Land use context and natural soil controls on plant community composition and soil nitrogen and carbon dynamics in urban and rural forests

Forests embedded in an urban matrix are a useful venue for investigating the effects of multiple factors such as climate change, altered disturbance regimes and species invasions on forest ecosystems. Urban forests also represent a significant land area, with potentially important effects on landscape and regional scale nitrogen (N) and carbon (C) storage and flux. We measured forest community composition, litterfall, leaf area index, soil chemical properties, in situ net N mineralization and nitrification, soil and soil solution inorganic N pools, and soil:atmosphere fluxes of nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) in eight forest stands that differed in their exposure to urban atmospheric conditions and natural soil conditions (high versus low fertility). Our objectives were (1) to compare the influence of urban land use context and natural soil controls on forest composition and C and N cycling processes and (2) to evaluate the importance of “natural” N cycle processes relative to anthropogenic N fluxes in the urban landscape. Forest productivity and N cycling varied more with soil type than with proximity to urban land use, while forest composition and soil:atmosphere fluxes of CO2 and CH4 were more strongly influenced by exposure to an urban land use matrix and atmosphere. The magnitude of natural processes was important in the context of urban and suburban landscapes, i.e. production of, and annual variation in, inorganic N in forest patches was large relative to watershed-scale atmospheric deposition, fertilizer use and food/sewage fluxes that have been measured in other studies in these study landscapes.