Crab bioturbation leads to more CO2 efflux in native than in invasive salt marshes

Coastal wetland ecosystems harbor diverse plant and macrobenthic species but are highly susceptible to invasion by non-native species. The combined impact of macrobenthic organisms and plant invasions on carbon cycling, however, remains insufficiently understood. This study investigated four herbivorous, burrowing crab species inhabiting Spartina alterniflora (SA) and Phragmites australis (PA)-dominated salt marshes in China and the USA to evaluate their influence on carbon fluxes under varying conditions of plant invasion. Key findings include the following: (1) In native S. alterniflora marshes, Sesarma crabs significantly reduced plant biomass through herbivory, whereas fiddler crabs in the USA and Chiromantes crabs in China displayed omnivorous feeding behaviors that contributed to maintaining ecological balance with vegetation. (2) Salt marshes dominated by Sesarma crabs exhibited signs of ecosystem degradation, characterized by elevated soil CO₂ fluxes compared to other S. alterniflora sites (5.083 ± 2.59, 3.761 ± 2.97, and 2.555 ± 1.63 μmol m⁻² s⁻¹ in die-off native, healthy native, and healthy invasive S. alterniflora sites, respectively; P > 0.05). (3) Plant invasion emerged as a more significant driver of soil CO₂ flux than crab feeding behavior, with CO₂ emissions being 2.116 μmol m⁻² s⁻¹ higher in native habitats (P < 0.05) and 1.675 μmol m⁻² s⁻¹ higher in sites dominated by herbivorous crabs (P > 0.05). Additionally, the positive correlation between crab abundance and burrow density, along with the negative correlation between burrow density and root biomass, highlights the complex interactions shaping carbon dynamics. This study addresses a critical knowledge gap regarding the role of crab activity in influencing coastal wetland carbon emissions and provides valuable insights for quantifying and modeling CO₂ fluxes in these ecosystems.