Tidal freshwater wetlands (TFW) are at significant risk of loss or alteration due to global climate change, and saltwater intrusion from sea level rise is of particular concern for these habitats due to their proximity to coastal areas. A space-for-time model was used to investigate the effects of saltwater intrusion on soil methanogen communities along naturally occurring salinity gradients on the Waccamaw, James, and Hudson Rivers. Amplification of the methyl coenzyme-M reductase (mcrA) functional gene was used in qPCR, reverse transcription qPCR, and T-RFLP to measure the abundance, activity, and community composition of soil methanogens. Both the abundance and activity of methanogens decreased with increasing salinity, and the both total and active methanogen community composition shifted in response to changes in salinity. This research demonstrates that saltwater intrusion will alter carbon cycling in TFWs, potentially altering their ability to sequester carbon and keep pace with rising sea level.

Tidal freshwater marshes (TFMs) are unique ecosystems that bridge the gap between terrestrial and aquatic ecosystems and are important in the sequestration of soil organic carbon. With the ever changing global climate, TFMs are left vulnerable to downstream effects of rising sea level and salt water intrusion due to increases in flooding by saline waters. These changes often act over large spatial and temporal scales resulting in significant impacts to local and regional environments. This multidisciplinary study assessed the amount and lability of desorbed organic carbon in tidal freshwater marsh soils from the Waccamaw River Marsh, South Carolina and Sweet Hall, a marsh on the Pamunkey River, Virginia. Soils from each marsh were extracted at 0-35 practical salinity units (psu) and the dissolved organic carbon (DOC) concentration, and carbon lability of the leachates were measured. At increasing levels of salinity, soil desorption amounts were higher in the Waccamaw River marsh interior and similar between the Waccamaw River creekbank and Sweet Hall levee. A larger fraction of desorbed DOC was consumed in the more organic soils from the Waccamaw River marsh in comparison to the more mineral soil from Sweet Hall Marsh. Finally, the rate of decay of the desorbed carbon was highest in the Sweet Hall levee soils, indicating more labile desorbed carbon, while the Waccamaw River Marsh soils had lower decay rates indicating less labile desorbed carbon. By understanding how salt water intrusion affects desorption and lability of soil organic carbon, in coastal marshes, we may be able to better understand how increasing sea levels may affect carbon storage in coastal ecosystems.