Anthropogenic feed and food trade and hydrological factors influence river N and P export and stoichiometry from heavily exploited watersheds

Over the last century, agricultural intensification and urban expansion have significantly disrupted nitrogen (N) and phosphorus (P) cycles, with detrimental effects on the integrity of aquatic ecosystems. Nonetheless, current understanding of riverine nutrient dynamics -naturally influenced by geo-hydrological and climatic features - is still incomplete. This study aims to understand how the composition of the anthropogenic nitrogen and phosphorus load and geo-hydrological watershed characteristics influence riverine loads. To investigate these aspects, a trophic classification was applied to distinguish autotrophic (production-driven) watersheds typically associated with fertilizer use from heterotrophic (consumption-driven) systems, which rely on imported feed and food. Our analysis focused on the Po River Hydrographic District, one of the regions most impacted by N and P contamination in Europe. Net Anthropogenic Nitrogen and Phosphorus Inputs (NANI and NAPI), riverine N and P exports, and geo-hydrological characteristics were assessed to understand export variability between trophic groups. In autotrophic systems, nitrogen export was more influenced by hydrology than by NANI levels. Conversely, nitrogen loads were closely associated with manure and feed imports in heterotrophic watersheds, with runoff and leaching acting as primary pathways. Phosphorus export was largely governed by erosion and soil retention, showing a weak correlation with NAPI. Although N:P input ratios differed between groups, output ratios tended to converge, suggesting that different processes regulate nutrient stoichiometry.