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