Alterations to tidal marsh biogeochemical cycling and greenhouse gas exchange in response to sea-level rise and salinity intrusion
Tidal marshes are highly productive ecosystems with the potential to sequester large amounts of carbon. However, tidal wetlands may be sources of the powerful greenhouse gases (GHGs) methane (CH4) and nitrous oxide (N 2O), which are produced via microbial metabolic processes. As global climate changes, it is increasingly important to understand the factors that control ecosystem productivity, GHG fluxes, and potential feedbacks between global change factors, C cycling, and marsh resilience to sea-level rise (SLR). Marshes along the salinity gradient from tidal freshwater marshes (TFM) to salt-marshes will likely respond in unique ways to changing environmental drivers, especially salinity intrusion. Field measurements were undertaken to quantify rates of GHG (CO2 and CH4) exchange rates, plant biomass, microbial sulfate reduction and methanogenesis rates, and soil biogeochemistry at three tidal wetland sites along the salinity gradient in the Delaware River Estuary over four years. The impacts of salinity intrusion on TFM soil C cycling were further investigated in a controlled one year laboratory experiment. Finally, a multi-year field manipulation of sea-level and salinity regime was undertaken to evaluate the response of marshes along the salinity gradient to SLR, and TFMs specifically to SLR coupled with salinity intrusion.
Despite similar plant productivity between marsh types, differences in microbial processes largely determined the GHG source/sink status of the wetland types. The mesohaline salt-marsh consistently sequestered C and was a GHG sink. In contrast, the TFM sequestered C but, because of appreciable release of CH4, was GHG neutral. The oligohaline marsh experienced significant seasonal salinity intrusion in the late summer, resulting in major alterations to marsh C cycling. The oligohaline marsh did not sequester C in part due to surprisingly high rates of CH4 release, and was a significant source of GHG to the atmosphere. In the laboratory experiment, microbial organic matter mineralization and release of CO2 increased following simulated salinity intrusion. Rates of CH4 release were also significantly greater from soils following salinity intrusion. In the field manipulation, N2O emissions decreased while CH4 flux increased with flooding corresponding to measured increases in microbial methanogenesis. There were complex interactions between changes in plant production and microbial organic matter decomposition with both SLR and salinity intrusion, and TFMs experiencing both simultaneously had reduced C sequestration and increased GHG release. SLR and salinity intrusion therefore limit the vertical accretion potential of TFMs, put TFMs at risk of permanent submergence, and produce a feedback to atmospheric GHG concentrations.