Marsh ecosystem greenhouse gas fluxes in a warmer world
Salt marshes are large net sinks of carbon (C), due to relatively high rates of primary productivity and slow decomposition rates. In addition, marshes typically emit low levels of nitrous oxide (N2O), a powerful GHG produced via microbially-mediated processes (e.g. nitrification and denitrification). In this study we examine how increased temperatures may alter net greenhouse gas (GHG) fluxes (i.e. CO2, CH4, N2O) in New England salt marshes. Because many of the processes controlling GHG fluxes are sensitive to temperature, we hypothesize that higher temperatures accompanying climate change may increase the fluxes of GHGs from marshes to the atmosphere. To test this hypothesis we experimentally warmed a relatively undisturbed salt marsh located in the Waquoit Bay National Estuarine Research Reserve (USA) using passive open top chambers (OTCs). We used the cavity ring-down spectroscopy method (Picarro and LGR) to measure in-situ fluxes of CO2, CH4, N2O monthly in light and dark conditions.
On average OTCs significantly (p<0.01) warmed air temperatures by 1.54 ˚C and 1.62 ˚C, in the low and high marsh respectively (July-October 2014). Unlike air temperatures, sediment temperatures were not significantly warmer inside the OTCs. Monthly net ecosystem exchange (NEE) values always indicated a net C sink, ranging from -11.2 to -0.04 μmol m-2 s-1 across all plots. Ecosystem respiration (Re) ranged from 0.26 to 7.7 μmol m-2 s-1 across all plots, with no significant differences in NEE or Re observed between treatments during 2014. Our experiment provides a unique tool to examine the importance of temperature in driving GHG emissions in salt marshes, with implications for future marsh C storage in a warmer world.