Abstract. Climate warming perturbs ecosystem carbon (C) cycling, causing both positive
and negative feedbacks on greenhouse gas emissions. In 2016, we began a
tidal marsh field experiment in two vegetation communities to investigate
the mechanisms by which whole-ecosystem warming alters C gain, via
plant-driven sequestration in soils, and C loss, primarily via methane
(CH4) emissions. Here, we report the results from the first 4 years.
As expected, warming of 5.1 ∘C more than doubled CH4
emissions in both plant communities. We propose this was caused by a
combination of four mechanisms: (i) a decrease in the proportion of CH4
consumed by CH4 oxidation, (ii) more C substrates available for
methanogenesis, (iii) reduced competition between methanogens and sulfate-reducing bacteria, and (iv) indirect effects of plant traits. Plots
dominated by Spartina patens consistently emitted more CH4 than plots dominated by
Schoenoplectus americanus, indicating key differences in the roles these common wetland plants play
in affecting anaerobic soil biogeochemistry and suggesting that plant
composition can modulate coastal wetland responses to climate change.