Hidden nitrogen pathways driving pollution from farms to cities

Peer-Reviewed Publication

Maximum Academic Press

Using an integrated analytical framework that separates surface flow, subsurface flow, and baseflow, the team quantified how each pathway contributes unevenly to nitrogen export. Their findings show that urban watersheds release most ammonium through baseflow linked to leaking sewer systems, while agricultural regions shift seasonally between groundwater-dominated nitrate transport and storm-driven dissolved organic nitrogen pulses.

Nitrogen enrichment from agriculture, urban development, atmospheric deposition, and wastewater has reshaped global biogeochemical cycles and intensified eutrophication. While surface runoff during storms is often viewed as the main export mechanism, recent studies show that subsurface pathways and baseflow can transport more than half of the annual N load in many basins. However, conventional analytical tools have limitations: hydrograph separation equates water volume with pollutant contribution, while typical chemical tracer methods lack hydrological context. These challenges hinder efforts to pinpoint when and how nitrogen leaves landscapes, particularly across rural–urban gradients where land use intensity, drainage systems, and pollution sources diverge sharply.

A study (DOI:10.48130/nc-0025-0006) published in Nitrogen Cycling on 17 October 2025 by Yongqiu Xia’s team, Hohai University, demonstrates that effective nitrogen management must recognize not only where pollution originates, but also how it travels through the watershed.

Using an integrated analytical approach that combined three-component hydrograph separation with pathway-specific End-Member Mixing Analysis (EMMA), the study first quantified hydrological pathways and then linked each pathway to N export patterns across three contrasting watersheds. Hydrograph separation established the proportional contributions of surface flow, subsurface flow, and baseflow at both annual and seasonal scales, while LOADEST-simulated continuous nitrogen concentrations and stratified end-member sampling enabled the construction of concentration–discharge (C–Q) relationships for each pathway. This method revealed clear hydrological contrasts: annually, baseflow dominated in the urban watershed (40.2%), whereas surface flow prevailed in traditional (39.1%) and intensive agricultural watersheds (39.9%), with subsurface flow remaining stable across sites (21.6%–28.4%). Seasonal analysis showed pronounced shifts, with baseflow dominating dry-season runoff (up to 58.7%) and surface flow becoming the primary wet-season pathway in agricultural systems (over 40% of runoff). Pathway-specific EMMA further demonstrated distinct geochemical signatures: intensive agriculture exhibited the highest N concentrations across all pathways, with subsurface flow showing particularly elevated total dissolved nitrogen (median 9.07 mg L⁻¹). In traditional agriculture, baseflow carried most nitrate (median 1.87 mg L⁻¹), while surface flow displayed large fluctuations in dissolved organic nitrogen (DON). Urban watersheds showed a consistent gradient of baseflow > subsurface flow > surface flow for both TDN and ammonium. Annual nitrogen load estimates reflected these hydrological and chemical differences: the urban watershed exported the greatest TDN load (33,287.1 t), while smaller discharges limited loads in the intensive agricultural site despite higher concentrations. Baseflow dominated nitrate export in traditional agriculture (50.2%) and ammonium export in urban systems (46.4%), whereas surface flow controlled DON transport in both agricultural and urban landscapes. Seasonal switching was also evident, with dry-season N export governed by baseflow and wet-season export shifting to surface runoff pathways. Monte Carlo uncertainty tests confirmed the robustness of these pathway distinctions and their statistical significance.

The findings demonstrate that nitrogen management based solely on land use or total runoff ignores crucial transport dynamics. Traditional agricultural watersheds require dual interventions: controlling surface runoff to reduce DON surges during storms and improving soil N retention to limit nitrate leaching into groundwater. Intensive agriculture demands stricter fertilizer regulation and integrated treatment systems targeting both surface and subsurface pathways. Urban nitrogen management must pivot toward repairing sewer leakage and enhancing subsurface filtration—since baseflow and shallow interflow, not stormwater runoff, dominate nitrogen delivery.