ABSTRACT
Biogeochemical fluxes-the movement of materials among components of ecosystems and between different ecosystems-are fundamental ecological processes. Biogeochemical fluxes usually are defined for elements, rather than compounds. Biogeochemical cycles involve changes in form due to chemical, physical and organism-mediated processes, physical or organism-mediated translocation, and utilization in biotic production. Mass is conserved in biogeochemical cycles, and, in theory, any atom released by biota to the environment will eventually enter the biotic component of biogeochemical cycles again. Investigation of the reciprocal controls of elements and biotic processes at a variety of scales has been fertile ground for ecological investigation for decades. Human activities have accelerated biogeochemical cycles, especially over the past 150 years, and cities in particular are implicated as foci of altered nutrient and non-essential element cycles. People (who are concentrated in cities) alter urban biogeochemistry in two main ways: (1) directly by creating new fluxes of materials or enhancing existing ones; and (2) indirectly by altering environmental conditions and thereby rates of biogeochemical processes. In this chapter we discuss urban biogeochemical fluxes at various scales and present examples of approaches to quantifying biogeochemical fluxes that are unique to urban systems.
