Urban landscapes contain a mix of land-use types with different patterns of carbon (C) and nitrogen (N) cycling and export. These patterns affect interactions between ecosystems and the atmosphere.  We have measured soil:atmosphere fluxes of carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) in four urban grassland and eight forested long-term study plots in the Baltimore, MD USA metropolitan area monthly since 1998. We have evaluated ancillary controls on these fluxes by measuring soil temperature, moisture, organic matter content , microbial biomass and potential net N mineralization and nitrification. Soil:atmosphere fluxes of CO₂, or total soil respiration, ranged from 0 to 0.244 g C m^-2 h^-1, with marked seasonal patterns (highest rates in summer) and few differences between forest and grasslands (grass plots had higher fluxes than forest plots in the wet years of 2003 and 2005). Urban forest plots had higher CO₂ fluxes than rural forest plots.  N₂O fluxes ranged from 0.05 to more than 0.3 g N m^-2 y^-1 with few differences between grass and forest plots, and markedly higher fluxes in wet years. Differences in N₂O flux between forests and grasslands were not as high as expected given the higher frequency of disturbance and fertilization in the grasslands. The fact that CO₂ flux, organic matter and microbial biomass were as high or higher in urban grasslands than in forests suggests that active carbon cycling creates sinks for N in vegetation and soil in these ecosystems leading to low N₂O fluxes in the grasslands, despite high N inputs.  Net CH₄ flux ranged from -2.4 to 1.0 mg m^-2 d^-1 and was higher in urban forests than rural forests and was negligible in urban grasslands.  CH₄ flux appears to be much more sensitive to N enrichment than either CO₂ or N₂O flux.