Understanding, managing, and minimizing urban impacts on surface water nitrogen loading.

The concentration of materials and energy within cities is an inevitable consequence of dense populations and their per capita requirements for food, fiber, and fuel. As the world population becomes increasingly urban over the coming decades, urban areas will dramatically affect the distribution of nutrients across the face of the planet. In many cities, technological developments and urban planning have been effective at reducing the amount of waste nitrogen that is ultimately exported to downstream surface waters, largely through investments in sanitary sewer infrastructure and wastewater treatment. There are, however, still large cities throughout the developed world that have failed to take advantage of these obvious innovations to reduce their impact on downstream ecosystems. In addition, very few cities have adequately addressed the problems of diffuse nitrogen pollution, instead city infrastructure is often designed to route this N directly into downstream ecosystems. In the developing world, many of these problems are more acute, as rapidly growing urban populations exceed the capacity of limited municipal infrastructure. Reducing urban N pollution of groundwaters and surface waters both locally and globally can only be achieved through cultural and political adaptation in addition to technological innovations. In this review, we will focus on the implications of an increasingly urban world population on local, regional, and global nitrogen cycles and propose a variety of approaches for minimizing and mitigating the impacts of urban N concentration.