Simulated heatwave increases CO₂ efflux from unvegetated saltpans and amplifies CH₄ efflux from vegetated saltmarshes

Sigit Deni Sasmito, Alexander Barkley, Lamberto Montagna, Brodie O'breza, Most Linnas, Amrit Mishra, Clement Duvert, Rory Mulloy, Paula Cartwright, Emily Douglas, Catherine E. Lovelock, Nathan Waltham

Atmospheric heatwaves are increasing in frequency, intensity and duration under anthropogenic climate change, yet their effects on greenhouse gas (GHG) fluxes from coastal wetlands remain poorly understood. Coastal wetlands, including saltmarshes and saltpans (also called salt flat and sabkha), are globally significant carbon stores, but their vulnerability to extreme heat has rarely been tested experimentally. Here we report CO₂ and CH₄ fluxes over multiple days of simulated heatwaves using open-top chambers (OTCs) in three coastal wetland habitats in the dry tropics of Australia: unvegetated saltpan, and saltmarsh dominated by Sarcocornia quinqueflora or Sporobolus virginicus. We compared these with dry-season fluxes from the same site. OTCs warmed the surface by 3.3–4.5 °C in all habitats. Simulated heatwave shifted the saltpan from CO₂ uptake to emissions (p<0.05), whereas CO₂ efflux in both saltmarshes did not change significantly under OTCs. In S. virginicus, CO₂ efflux was insensitive to warming, indicating limited thermal effects on respiration. We identified vegetation cover, soil temperature and soil moisture as the main drivers of CO₂ efflux. CH₄ effluxes were low (<0.4 nmol m⁻² s⁻¹) across sampled habitats, but increased three to five times from the dry to the wet season, and were further increased during simulated heatwaves in S. quinqueflora (p<0.05). These contrasting responses suggest that vegetation shapes simulated heatwave effects on coastal wetland GHG fluxes through both physical and biological controls. Unvegetated saltpans may be particularly vulnerable to heatwave-driven CO₂ losses, while warming may increase CH₄ efflux from vegetated saltmarsh. Our results have implications for blue carbon conservation and managing coastal wetland carbon stocks in a warming world.