The science and politics of increasing nitrogen pollution from human activity : case study of the Aberjona watershed

Abstract

The biogeochemical cycling of nitrogen has critical implications for all life on earth. The Haber-Bosch process (1909) paved the way for the industrial fixation of NH3 from unreactive atmospheric dinitrogen, a phenomenon previously restricted to the enzyme nitrogenase and to a lesser extent lightning. Perhaps the most profoundly catalytic invention of the 20th century, Haber-Bosch has greatly increased the global anthropogenic throughput of nitrogen that now rivals the 160 million metric tons fixed annually via nitrogenase. Notably 40% of the world's 6 billion people alive today owe their body's kilogram of nitrogen to synthetic fertilizer created via the Haber-Bosch process (Smil 2001). The intensification of agriculture and the growth of urban centers during the 20th century has also resulted in substantive increases in nitrogen loads to the environment. Nitrogen transported by surface and groundwater is ultimately concentrated in rivers and coastal areas, with consequences such as eutrophication and the "dead zones" of hypoxia found throughout the world. Microbial metabolism of nitrogen via the nitrification and denitrification pathways also produces N20, a potent greenhouse gas currently responsible for 5% of total atmospheric radiative forcing.

(cont.) This thesis examines the environmental implications of increased nitrogen throughput and the regulatory frameworks that are evolving to cope with nitrogen pollution. A case study of the Aberjona river, draining a heavily polluted urban watershed north of Boston, is examined in terms of the correlation between urban intensity and nitrogen yields (r2=0.79), the internal processing of nitrogen via major biological pathways, and the role of legacy pollutants as a source of nitrogen. The high fraction of organic nitrogen (34% of TOT N export) suggests that the effects of urban intensification are not limited to the export of inorganic species, and should be included in urban nitrogen budgets for effective watershed management.