Soil properties and sediment accretion modulate methane fluxes from restored wetlands

Wetlands are the largest source of methane (CH₄ ) globally, yet our understanding of how process-level controls scale to ecosystem fluxes remains limited. It is particularly uncertain how variable soil properties influence ecosystem CH₄ emissions on annual time scales. We measured ecosystem carbon dioxide (CO₂ ) and CH₄ fluxes by eddy covariance from two wetlands recently restored on peat and alluvium soils within the Sacramento-San Joaquin Delta of California. Annual CH₄ fluxes from the alluvium wetland were significantly lower than the peat site for multiple years following restoration, but these differences were not explained by variation in dominant climate drivers or productivity across wetlands. Soil iron (Fe) concentrations were significantly higher in alluvium soils, and alluvium CH₄ fluxes were decoupled from plant processes compared with the peat site, as expected when Fe reduction inhibits CH₄ production in the rhizosphere. Soil carbon content and CO₂ uptake rates did not vary across wetlands and, thus, could also be ruled out as drivers of initial CH₄ flux differences. Differences in wetland CH₄ fluxes across soil types were transient; alluvium wetland fluxes were similar to peat wetland fluxes 3 years after restoration. Changing alluvium CH₄ emissions with time could not be explained by an empirical model based on dominant CH₄ flux biophysical drivers, suggesting that other factors, not measured by our eddy covariance towers, were responsible for these changes. Recently accreted alluvium soils were less acidic and contained more reduced Fe compared with the pre-restoration parent soils, suggesting that CH₄ emissions increased as conditions became more favorable to methanogenesis within wetland sediments. This study suggests that alluvium soil properties, likely Fe content, are capable of inhibiting ecosystem-scale wetland CH₄ flux, but these effects appear to be transient without continued input of alluvium to wetland sediments.