Abstract
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.
Highlights
Anthropogenic activities enrich the atmosphere with CO2, but the extent to which the biosphere can absorb this increase remains uncertain[1,2]
S. patens was primarily responsible for these differences, as it exhibited mean uptake rates up to 3 times greater than those of P. australis or S. americanus (Fig. 1a,c) and separated from both species in pairwise comparisons (Table 2)
For NH4+, S. americanus had 20–30% greater mean Vuptake across the range of N concentrations than did P. australis (Fig. 1a). These interspecific differences manifested in the parameter values of maximal uptake rate (Vmax), the maximal uptake rate, when Michaelis-Menten curves were fit to the data; in this context, Vmax reflects a species’ capacity for N uptake under saturating N conditions
Summary
Anthropogenic activities enrich the atmosphere with CO2, but the extent to which the biosphere can absorb this increase remains uncertain[1,2]. While the effects of elevated CO2 on plant productivity are well studied in a variety of ecosystems[19,20] including saltmarshes[21,22], our understanding of these responses is primarily based on changes in aboveground biomass, CO2 assimilation[23,24], and shifts in species composition that reflect competition between plant functional groups[10]. In the context of anthropogenically-induced changes to the carbon and nitrogen cycles in wetland ecosystems, information on physiological responses of N uptake to elevated CO2 could be highly relevant for understanding these species shifts and how they influence critical ecosystem-level phenomena such as resilience to sea level rise and carbon sequestration[44]
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