Methane production efficiency and production pathway in restored and natural wetlands of the Sacramento-San Joaquin Delta

Background/Question/Methods

Wetland methane (CH₄) production efficiency (MPE) is defined as the relative rate of CH₄ to carbon dioxide (CO₂) production, and provides an index of the importance of alternative biogeochemical pathways for carbon (C) degradation. Wetland MPE varies widely and impacts wetland greenhouse gas budgets but controls are still uncertain. Environmental variables such as pH, redox, and substrate quality are known to control the underlying processes of respiration, methanogenesis, and fermentation and thus are likely to control MPE. We measured MPE in freshly collected soils from restored (2 and 10-years old) and natural rich-fen wetlands located in the Sacramento-San Joaquin Delta, California. Sites were chosen to cover a range in environmental conditions. Laboratory incubations were used to quantify gross CH₄ fluxes using stable isotope pool dilution. Soil chemical characteristics including HCl-extractable iron (Fe) and sulfur (S), salt extractable mineral nitrogen (NH⁺, NO₃^-), pH and salinity, dissolved organic carbon (DOC), and soil CN composition were measured as explanatory variables. In a subsequent experiment we manipulated redox by incubating soil under an anaerobic headspace and investigated the effects on the MPE and pathway of CH₄production.

Results/Conclusions

Gross CH₄ production rates were high and ranged from 37.62 ± 19.81 ng C g^-1 hr^-1 at the newly restored site to 125.52 ± 24.33 ng C g^-1 hr^-1 at the older site. Methane production efficiency ranged from 2.6 ± 1.3 % at newly restored and natural wetlands, to 12 ± 1.1 % at older restored site. Almost all Fe was reduced at all wetlands and thus did not explain differences in MPE across sites however soil NH₄⁺ showed a strong positive correlation with MPE (R² = 0.75), consistent with a redox control. In the anoxic incubation MPE increased over time in all soils, reaching 25-28% by day 30. Notably, MPE increased due to a drop in absolute CO₂ production rather than an increase in CH₄ production. Measurement of δ¹³CH₄ and δ¹³CO₂ indicated that CO₂-reduction was the predominant methanogenic pathway consistent with a tidal influence, but unusual for rich fen wetlands. We conclude that wetland MPE can vary substantially in deltaic fens and our data support soil redox as a controlling factor.