Riparian seasonal water quality and greenhouse gas dynamics following stream restoration

Stream restoration, such as channel reconfiguration and riparian zone revegetation, is increasing in response to aquatic impairment in agroecosystems. Though riparian zones effectively remove nitrogen (N), conditions that foster nitrate (NO3−) removal (e.g., soil saturation, low oxygen, available carbon) may facilitate the release of soluble reactive phosphorus (SRP) and greenhouse gases (GHGs—carbon dioxide [CO2], methane [CH4], and nitrous oxide [N2O]). In this study, water quality and GHG fluxes were quantified seasonally at agricultural restored, unrestored, and forested riparian zones via networks of wells and static chambers. Carbon dioxide comprised the majority of GHG emissions, and CO2 fluxes were lower at the restored site than the forested site (medians: 0.71 g C m−2 day−1 and 1.29 g C m−2 day−1, respectively). The restored site was a net CH4 source (median: 1.20 mg C m−2 day−1), while the other sites were net CH4 sinks (medians: − 1.00 to − 1.76 mg C m−2 day−1). Sites had comparable N2O emissions. Subsurface SRP was lower at the restored than unrestored agricultural sites, while median NO3− removal efficiency was higher (restored: 91%, unrestored: 35–66%). Over time, NO3− removal and CO2 fluxes increased post-restoration. Methane emissions were elevated when the restored riparian zone was a CO2 and N2O sink and NO3− removal was high. Despite variability within and between sites, seasonal conditions (temperature, water table height, precipitation) and site characteristics (geomorphology, soils) remained key explanatory variables for riparian N removal and GHG release. Overall, riparian restoration may promote N removal without increasing total GHG emissions.