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 (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 ¹⁵N uptake kinetics of three dominant plant species in North American coastal wetlands (Spartina patens, C₄ grass; Phragmites australis, C₃ grass; Schoenoplectus americanus, C₃ 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.