Interacting effects of warming and CO2 on marsh stability and carbon accumulation

Abstract:

Salt marshes are globally important, carbon dense ecosystems that are simultaneously maintained and threatened by sea level rise. Many field experiments attempt to simulate future climates by manipulating single global change drivers in isolation, but this approach neglects non-additive interactions between drivers. To assess the interactive effects of multiple global changes on wetland stability and soil carbon accumulation, we actively increased aboveground plant-surface temperature, below-ground soil temperature, and atmospheric CO2 in a brackish salt marsh. Negligible sedimentation within the marsh allowed us to use elevation change as a proxy for the balance between below-ground carbon gain through plant production and loss through decomposition. We find that a moderate amount of warming (1.7 degrees above ambient temperatures) maximizes root growth, marsh elevation gain, and below-ground carbon accumulation. At higher warming treatments, effects were negligible or slightly negative from reduced root growth and increased carbon mineralization. Between plant communities, C4 plant communities were found to be more reactive to warming than C3 communities. Interestingly, elevated CO2 had a negligible effect on marsh elevation and carbon accumulation despite having a large increase in root production, which we attribute to enhanced carbon mineralization associated with oxygen delivery from more extensive roots (i.e. O2-induced priming). Maximized elevation and below-ground carbon accumulation for moderate warming scenarios may indicate enhanced function and resiliency of marshes under some climate conditions, but our work identifies important non-linearities in the response, and suggests these benefits may be temporary as global temperatures continue to rise.