Managing excess nutrients remains a major obstacle to improving ecosystem service benefits of urban waters. We compared watershed inputs, outputs, and retention for nitrogen (N) and phosphorus (P) in seven subwatersheds of the Mississippi River, in Saint Paul, Minnesota, USA towards improving management of urban nutrient pollution. We quantified watershed inputs of nutrients from residential and non-residential fertilizer, atmospheric deposition, dog waste, biological N fixation, compost, and weathering using modeling, survey data, and other measurements. We quantified watershed nutrient outputs via yard waste, street sweeping, and storm drain exports, and compared inputs to outputs to estimate retention of N and P.
Watersheds retained only 18% of net P inputs versus 78% of net N inputs (watershed area-weighted average, where net inputs equal inputs minus biomass removal) despite relatively low P inputs related to a statewide restriction on lawn P fertilizer use. In contrast to many non-urban watersheds that exhibit high P retention, these urban watersheds have high street density that enhanced transport of P-rich materials from landscapes to stormwater. High P exports in storm drainage networks and yard waste resulted in net loss of P in some watersheds. Comparisons of the N:P stoichiometry of net inputs versus storm drain exports implicated denitrification or leaching to groundwater as likely fates for retained N. Thus, these urban watersheds exported high quantities of N and P, but via contrasting pathways: P was exported primarily via stormwater runoff, contributing to surface water degradation, whereas N losses additionally contribute to ground water pollution. Consequently, N and P management require different strategies, with N management focusing on reducing watershed inputs, and P management additionally focusing on reducing P movement from vegetated landscapes to streets and storm drains