Coastal wetlands are some of the most effective ecosystems at storing organic carbon (C) due to high productivity and slow breakdown in anaerobic soils, making them globally important sinks of “blue carbon.” Wetland soil C therefore plays a role in mitigating climate change globally and locally the storage of organic matter helps maintain relative elevation of wetlands in the face of sea level rise and storms. Climate change is driving vegetation shifts globally and in many coastal wetlands, mangroves are expanding poleward due to increases in minimum temperature. In the Gulf of Mexico, black mangroves (Avicennia germinans) are encroaching into short-statured salt marshes. Changes in vegetation type can alter C inputs (altering autochthonous organic inputs and mediating the trapping of allochthonous organic matter) and C breakdown rates (changing the quality of organic matter, altering microclimate and priming and oxidizing soil C). Thus, understanding how vegetation changes alter C storage is essential given large-scale vegetation regime shifts in coastal wetlands.
We manipulated black mangrove cover in 10 experimental plots (24 m x 42 m) to represent an ecotonal gradient from herbaceous marsh to mangrove-dominated wetlands (0-100% mangrove cover) in Port Aransas, Texas, USA. We measured differences in organic C standing stocks (above and belowground biomass and soil %C), changes in inputs of organic C (root productivity and surface accretion) and changes in the breakdown of organic matter (roots, leaves and standard substrates) along the gradient of mangrove cover. As mangrove cover increased, above and belowground biomass and %C in surface soils increased. Mangrove cover did not affect belowground productivity but increasing mangrove cover decreased surface accretion due to entrainment of allochthonous subsidies along the coastal margin rather than the wetland interior. Mangroves reduced litter and root breakdown by producing more recalcitrant organic matter than marsh plants and by reducing soil surface temperature and light. However, mangroves increased the breakdown of standard substrates belowground, indicating that increasing mangrove cover may enhance breakdown of existing belowground soil C. Our findings illustrate how changes in the identity and density of functional vegetation types in wetland ecosystems (marsh vs. mangroves) alter the mechanisms of ecosystem C storage and accretion.