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Title: Influence of transient flooding on methane fluxes from subtropical pastures

item CHAMBERLAIN, SAMUEL - Cornell University
item GOMEZ-CASANOVAS, NURIA - University Of Illinois
item WALTER, M TODD - Cornell University
item BOUGHTON, ELIZABETH - Macarthur Agro-Ecology Research Center
item Bernacchi, Carl
item DELUCIA, EVAN - University Of Illinois
item GROFFMAN, PETER - City University Of New York
item KEEL, EARL - Macarthur Agro-Ecology Research Center
item SPARKS, JED - Cornell University

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2016
Publication Date: 4/1/2016
Citation: Chamberlain, S.D., Gomez-Casanovas, N., Walter, M., Boughton, E., Bernacchi, C.J., DeLucia, E., Groffman, P.M., Keel, E.W., Sparks, J.P. 2016. Influence of transient flooding on methane fluxes from subtropical pastures. Journal of Geophysical Research-Biogeosciences. 121:965-977.

Interpretive Summary: Methane is a potent greenhouse gas that contributes to global warming. Subtropical pastures are a major methane source particularly when flooded. Little is known, however, about the mechanisms behind methane production in these ecosystems and how they will influence emissions under future climate changes. This research linked field-based measurements with laboratory analyses to understand the emission rates, and the underlying mechanisms, for soils in subtropical pastures. The results show (1) that the laboratory analyses matched the field measurements and (2) that surface soils were more important in methane production than from deeper soils. These results suggest that more surface flooding of subtropical pastures, a likely scenario for subtropical biomes as climate change progresses, can link directly to higher methane emissions.

Technical Abstract: Seasonally flooded subtropical pastures are major methane (CH4) sources, where transient flooding drives episodic and high-magnitude emissions from the underlying landscape. Understanding the mechanisms that drive these patterns is needed to better understand pasture CH4 emissions and their response to global change. We investigated belowground CH4 dynamics in relation to surface fluxes using laboratory water table manipulations and compared these results to field-based eddy covariance measurements to link within-soil CH4 dynamics to ecosystem fluxes. Ecosystem CH4 fluxes lag flooding events, and this dynamic was replicated in laboratory experiments. In both cases, peak emissions were observed during water table recession. Flooding of surface organic soils and precipitation driven oxygen pulses best explained the observed time lags. Precipitation oxygen pulses likely delay CH4 emissions until groundwater dissolved oxygen is consumed, and emissions were temporally linked to CH4 production in surface soil horizons. Methane accumulating in deep soils did not contribute to surface fluxes and is likely oxidized within the soil profile. Methane production rates in surface organic soils were also orders of magnitude higher than in deep mineral soils, suggesting that over longer flooding regimes CH4 produced in deep horizons is not a significant component of surface emissions. Our results demonstrate that distinct CH4 dynamics may be stratified by depth and flooding of surface organic soils drives CH4 fluxes from subtropical pastures. These results suggest that small changes in pasture water table dynamics can drive large changes in CH4 emissions if surface soils remain saturated over longer time scales.