Biogeochemical Processes

Abstract

Urbanization affects Earth's ecosystems by changing fundamental processes that control the cycling of elements. Biological, hydrological, atmospheric, and geological processes play essential roles in terrestrial biogeochemical cycles by regulating the synchrony between release and uptake of nutrients by microorganisms and plants (NRC 1986, Melillo et al. 2003, Dahlgren 2006). Across multiple scales from molecular to the entire ecosystem level, biological processes regulate nutrient cycles by providing fuel and materials to Earth's ecosystems. Humans have altered biogeochemical processes in fundamental ways, by burning fossil fuels, changing land uses, extracting metals, and producing and applying synthetic chemicals (Figure 6.1) (Vitousek et al. 1997). Scientists have long recognized the global implications of the human impact on biogeochemical processes, but we have only a limited understanding of how urban ecosystems contribute to these changes and are affected by them. Today we have more available data and more sophisticated measurement techniques and analytic tools that allow us to explore relationships between human and biogeochemical processes in ways not possible before. Urban ecology scholars have challenged biogeochemical models, suggesting that they are too simplified to apply in urban ecosystems (McDonnell et al. 1997). Scholars in Phoenix and Baltimore suggest that a distinct urban biogeochemistry is emerging due to human-controlled fluxes of energy and elements (Groffman et al. 2004, Grimm et al. 2005, Kaye et al. 2006). Despite the great variability across the bio-physical settings and socioeconomic activities of cities, the coupling of human and natural processes may be creating a unique biogeochemistry. Humans in urbanizing regions affect biogeochemical processes directly by adding nutrients, and indirectly by modifying the mechanisms that control the spatial and temporal variability of nutrient sources and sinks. Kaye et al. (2006) propose that in cities, complex interactions between society and the environment are mediated by distinctive factors such as the degree of impervious surface, the built infrastructure, demographic trends, and the individual choices of households. If we are to fully understand the role that cities play in the cycling of chemicals, I propose that urban ecologists must also examine how the distinctive spatial heterogeneity caused by human choices, including patterns of urban land use and infrastructure, defines urban biogeochemistry.