ARS | Publication request: Climate change reduces the net sink of CH4 and N2O in a semiarid grassland

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 17, 2013
Publication Date: N/A

Interpretive Summary: As greenhouse gases continue to increase in Earth’s atmosphere and climate change unfolds, attention is being focused on how natural ecosystems emit or take up these greenhouse gases and whether or not climate change is affecting these more natural gas fluxes. Infrastructure that increases ambient temperatures and increased concentrations of the carbon dioxide was installed into a native semi-arid grassland in southeastern Wyoming to simulate the future environment of grasslands in regards to climate change. Land to atmosphere fluxes of two important greenhouse gases, methane and nitrous oxide, were monitored for five years in the experimental plots to see how the altered conditions affected them. Fluxes of both gases were sensitive to the changes in CO2 and temperature, and the combined effect of warming and rising CO2 was to reduce the capacity of this semi-arid grassland to take up these greenhouse gases from the atmosphere. This suggests that the problem of human-induced greenhouse gas emissions to the atmosphere is causing a feed-back in native grasslands, essentially re-forcing the release of greenhouse gases into the atmosphere, with additional effects on Earth’s climate.

Technical Abstract: Methane (CH4) and nitrous oxide (N2O) are potent greenhouse gases; their concentrations in the atmosphere have increased because of human activity. Soils are important sources and sinks of both gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO2 and temperature affected CH4 and N2O fluxes in a semiarid grassland during five growing seasons. We hypothesised that responses of CH4 and N2O fluxes to elevated CO2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO2 and warming would have opposing effects on CH4 and N2O fluxes. Methane was taken up throughout the growing season in all five years. A bell-shaped relationship was observed with soil moisture in which the lowest CH4 uptake occurred at low and high soil moisture and the highest CH4 uptake at intermediate soil moisture. Both N2O emission and uptake occurred at our site with some years showing cumulative N2O emission and other years showing cumulative N2O uptake. Nitrous oxide fluxes were positively related to soil moisture with N2O uptake mostly occurring in dry soils and N2O emission in wet soils. In contrast to our hypothesis, both elevated CO2 and warming reduced the sink of CH4 and N2O expressed in CO2 equivalents (across five years by 7 and 11% for elevated CO2 and warming respectively). We conclude that CH4 and N2O fluxes are sensitive to soil moisture, and that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH4 and N2O expressed in CO2-equivalents.