Growing season soil respiration produces a significant amount of urban CO2.
Soil CO2 efflux is spatially and temporally variable across urban areas.
Areas with high pervious residential land cover are dominant areas of soil CO2 efflux.
Soil amendments by homeowners may be responsible for high urban soil CO2 efflux.
Assessing and attributing urban CO2 fluxes requires quantifying biological CO2 efflux.