Spartina alterniflora has the highest methane emissions in a St. Lawrence estuary salt marsh

Abstract

Salt marshes have the ability to store large amounts of ‘blue carbon’, potentially mitigating some of the effects of climate change. Salt marsh carbon storage may be partially offset by emissions of CH4, a highly potent greenhouse gas (GHG). Sea level rise and invasive vegetation may cause shifts between different elevation and vegetation zones in salt marsh ecosystems. Elevation zones have distinct soil properties, plant traits and rhizosphere characteristics, which affect CH4 fluxes. We investigated differences in CH4 emissions between four elevation zones (mudflat, Spartina alterniflora, Spartina patens and invasive Phragmites australis) typical of salt marshes in the northern Northwest Atlantic. CH4 emissions were significantly higher from the S. alterniflora zone (17.7±9.7 mg C m-2 hr-1) compared to the other three zones, where emissions were negligible (< 0.3 mg C m-2 hr-1). These emissions were high for salt marshes and were similar to those typically found in oligohaline marshes with lower salinities. CH4 fluxes were significantly correlated with soil properties (salinity, water table depth (WTD), bulk density and temperature), plant traits (rhizome volume and biomass, root volume and dead biomass volume all at 0-15 cm) and CO2 fluxes. The relationships between CH4 emissions, and rhizome and root volume suggest that the aerenchyma tissues in these plants may be a major transport mechanism of CH4 from anoxic soils to the atmosphere. This may have major implications for the mitigation potential carbon sink from salt marshes globally, especially as S. alterniflora is widespread. This study shows CH4 fluxes can vary over orders of magnitude from different vegetation in the same system, therefore, specific emissions factors may need to be used in future climate models and for more accurate carbon budgeting depending on vegetation type.

Empirical Dynamic Modeling Reveals Complexity of Methane Fluxes in a Temperate Salt Marsh

“…Evidence to support this phenomenon exists from a variety of wetland studies that contain S . alterniflora and emit methane above or at the high‐end of a widely accepted CH 4 ‐sulfate threshold (Comer‐Warner et al., 2022; Huertas et al., 2019; Oremland et al., 1982; Poffenbarger et al., 2011). We propose that the elevated emissions, especially during senescence, are tied to the release of secondary plant compounds (osmolytes) associated with S .…”

“…While salinity causes direct inhibitory effects for acetoclastic and hydrogenotrophic methanogenesis by limiting the availability of substrates via microbial competition in most wetlands (Reddy et al, 2022), there is evidence for the presence of methylotrophic bacteria in salt marshes which produce CH 4 from non-competitive substrates resulting in elevated emissions despite the presence of sulfate (Capooci & Vargas, 2022;Capooci et al, 2024;Seyfferth et al, 2020). Evidence to support this phenomenon exists from a variety of wetland studies that contain S. alterniflora and emit methane above or at the high-end of a widely accepted CH 4 -sulfate threshold (Comer-Warner et al, 2022;Huertas et al, 2019;Poffenbarger et al, 2011). We propose that the elevated emissions, especially during senescence, are tied to the release of secondary plant compounds (osmolytes) associated with S. alterniflora (which we did not measure) including glycine betaine and dimethyl sulfoniopropionate which are broken down via fermentation to trimethylamine and dimethyl sulfide and are exclusively utilized by methylotrophic methanogens (Husband & Kiene, 2007;Jones et al, 2019).…”

The hidden roots of wetland methane emissions

“…Root CH 4 uptake is dependent on root wall permeability for gas diffusion, which is greatest at the root tips and fine roots (Henneberg et al., 2012). Thus, traits affecting CH 4 transport include root porosity, tissue density, diameter, surface area and length (proxy for root permeable area; Henneberg et al., 2012) (Colmer, 2003), root tip density (number of root tips per unit volume of soil) (Gerard & Chanton, 1993), and rhizome biomass (e.g., van der Nat & Middelburg, 1998b), volume (Comer‐Warner et al., 2022), diameter (Tripathee, 2014; Tripathee & Schäfer, 2015), porosity and length (Armstrong et al., 1988), as well as presence and number of aerial roots (pneumatophores) that can facilitate transport of O 2 (Freschet, Roumet, et al., 2021; Zhang et al., 2022), and CH 4 (Pulliam, 1992; Purvaja et al., 2004; Zhang et al., 2022; but see Allen et al., 2007). Increases in each of these traits (except root tissue density) could potentially increase gas transport (Figures 1 and 2) (Watson et al., 1997).…”

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

“…Spartina alterniflora have complex aerenchyma tissue systems for enhanced gas exchange between the roots and shoots which allows this species to persist in flooded conditions (Jackson & Armstrong, 1999;Maricle & Lee, 2002). These aerenchyma tissues also serve as a conduit for CH 4 transport from the rhizosphere to atmosphere (Comer-Warner et al, 2022) and thus it is possible that this shift in vegetation composition may play a role in increased CH 4 efflux from the TLP plots. Lastly, related studies report that degraded or disturbed vegetated coastal ecosystems were likely to have greater CH 4 fluxes than restored or pristine systems He et al, 2023;Rosentreter et al, 2021) As salt marshes are valued for their carbon sequestration function and their ability to have a net cooling impact on the atmosphere, we wanted to understand how TLP might alter this capacity.…”