| As sea-level rise accelerates to the highest rates in the last 5000 years, ocean inundation threatens low-lying coastal ecosystems. Some ecosystems such as tidal wetlands have natural mechanisms to accumulate soil and gain elevation to keep up with sea-level rise, but the limits of this process are unknown. Other global change factors can strongly alter plant productivity and soil organic matter (SOM) decomposition, the primary determinants of elevation change in peat-forming marshes. I investigated the effects of sea-level rise, elevated atmospheric CO2 concentration, and N-fertilization on productivity of Schoenoplectus americanus and Spartina patens, SOM decomposition, and soil redox potential. Plant productivity, SOM decomposition, and soil redox potential tended to decrease with increasing relative sea level. Plant biomass responded more strongly to sea-level rise than SOM decomposition or soil redox potential, suggesting that vertical accretion rates will slow under sea-level rise conditions. Elevated CO2 increased Sc. americanus shoot and root biomass, and N-fertilization incrased shoot biomass of both species, but only within a limited range of relative sea levels. Pore-water [NH4+] increased as relative sea level rose, likely due to diminished plant demand, and elevated CO2 increased soil respiration rate. Counter to previous predictions, the CO2 and N treatments appeared to provide only marginal benefits to plant productivity under increasing flooding stress. |
