Effects of Urban Land-Use Change on Biogeochemical Cycles

Urban land-use change, the conversion of agricultural and natural ecosystems to human settlements, has become an important component of global change. Virtually all of the projected increase in the world’s population is expected to occur in cities so that by the year 2007 more than half of the global population is expected to live in urban areas (United Nations 2004). Yet, urban settlements and surrounding areas are complex ecological systems that have only recently been studied from a rigorous ecological perspective (Pickett et al. 2001). Clearly, urban areas represent ecosystems with modified biogeochemical cycles such that fluxes and pools of matter, energy, and organisms differ greatly from the previous ecosystem (Collins et al. 2000). Our knowledge of the magnitude and extent of these biogeochemical changes at local, regional, and global scales is a major area of uncertainty in our understanding of global change. In the ecological sciences, the ecosystem approach has been used successfully in studying the effects of disturbance and exogenous inputs of nutrients and pollutants on biogeochemical cycles (Likens 1992). To study biogeochemical cycles in urban areas, Grimm et al. (2003) recommended the ecosystem approach where: (1) urban ecosystems are viewed as spatially homogenous, i.e., “well-mixed reactors,” or (2) urban ecosystems are considered a heterogeneous assemblage of parts or “patches.” Indeed, cities have been viewed as well-mixed reactors in answering questions about how they affect biogeochemical cycles at regional and global scales (Fig. 5.1). In so doing, mass balances have been constructed for entire cities to assess their “ecological footprint,” that is, the amount of land resources acquired from other ecosystems needed to sustain the urban population (Rees 2003). Moreover, the mass balance approach allows investigations into the movement of nutrients and pollutants across ecosystem boundaries and the cycling of these materials within the system itself. For example, Baker et al. (2001) constructed a nitrogen balance for the Phoenix metropolitan area and found that inputs (human and