Nitrogen uptake kinetics and saltmarsh plant responses to global change.

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 (NO) and ammonium (NH) through roots can strongly influence N acquisition rates and their responses to environmental factors such as rising atmospheric CO and eutrophication. We examined the N uptake kinetics of three dominant plant species in North American coastal wetlands (Spartina patens, C grass; Phragmites australis, C grass; Schoenoplectus americanus, C sedge) under ambient and elevated CO 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 NO and NH under ambient conditions, suggesting that N uptake kinetics may underlie its strong productivity response to N in the field. Elevated CO increased NH and NO uptake rates for S. patens, but had negative effects on NO 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 CO-induced shifts, in combination with N proliferation, could alter ecosystem-scale productivity patterns of saltmarshes globally.