A Nitrogen Balance For The Central Arizona-Phoenix Ecosystem (poster)

BAKER, LARRY, Ying Xu, Lisa Lauver, Diane Hope, and Jennifer Edmonds

A preliminary N mass balance was constructed for the Central Arizona-Phoenix ecosystem, one of the first urban research sites in the NSF Long-Term Ecological Research (LTER) Program. Humans deliberately import large quantities of N in the form of human and animal food and fertilizer (27 x 106 kg/year) and mediate additional fixation by alfalfa (5.2 x 106 kg/year) and combustion (27.2 x 106 kg/year). Less than 10% of N enters the ecosystem through surface water or background atmospheric deposition. A substantial dairy industry produces milk and meat (3.7 x 106 kg N/year) that is mostly consumed within the ecosystem, providing about one-third of the protein in human food consumed within the ecosystem. Dairy cows excrete more N (11.5 x 106 kg N/year) than humans (10.6 x 106 kg N/year). Most N inputs accumulate in aquifers and the vadose zone, are re-exported as food, or are denitrified. More N is accumulated in groundwater (3.6 x 106 kg/year) and landfills (7.8 x 106 kg/year) than is exported via surface water (2.6 x 106 kg/year). Water management practices intended to conserve water (irrigation practices, wastewater management, urban flood control) also tend to reduce N export . A key uncertainty is the fate of combustion-derived NOx. Deposition of this N within the ecosystem or in the surrounding ecosystems may have important ecological effects that remain to be quantified