Increasing fertilization has pushed the nitrogen cycle beyond planetary boundaries, yet its fate remains uncertain owing to long-standing neglect of landscape water velocities in nitrogen models. Leveraging isotope-aided modeling across 3821 European catchments, we demonstrate that evolution of nitrogen cycling is strongly linked to shifts in landscape water velocities since 1980. We propose the concept of “wetness boundaries,” where hydrological transitions beyond boundaries amplify nitrogen accumulation and leaching, whereas conditions remaining within boundaries mitigate these processes. Applying this framework, we project reduced nitrogen leaching across 76% of Europe under mild hydrological shifts by 2100 but increasing nitrogen accumulation under pronounced deceleration of water cycling in Eastern and Southern Europe. These findings underscore emerging water quality risks under climate change and the need to mitigate extreme hydrological shifts.