Hydrological Connectivity Dominates NO3‐N Cycling in Complex Landscapes – Insights From Integration of Isotopes and Water Quality Modeling

Distributed water quality modeling often involves considerable uncertainty due to complexity and spatial heterogeneity of hydrological and biogeochemical processes. To address this, stable water isotopes were coupled into a water quality modeling framework (EcH2O‐iso‐nitrate) for 30‐year hydrological and NO3‐N simulations in a mixed‐land‐use catchment (Demnitz Mill Creek, DMC, Germany). The isotope‐aided model effectively simulated the (dis)connection of different flow paths and related biogeochemical transformations. Results highlight the role of transient hydrological states in NO3‐N cycling in addition to static landscape characteristics. First, hydrological connectivity controls N transformations by regulating soil moisture and available NO3‐N for processing from upstream inflows, which was demonstrated by the spatially‐explicit estimates of water ages and Damköhler number. Second, hydrological pathways determine where, when, and which NO3‐N storages were connected, thereby dictating NO3‐N concentrations in different flow paths. In DMC, subsurface flows were the primary pathway, but transition to near‐surface flow occurred in specific riparian “hot‐spots” due to the development of soil saturation along flow paths. Such transition was transient but exerted disproportional impacts on NO3‐N dynamics by creating a preferential “fast‐track” toward channel network. This explains why DMC, as a groundwater‐dominated catchment, has flashy NO3‐N peaks in stream flow. To further unveil the role of hydrological connectivity in management planning, scenario analysis was conducted to identify optimal locations for wetland construction, showing riparian hotspots with better connectivity had the greatest potential for NO3‐N removal. These findings underscore the necessity of considering hydrological heterogeneity in modeling and management planning, which can be revealed by tracer‐aided modeling.