A distinct urban biogeochemistry?
Grimm, N*,1, Kaye, J1, Hall, S2, Allen, J1, Lewis, D1, 1 Arizona State University, Tempe, AZ, USA2 The Colorado College, Colorado Springs, CO, USA
ABSTRACT- Urban ecosystems present challenges to biogeochemical theory. Do patterns and processes of urban biogeochemistry differ fundamentally from those of non-urban ecosystems? Ecosystem theory holds that, on annual time scales in terrestrial ecosystems, the importance of internal N recycling far outweighs external inputs. Theory also holds that C cycles are relatively open, and C fixation by plants fuels most heterotrophic microbial growth. Little is known about ecosystem effects of vastly increased rates of N deposition from urban atmospheres (up to 30 kg ha-1 y-1 downwind of Phoenix). Moreover, atmospheric organic C particles from cooking and combustion may be a significant flux into urban ecosystems, and may be accessible to heterotrophic microorganisms in soil. Intensive, localized management also contributes enhanced nutrient inputs. We hypothesize that these and other patterns produce nutrient cycles in urban ecosystems that differ from those of non-urban ecosystems. For example, soil microbes may be decoupled from plants owing to high organic C and inorganic N deposition to soils. Under high inorganic N deposition, N supply may increase to the point that P limits plant growth. Finally, abundant metals from industrial activity may alleviate micronutrient limitations present in non-urban ecosystems, but fundamentally new cycles may result from increased loading of heavy metals, toxic organic compounds, or nutrients. In preliminary investigations, we used published atmospheric modeling results to predict locations of low and high deposition within and outside the urban area of Phoenix, Arizona. At these desert and desert-remnant sites, we measured short-term dry deposition, soil nutrient storage and N transformations, and plant tissue chemistry. Plant stoichiometry and soil processes differed among sites with different atmospheric N deposition rates. We further compared nutrient and metal exports from a range of urban watersheds, and found that watershed characteristics interacted with storm features in controlling chemistry of export. We maintain that whether cities exhibit a distinct urban biogeochemistry depends upon the extent to which high or unique material inputs alter interactions among element cycles.
Key words: land-water interaction, urbanization, atmosphere-land interaction, nutrient cycling
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