Spatial-temporal synchrony between Nitrate and Discharge Varies with environmental and anthropogenic controls

Nitrogen cycling has been dramatically altered by anthropogenic activities, impacting water quality and ecosystem functioning of river systems worldwide. Understanding the (a)synchrony between discharge (Q) and nitrate concentration (N) is crucial to revealing the temporal-spatial processes that govern nitrogen dynamics and identify the controlling factors to improve monitoring and management strategies. Here data collected from 66 river catchments across England, spanning 20 years, were analysed to characterise Q-N synchrony patterns and assess spatiotemporal variability. QMax-synced catchments (i.e. max N occurred with max Q, accounting for 28.8% of catchments) are mainly smaller, agricultural catchments, where high-flow conditions can mobilise accumulated nitrate from agricultural soils. By contrast, QMin-synced catchments (i.e. max N occurred with min Q, 25.8% of catchments) have higher proportions of urban area and displayed stronger point-source influences. These catchments are generally larger and have a higher proportion of surface runoff during high-flow periods, diluting nitrate-rich point sources and shifting peak nitrate concentrations to the period of lowest flow. Asynced catchments (46.8%) are also generally larger but with a larger mixture of land use and therefore point and diffuse nitrate sources. Furthermore, the synchrony variability is primarily influenced by sharp topography in QMax-synced catchments, while anthropogenic activities like sewage treatment plant density mainly impact that in QMin-synced and Asynced catchments. Our results demonstrate that the seasonal timing of peak nitrate has a strong and highly contrasting dependence on hydrological conditions that shift with catchment size and availability of diffuse and point sources, with important implications for monitoring and management.