Abstract
The rapid increase in atmospheric CO 2 concentrations ( C a ) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO 2 between the atmosphere and the biosphere ( NEE ), under both ambient and elevated C a . Here we report data on annual sums of CO 2 ( NEE net ) for 19 years on a Chesapeake Bay tidal wetland for S cirpus olneyi (C 3 photosynthetic pathway)‐ and Spartina patens (C 4 photosynthetic pathway)‐dominated high marsh communities exposed to ambient and elevated C a (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated C a on seasonally integrated CO 2 assimilation ( NEE net = NEE day + NEE night , kg C m −2 y −1 ) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEE net averaged 1.9 kg m −2 y −1 in ambient C a and 2.5 kg m −2 y −1 in elevated C a , for the C 3 ‐dominated community. Similarly, elevated C a significantly ( P 4 ‐dominated community, as NEE net averaged 1.5 kg m −2 y −1 in ambient C a and 1.7 kg m −2 y −1 in elevated C a . This resulted in an average CO 2 stimulation of 32% and 13% of seasonally integrated NEE net for the C 3 ‐ and C 4 ‐dominated communities, respectively. Increased NEE day was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated C a , while decreased NEE night was associated with lower canopy nitrogen content. These results suggest that rising C a may increase carbon assimilation in both C 3 ‐ and C 4 ‐dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.