Hydrological thresholds govern methane flux variability across wetland-cropland transition landscapes

Hydrological variability is a key regulator of greenhouse gas (GHG) fluxes across wetland-cropland transitions in cultivated landscapes, acting directly through water table dynamics and indirectly via land-use change, yet the balance of these effects is poorly understood. In these landscapes, rapid shifts in soil moisture which may extend to fully saturated conditions, can trigger highly dynamic methane (CH₄) and carbon dioxide (CO₂) responses, particularly within transition zones. In very flat and highly cultivated regions such as the Argentinian Pampas, widespread flooding and land-use reversion to wetlands have been associated with hydrological changes linked to the historical expansion of croplands. The effects of this large-scale ecohydrological transformation on biogeochemical functioning are still unclear.

We measured CH₄ and CO₂ fluxes across wetland-cropland transitions spanning multiple land uses and moisture regimes using in situ GHG monitoring combined with a broad suite of soil physical and chemical parameters across multiple field campaigns. This approach captured a wide range of water table positions and trends and allowed assessment of hydrology-, soil-, and carbon-related drivers of flux variability.

Across the landscape, water table depth was the dominant control on CH₄ fluxes, with wetlands exhibiting the highest values. CH₄ fluxes displayed a clear nonlinear response to hydrological conditions, with sharp increases once the water table approached the soil surface (-24 cm), indicating a strong threshold behaviour. While accounting for water table position reduced apparent differences among land uses, CH₄ fluxes remained systematically higher in wetlands and transitional zones than in croplands and pastures, demonstrating additional modulation by land-use–specific soil properties. Moreover, the sensitivity of CH₄ emissions to water table changes differed among land uses, with transitional zones and wetlands showing the strongest responses, highlighting their vulnerability to small hydrological shifts.

In contrast, CO₂ fluxes were primarily controlled by temperature and dissolved organic carbon availability and showed a comparatively weaker and more gradual response to moisture gradients, without clear threshold behaviour.

Overall, our results show that water table dynamics are the primary control on CH₄ flux variability at the landscape scale, while land use determines how strongly soils respond to hydrological change. These findings emphasize the importance of accounting for both hydrological variability and land-use transitions when assessing GHG emissions from ecosystems.