Nitrogen uptake kinetics and saltmarsh plant responses to global change

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.

Figure 1

Rates of ¹⁵N uptake by Spartina patens, Phragmites australis, and Schoenoplectus americanus during assays. Points are means (±SE) for replicate plants at the six N concentrations used; horizontal jitter has been added to reduce overlap. Shaded bands show the range of Michaelis-Menten curves corresponding to the bootstrapped 95% confidence interval for V_max.

Figure 2

Median of bootstrapped estimates for the Michaelis-Menten parameters V_max and K_m. Error bars depict the central 95% of estimates from across all bootstrapped fits (n = 999).

Figure 3

Mean (±SE) stimulation effects (i.e., difference from ambient experimental treatments) for (a) aboveground biomass production and (b) rhizome biomass production in experimental plots Global Change Research Wetland. The three treatments were CO₂, elevated atmospheric CO₂; N, nitrogen fertilization; and CO₂ + N, both elevated CO₂ and N fertilization. Means are calculated using the first 5 years’ worth of data from two long-term field experiments (see text).