Understanding the controls on floodplain carbon (C) cycling is important for assessing greenhouse gas emissions and the potential for C sequestration in river-floodplain ecosystems. We hypothesized that greater hydrologic connectivity would increase C inputs to floodplains that would not only stimulate soil C gas emissions but also sequester more C in soils. In an urban Piedmont river (151 km² watershed) with a floodplain that is dry most of the year, we quantified soil CO₂, CH₄, and N₂O net emissions along gradients of floodplain hydrologic connectivity, identified controls on soil aerobic and anaerobic respiration, and developed a floodplain soil C budget. Sites were chosen along a longitudinal river gradient and across lateral floodplain geomorphic units (levee, backswamp, and toe slope). CO₂ emissions decreased downstream in backswamps and toe slopes and were high on the levees. CH₄ and N₂O fluxes were near zero; however, CH₄emissions were highest in the backswamp. Annual CO₂ emissions correlated negatively with soil water-filled pore space and positively with variables related to drier, coarser soil. Conversely, annual CH₄ emissions had the opposite pattern of CO₂. Spatial variation in aerobic and anaerobic respiration was thus controlled by oxygen availability but was not related to C inputs from sedimentation or vegetation. The annual mean soil CO₂ emission rate was 1091 g C m⁻² yr⁻¹, the net sedimentation rate was 111 g C m⁻² yr⁻¹, and the vegetation production rate was 240 g C m⁻² yr⁻¹, with a soil C balance (loss) of −338 g C m⁻² yr⁻¹. This floodplain is losing C likely due to long-term drying from watershed urbanization.