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

Background/Question/Methods

Methane (CH₄) is a globally important greenhouse gas, however its emission sources remain poorly quantified and understood. We have a particularly poor understanding of biogenic CH₄ fluxes from seasonally flooded ecosystems because emissions are highly variable between wet and dry periods, and surface fluxes are infrequently assessed in relation to subsurface production and consumption processes. Elucidating the impact of subsurface processes on surface fluxes is needed to better understand variable CH₄ emissions and their potential response to environmental change. We investigated subsurface CH₄ production and consumption in relation to net surface fluxes from seasonally flooded pasture soils through laboratory water table manipulations. Intact 15 x 50 cm soil columns were excavated from a Florida pasture and water table levels were manipulated in experimental mesocosms. The water table in three replicate columns was increased from 55 cm below surface to the soil surface at 10 cm increments, and then reduced at the same interval to examine potential hysteresis in CH₄ fluxes. At each interval, we measured soil gas concentrations and isotope ratios every 10 cm throughout the columns, as well as surface fluxes. These results were compared to eddy covariance-measured fluxes to link within-soil CH₄dynamics to ecosystem fluxes.

Results/Conclusions

No CH₄ emissions were observed until the water table reached the soil surface. However, as the water table receded, surface emissions persisted until the water table was 35 cm below the surface. Methane concentrations in surface profiles (0-10 cm) followed observed trends in surface emissions. Methane was also produced in deep profiles, most notably 40 cm depth, where concentrations rose to ~600 ug/L. Concentrations at 40 cm depth were 10-20x higher than surface profile concentrations, however deep profile CH₄ production did not influence surface emissions. This observation, along with the observed enrichment of δ¹³C-CH₄ at depth, suggests oxidation and limited transport of CH₄ produced in deep profiles. Overall, surface emissions were positively correlated to CH₄ production at 10 cm depth (r² = 0.55) and were unrelated to production at other soil depths. The mesocosm results support ecosystem observations where CH₄ emissions are rarely observed during a rising water table, and sustained ecosystem emissions are observed during retreating water tables. Under fluctuating water table conditions, it appears that pasture CH₄ emissions are driven by CH₄ production in the top 10 cm profiles, and CH₄ produced at depth is limited by vertical transport or oxidized before being emitted to the atmosphere.