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

Abstract

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 μmol m−2 hr−1) emitted up to 7 to 22 times more CH4 compared to control (5.77 ± 0.74 μmol m−2 hr−1) and reference (1.63 ± 0.75 μmol 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.