Climate change reduces the net sink of CH 4 and N 2 O in a semiarid grassland

Abstract

Atmospheric concentrations of methane ( CH 4 ) and nitrous oxide ( N 2 O ) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse 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 CO 2 and temperature affected CH 4 and N 2 O fluxes in a well‐drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH 4 and N 2 O fluxes to elevated CO 2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO 2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO 2 and warming would have opposing effects on CH 4 and N 2 O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell‐shaped relationship was observed with soil moisture with highest CH 4 uptake at intermediate soil moisture. Both N 2 O emission and uptake occurred at our site with some years showing cumulative N 2 O emission and other years showing cumulative N 2 O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO 2 and warming reduced the sink of CH 4 and N 2 O expressed in CO 2 equivalents (across 5 years by 7% and 11% for elevated CO 2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH 4 and N 2 O expressed in CO 2 ‐equivalents.

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