Elevated CO2 and nitrogen addition accelerate net carbon gain in a brackish marsh
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Wetlands have an inordinate influence on the global greenhouse gas budget, but how global changes may alter wetland contribution to future greenhouse gas fluxes is poorly understood. We determined the greenhouse gas balance of a tidal marsh exposed to nine years of experimental carbon dioxide (CO2) and nitrogen (N) manipulation. We estimated net carbon (C) gain rates by measuring changes in plant and soil C pools over nine years. In wetland soils that accrete primarily through organic matter inputs, long-term measurements of soil elevation, along with soil C density, provide a robust estimate of net soil C gain. We used net soil C gain along with methane and nitrous oxide fluxes to determine the radiative forcing of the marsh under elevated CO2 and N addition. Nearly all plots exhibited a net gain of C over the study period (up to 203 g C m−2 year−1), and C gain rates were greater with N and CO2 addition. Treatment effects on C gain and methane emissions dominated trends in radiative forcing while nitrous oxide fluxes in all treatments were negligible. Though these soils experience salinities that typically suppress methane emissions, our results suggest that elevated CO2 can stimulate methane emissions, overcoming positive effects of elevated CO2 on C gain, converting brackish marshes that are typically net greenhouse gas sinks into sources. Adding resources, either CO2 or N, will likely increase “blue carbon” accumulation rates in tidal marshes, but importantly, each resource can have distinct influences on the direction of total greenhouse forcing.
KeywordsCarbon gain CO2 enrichment Nitrous oxide Greenhouse gases Methane Nitrogen pollution
The authors thank J. Duls, G. Peresta, and A. Peresta for assistance with data collection and maintenance of the experiment and treatments at the Smithsonian Global Change Research Wetland. We also thank M. Vile for technical insight, as well as two anonymous reviewers for the helpful insights. This work was supported by National Science Foundation-LTREB Program Grants DEB-0950080 and DEB-1457100, the Smithsonian Institution, and Villanova University.
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