Influence of transient flooding on methane fluxes from subtropical pastures

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.