Salt marshes are important ‘Blue Carbon’ systems that play a significant role in climate change mitigation through their ability to store carbon. However, salt marsh survival is being threatened as marsh net elevation gain cannot keep pace with sea level rise. Thin layer placement (TLP) is one proposed strategy to boost elevation by applying sediment to the marsh surface. While TLP can boost salt marsh elevation, we know little about its impact on biogeochemical cycling and greenhouse gas (GHG) dynamics. We addressed this knowledge gap by measuring GHG fluxes (methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2)) in experimental TLP plots in Coggeshall Marsh, Narragansett Bay National Estuarine Research Reserve. We measured GHGs 6 times over the 2020 growing season (June, July, August, September, and October). Gas concentrations were determined using cavity ring down spectroscopy in 20 experimental plots with the following TLP treatments: 7 cm and 14 cm of added sediment, control plots that didn’t receive sediment, and reference plots that represent elevational end goal targets for the TLP plots.
Results/Conclusions
While not significantly different, TLP plots emitted more CH4 (7 cm: 35.7 µmol m-2 hr-1 ± 12.7, 14 cm: 19.8 µmol m-2hr-1 ±3.5) compared to control (5.8± 0.7) and reference plots (1.6±0.7). However, TLP plots took up significantly more CO2 (7 cm: -6.8 ± 0.9, 14 cm: -6.4 ± 0.6) than reference plots (-2.7 ± 0.8) and comparable amounts to control plots (-7.2 ±0.7). N2O fluxes were negligible for all treatments and had median values of 0 ± 0. To determine the net atmospheric warming or cooling impact of these plots, global warming potential and sustained global warming potential values were used to convert CH4 and N2O fluxes to CO2 equivalents. CO2 equivalents for each treatment were then compared to 3 C burial rates: 1 global rate and 2 localized rates. The results of this analysis showed that if using mean values and local C burial rates, the TLP plots can be considered to have a net warming impact on the atmosphere. Finally, we examined what environmental parameters may help explain observed fluxes. Overall, this study demonstrates differences in GHG dynamics between TLP and control and reference plots and underscores the importance of tracking biogeochemical development of TLP projects.
