Nonlinear controls on nitrogen export across heterogeneous riparian zones in a karst catchment: Insights from an integrated habitat classification framework

Identifying the spatial heterogeneity of riparian zones and its regulatory mechanisms on nitrogen migration is crucial for the prevention and control of non-point source pollution in watersheds. However, the influence of complex habitat characteristics in karst watersheds on nitrogen migration and its nonlinear “structure-process-function” response mechanism remains unclear. This study, focusing on the riparian zone of the Lianjiang River, constructed a comprehensive classification framework of “land use-geomorphic background-riparian matrix,” and combined in-situ simulated runoff scour experiments and generalized additive model to assess the runoff-infiltration characteristics and nitrogen concentrations driving mechanisms of different riparian types. The results showed significant spatial heterogeneity of the riparian zone in the study area, with agricultural land dominating and significant differences in the physicochemical properties of different soil types. The cumulative infiltration rate was: urban open space > forest land > agricultural land > grassland. With increasing scour events, the concentrations of TN, NH 4 + -N, and NO 3 − -N generally decreased, but the characteristics and driving pathways differed across the various nitrogen forms. NH 4 + -N export concentrations is mainly controlled nonlinearly by the intensity of human activities, explaining 56.9% of the variance. NO 3 − -N export concentrations is mainly driven by surface runoff, explaining 36.5% of the variance. TN export concentrations shows a weak response to a single environmental gradient and is controlled by the coupling of multiple “structure-process” factors. These results highlight the crucial role of complex habitat characteristics in regulating nitrogen migration along riparian zones in karst watersheds and provide a solid scientific foundation for the precise control of non-point source nitrogen pollution at the watershed scale.