Mediterranean riparian zones as hotspots of greenhouse gases: effects of vegetation, distance to the riverbank and wet periods

The ability of riparian forests to reduce nutrient flows from crops to streams is well known. However, their role as sources and sinks of greenhouse gases (GHGs) has been less studied, particularly in Mediterranean environments. We assessed the changes in daily soil N₂O and CH₄ fluxes between 2021 and 2023 in a riparian zone located in Central Spain. We also evaluated if cumulative fluxes of such GHGs depended on soil NO₃⁻, NH₄⁺ and DOC contents, and water-filled pore space (WFPS). Their dependence to different vegetation types and distances to the riverbank, as well as on the existence of wet periods, which include occasional floods, was also analysed. Daily N₂O and CH₄ fluxes were both low. Those of N₂O were positive whereas those of CH₄ were mostly negative, but positive fluxes of this GHG were nevertheless observed in late autumn 2021 and in spring 2022. Cumulative fluxes were mainly driven by soil NH₄⁺ contents in the case of N₂O and by WFPS in the case of CH₄, and influenced by the distance from the riverbank in both cases. The relationships between cumulative fluxes of N₂O and its major drivers were positively and significantly influenced by the wet periods. Our results indicate that our riparian zone acted as a net source of N₂O and a net sink of CH₄, but it became a net source of CH₄ in cold and wet periods, where anoxic conditions in which methanogenesis occurs are favoured. Soil N₂O emissions mainly originate as a by-product of nitrification but also from incomplete denitrification after heavy rainfall events in warm months. Thus, we advocate for preventive strategies to reduce nitrogen flows from cropping systems to reduce soil N₂O emissions in Mediterranean riparian zones.