Methane production and emissions from a tidal wetland under multifactored global change

Background/Question/Methods

Methane, a potent greenhouse gas, may contribute to climatic feedbacks that are currently difficult to predict given our poor understanding of the behavior of natural sources and sinks. Unmanaged wetlands are vulnerable to many global changes that may positively feedback on climate change by increasing methane emissions.  The goal of this study was to determine how nitrogen fertilization, elevated CO₂, and changes in relative sea level alter net methane emissions and soil methane production in a brackish tidal marsh.  Net emissions were measured using the static chamber method from mesocosms growing Schoenoplectus americanus and Spartina patens with manipulated sea level, atmospheric CO₂, and nitrogen treatments. Soil samples were later collected and incubated anaerobically to estimate methane production potentials. 

Results/Conclusions

Relative sea level had the greatest effect on net methane emissions, and methane emissions increased as flooding increased.  Soil incubations indicate that sea level had an indirect effect on methanogenesis; sea level was correlated with belowground biomass (r²= 0.65), and in the samples with the most inundation, methane production was correlated with belowground biomass (r²= 0.47) indicating carbon limitation of methanogenesis.  Elevated CO₂ increased net methane emissions but had no effect on potential methane production suggesting that the CO₂ effect may depend on the activity of live plants.  Nitrogen  had no observed effect on net methane emissions or production potentials.  There were no interactions between CO₂, N, or sea level treatments for net methane emissions.  These results indicate that rising sea levels and increasing atmospheric carbon dioxide may increase the methane flux from tidal wetlands, generating a positive feedback to warming.