonium (NH4+) from soil and causing salinity and hydrogen sulfide stress to nitrifiers and denitrifiers. This experiment tests the soil biogeochemical impacts of artificial seawater amendments to intact sediment cores taken from a freshwater coastal wetland on the Albemarle Peninsula of North Carolina. Intact cores were assigned to one of four experimental treatments designed to compare the impact of surface and subsurface saltwater exposure on sediment biogeochemistry. Cores received surface water treatments 2-3 times per week and were exposed continuously to subsurface treatments. Gas flux and porewater were sampled 9 times over the 20-week experiment. Saltwater added to the soil surface raised soil solution to 10.4 ppt average salinity by the end of the experiment and led to significant increases in NH4+ concentration and significant declines in dissolved organic carbon (DOC). In surface saltwater exposed cores we measured a significant increase in nitrous oxide (N2O) production but no significant change in carbon dioxide (CO2) or CH4 flux. Surface exposure treatments led to significant reductions in microbial biomass, and all salinity treatments (regardless of direction of exposure) had significant reductions in carbon mineralization and respiration efficiency by the end of the experiment. Our results suggest that saltwater exposure altered microbial biomass and function, and that surface water salinization will have more immediate and measurable impacts on biogeochemical cycling in these soils than exposure to saline ground water.