Elevated CO sub(2) and nitrogen addition accelerate net carbon gain in a brackish marsh

• Published in: Biogeochemistry 2017-03, Vol.133 (1), p.73-87
• Main Authors: Pastore, Melissa A, Megonigal, JPatrick, Langley, JAdam
• Format: Article
• Language: English
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-017-0312-2

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 (CO sub(2)) 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 CO sub(2) and N addition. Nearly all plots exhibited a net gain of C over the study period (up to 203 g C m super(-2) year super(-1)), and C gain rates were greater with N and CO sub(2) 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 CO sub(2) can stimulate methane emissions, overcoming positive effects of elevated CO sub(2) on C gain, converting brackish marshes that are typically net greenhouse gas sinks into sources. Adding resources, either CO sub(2) 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.