Flooding Alters Plant‐Mediated Carbon Cycling Independently of Elevated Atmospheric CO 2 Concentrations

Plant-mediated processes determine carbon (C) cycling and storage in many ecosystems; how plant-associated processes may be altered by climate-induced changes in environmental drivers is therefore an essential question for understanding global C cycling. In this study, we hypothesize that environmental alterations associated with near-term climate change can exert strong control on plant-associated ecosystem C cycling and that investigations along an extended hydrologic gradient may give mechanistic insight into C cycling. We utilize a mesocosm approach to investigate the response of plant, soil, and gaseous C cycling to changing hydrologic regimes and elevated atmospheric carbon dioxide (CO2) concentrations expected by 2100 in a coastal salt marsh in Louisiana, USA. Although elevated CO2 had no significant effects on C cycling, we demonstrate that greater average flooding depth stimulated C exchange, with higher rates of labile C decomposition, plant CO2 assimilation, and soil C respiration. Greater average flooding depth also significantly decreased the soil C pool and marginally increased the aboveground biomass C pool, leading to net losses in total C stocks. Further, flooding depths along an extended hydrologic gradient garnered insight into decomposition mechanisms that was not apparent from other data. In C-4 dominated salt marshes, sea level rise will likely overwhelm effects of elevated CO2 with climate change. Deeper flooding associated with sea level rise may decrease long-term soil C pools and quicken C exchange between soil and atmosphere, thereby threatening net C storage in salt marsh habitats. Manipulative studies will be indispensable for understanding biogeochemical cycling under future conditions.