Nitrogen balance for the central Arizona-Phoenix (CAP) ecosystem

A detailed fixed nitrogen (N) mass balance was constructed for the Central Arizona-Phoenix (CAP) ecosystem. Input of fixed N input to the ecosystem wa s 98 Gg y(-1). Of this, humans deliberately imported or mediated the fixati on of 51 Gg N y(-1); combustion processes added another 36 Gg y(-1). Fixati on by desert plants, wet deposition, and surface water input accounted for 11 % of total N input. Total fixed N output was 78 Gg N y(-1), a large comp onent of which was gaseous N products of combustion and denitrification. Co mputed accumulation of N was 21 Gg y(-1) (total input minus total output) o r alternatively, 17 Gg y(-1) (summing individual accumulation fluxes). Key uncertainties include dry deposition of atmospheric N and changes in soil s torage. inputs to the urban and agricultural components of the ecosystem we re an order of magnitude higher than inputs to the desert. Human hydrologic modifications in this ecosystem promote the accumulation and volatilizatio n of N while keeping riverine export low (3% of input). Interplay among the form and amount of N inputs, edaphic and climatic characteristics of the s ystem, hydrologic modifications, and deliberate efforts to reduce N polluti on controls the fate of N in human-dominated ecosystems.