Extreme precipitation controls P export mechanisms and N:P stoichiometry in contrasting Po River agricultural basins

Diffuse agricultural pollution is a major driver of nutrient enrichment in European surface waters, and its impacts are expected to intensify as climate change alters the timing, magnitude, and intensity of precipitation events. The Po River basin, like many nutrient hotspots worldwide, exhibits a chronic excess of nitrogen (N) driven by the preferential export of highly mobile nitrate. Extreme precipitation events, however, can also mobilize less mobile nutrient pools through erosion-driven transport. Phosphorus (P), which preferentially accumulates in the solid phase due to its strong affinity for soil particles, is therefore mainly exported during high-flow conditions, leading to marked, event-driven shifts in nutrient stoichiometry. Yet, how these processes vary across basins with contrasting nutrient surpluses, land management, and physical settings remains poorly understood.

In this study, we analyzed N and P export dynamics in two agricultural basins of the Po River watershed that experienced similar climatic anomalies but differ strongly in their biogeochemical and physical characteristics. The Chiese basin is characterized by high livestock density and a marked nutrient surplus, whereas the Volano basin exhibits lower livestock pressure and overall nutrient deficit. The two basins further contrast in soil permeability and topography, providing a natural experiment to investigate control mechanisms on nutrient export.

Monthly samplings carried out at the basin outlets throughout 2024-2025 were combined with high-frequency autosampler measurements (every 3 hours) during hydrological extremes. Dissolved and particulate phosphorus, including its bioavailable fraction and particulate nitrogen, together with dissolved inorganic nitrogen species (NO₃^-, NO₂^-, NH₄⁺), were quantified and linked to continuous discharge records.

Both basins displayed elevated nutrient concentrations, with contrasting partitioning between dissolved and particulate forms linked to differences in hydrological functioning. Hydrological extremes exerted divergent controls on nutrient behaviour, resulting in dilution or strong mobilization responses. Event-specific concentration-discharge relationships varied with seasonality and rainfall intensity. Overall, extreme events induced variable but generally P-dominated shifts in nutrient stoichiometry, with short-lived pulses accounting for a substantial fraction of annual export and temporarily altering N:P ratios.