Leaching the good stuff: nitrogen and phosphorus in real and experimental urban agricultural settings

Urban agriculture provides multiple services, but its potentially adverse impact on water quality remains poorly understood. Here we present empirical data from three coordinated studies examining nitrogen and phosphorus leaching in Minneapolis-St. Paul, Minnesota, USA, and Linköping Sweden, ranging from controlled experiments to observational studies of garden practices. Using zero-tension lysimeters at 30 cm in the soil over multiple growing seasons, we found that although plots receiving nutrient inputs tended to leach more nutrients than those that did not, annual nutrient input rates were not systematically strong predictors of growing season leaching. Legacy effects from previous soil management and cumulative inputs help explain patterns observed for phosphorus leaching, with differences among treatments becoming visible over time. While urban agriculture can support nutrient circularity through organic waste recycling, careful management is needed to balance this benefit against leaching risks. Long-term monitoring is essential for understanding and managing nutrient losses from such systems.