Decadal mitigation and shifting drivers of aquatic CH₄ and N₂O emissions during urban stream restoration

Heavily polluted urban streams are increasingly recognized as important but poorly constrained sources of CH₄ and N₂O emissions. However, long-term evidence linking watershed restoration to sustained reductions in stream CH₄ and N₂O emissions and their controlling mechanisms remains limited. Here, we present a decadal (2014-2023) field investigation of a severely polluted urban stream (QS Stream) in Chongqing, China, that experienced successive stages of heavy pollution, pollution interception, and ecological restoration. By tracking dissolved concentrations and fluxes of CH₄ and N₂O across three successive restoration stages, we quantified stage-specific mitigation efficiencies and shifts in environmental controls. During the pre-restoration stage (2014), QS Stream functioned as a pronounced hotspot of CH₄ and N₂O emissions. Integrated watershed management reduced CH₄ and N₂O fluxes by >70% over the decade, with rapid declines following point-source pollution interception (2018) and further substantial mitigation during subsequent ecological restoration (2023). Correlation and random forest analyses revealed systematic shifts in dominant controls, from carbon-nitrogen dominated regulation under severely polluted conditions to nitrogen- and DO-associated controls after point-pollution interception. Notably, pH and phosphorus limitation emerged as key constraints on both CH₄ and N₂O emissions at the ecological restoration stage. Importantly, integrated efficiency analyses further indicate that organic carbon removal rate yields higher mitigation benefits of CH₄ and N₂O emissions than equivalent reduction rates in nitrogen or phosphorus, highlighting strong contrasts in climate co-benefits among different nutrient management strategies. Together, these results demonstrate that CH₄ and N₂O dynamics in urban streams are governed by evolving, stage-dependent biogeochemical constraints rather than a single set of drivers. Coordinated pollution managements, with priority given to organic carbon control alongside sustained regulation of oxygen conditions and phosphorus availability, is therefore critical for maximizing and maintaining climate benefits during long-term urban stream restoration.