Coastal wetlands are important carbon sinks globally, but their ability to store carbon hinges on their nitrogen (N) supply and N uptake dynamics of dominant plant species. In terrestrial ecosystems, uptake of nitrate (NO3-) and ammonium (NH4 ) through roots can strongly influence N acquisition rates and their responses to environmental factors such as rising atmospheric CO2 and eutrophication. We examined the N-15 uptake kinetics of three dominant plant species in North American coastal wetlands (Spartina patens, C-4 grass; Phragmites australis, C-3 grass; Schoenoplectus americanus, C-3 sedge) under ambient and elevated CO2 conditions. We further related our results to the productivity response of these species in two long-term field experiments. S. patens had the greatest uptake rates for NO3- and NH4 under ambient conditions, suggesting that N uptake kinetics may underlie its strong productivity response to N in the field. Elevated CO2 increased NH4 and NO3- uptake rates for S. patens, but had negative effects on NO3- uptake rates in P. australis and no effects on S. americanus. We suggest that N uptake kinetics may explain differences in plant community composition in coastal wetlands and that CO2- induced shifts, in combination with N proliferation, could alter ecosystem-scale productivity patterns of saltmarshes globally.
