Influence of water table fluctuations on subsurface methane dynamics and surface fluxes in seasonally flooded subtropical pastures.

Abstract:

Seasonally flooded subtropical pastures are major sources of methane (CH₄), and periodic flooding drives complex emission dynamics from these ecosystems. Understanding the mechanisms of belowground CH₄ dynamics driving soil surface fluxes is needed to better understand emissions from these systems and their response to environmental change. We investigated subsurface CH₄ dynamics in relation to net surface fluxes using laboratory water table manipulations and compared these results to eddy covariance-measured fluxes to link within-soil CH₄ dynamics to observed ecosystem fluxes. Pronounced hysteresis was observed in ecosystem CH₄ fluxes during precipitation driven flooding events. This dynamic was replicated in mesocosm experiments, with maximum CH₄ fluxes observed during periods of water table recession. Hysteresis dynamics were best explained by oxygen dynamics during precipitation recharge events and the oxidation of CH₄ produced in organic soil horizons during water table recession. We observed distinct CH₄ dynamics between surface organic and deeper mineral soil horizons. In surface organic soil horizons, high levels of CH₄ production were temporally linked to observed surface emissions. In contrast, high concentrations of CH₄ observed in deeper mineral soils did not contribute to surface fluxes. Methane production potentials in surface organic soils were orders of magnitude higher than in mineral soils, suggesting that over longer flooding regimes CH₄ produced in mineral horizons is unlikely to be a significant component of net surface emissions. Our results demonstrate that distinct CH₄ dynamics may be stratified by depth, and flooding of the near-surface organic soils drives the high magnitude CH₄ fluxes observed from subtropical pastures. These results suggest that relatively small changes in pasture water table dynamics can drive large changes in net CH₄ emissions if surface organic soils remain saturated over longer time scales.