Sediment Addition Leads to Variable Responses in Temperate Salt Marsh Greenhouse Gas Fluxes During the Growing Season

Salt marshes play an important role in coastal carbon cycling. Unfortunately, these systems are threatened by sea level rise. One strategy to increase the resilience of marshes is thin-layer placement of sediment (TLP). While TLP can boost elevation, little is known about how TLP alters greenhouse gas fluxes. We addressed this knowledge gap by measuring greenhouse gas fluxes in TLP plots that received either 7 cm (TLP-7 cm) or 14 cm of added sediment (TLP-14 cm), control plots that received no sediment, and reference plots that served as elevation end goal targets for the TLP plots. We found that mean (+/- standard error) CO2 uptake was comparable between control and TLP plots (control: -25.84 +/- 2.46; TLP-7 cm: -24.44 +/- 3.32; TLP-14 cm: -23.18 +/- 2.08 mmol m-2 hr-1) and significantly less in reference plots (-9.54 +/- 2.98 mmol m-2 hr-1). However, TLP plots (TLP-7 cm: 35.74 +/- 12.70, TLP-14 cm: 19.79 +/- 3.47 mu mol m-2 hr-1) emitted up to 7 to 22 times more CH4 compared to control (5.77 +/- 0.74 mu mol m-2 hr-1) and reference (1.63 +/- 0.75 mu mol m-2 hr-1) plots, respectively. N2O fluxes from the TLP plots exhibited both uptake (TLP-7 cm) and emission (TLP- 14 cm). Overall, the marsh remained a net greenhouse gas sink, at least during the times we measured-during the day and throughout the growing season. This research demonstrates the dynamics of greenhouse gas fluxes in marshes amended with sediment and highlights the need for future diel and annual measurements. Salt marshes are important ecosystems that play a key role in the global cycling and storage of carbon. However, although salt marshes store carbon they also emit other potent greenhouse gases such as methane and nitrous oxide. Currently there is uncertainty in the magnitude and variability of these fluxes. Further complicating this issue is the impact of sea level rise (SLR) and SLR adaptation strategies on greenhouse gas dynamics. This study explores the impact of sediment addition, a SLR adaption strategy where sediment is added to the surface of the marsh to boost elevation, on greenhouse gas fluxes. We found that sediment addition increased methane fluxes but that overall marsh areas amended with sediment still stored more greenhouse gases than they emitted. CO2 uptake was comparable to control plots across all treatments of sediment addition CH4 emissions were 4-7 times higher in sediment addition plots compared to control plots, and up to 22 times higher compared to reference plots N2O fluxes were low and variable and did not exhibit a consistent pattern with sediment addition