Excess nitrogen loading is a major threat to freshwater and downstream coastal ecosystems, particularly in human-impacted watersheds dominated by agricultural and urban land use. Nitrogen “impairment” can trigger a cascade of harmful ecological effects: eutrophication, biodiversity loss, and acidification, as well as broader negative impacts on atmospheric conditions, air quality, and human health. Storm events often contribute a disproportionately large fraction of annual nutrient exports, with pulses of mobilized nutrients overwhelming ecosystem function at higher flows. Yet storm-scale nitrogen dynamics remain poorly understood due to historical limitations in temporal data. As storm frequency and intensity increase under climate change, understanding what controls patterns of storm-driven nitrogen loading is increasingly important.
This study investigated how land use and hydroclimatic conditions jointly influence nitrate sources, transport pathways, and export during storm events along a mixed land-use stream continuum. We deployed high-frequency (15-minute) in situ sensors measuring nitrate, discharge, and specific conductance at three nested sites along College Brook (New Hampshire, USA), spanning agricultural, forested, and urban reaches. Storm event nitrate loading was quantified using concentration–discharge (C–Q) relationships, hysteresis metrics, and event-scale nitrogen fluxes.
We hypothesize that agricultural and urban reaches provide readily mobilized nitrate sources and minimal riparian buffering, which will generate flushing (increased concentrations) and positive hysteresis, whereas forested reaches exhibit dilution (decreased concentrations) due to source limitation and riparian buffering. We also hypothesize that longer antecedent dry periods between storms enable nitrate accumulation in soils via mineralization, resulting in stronger initial flushing and storm-scale nitrogen fluxes during subsequent storms.
We present comparisons of storm event nitrate loading across land-use reaches and evaluate how these responses vary with precipitation characteristics, seasonality, and antecedent moisture conditions. Preliminary results show greater flushing, fluxes, and positive hysteresis linked agricultural land use, before marked dilution by forested land continuing downstream. By capturing nutrient dynamics at multiple points along a stream continuum, this work disentangles effects of fine-scale land use that are often obscured in single downstream monitoring locations within complex, mixed land-use watersheds. These findings will inform stream restoration efforts in College Brook and help watershed managers identify high-priority locations for mitigation.