AbstractFloodplain ecosystems play a significant role in the global carbon (C) cycle, particularly due to their C sink potential in hardwood floodplain forests. However, in these forests, interactions between a heterogeneous micro‐relief and anthropogenic landscape changes make estimating C loss through soil CO₂ efflux difficult. To determine the drivers of soil CO₂ efflux, we selected six hardwood floodplain forests at the lower middle Elbe River, which were distributed among different relief positions (low‐lying or high‐elevated) in the active and former flooding zone. We measured soil CO₂ effluxes over a full year using the closed‐chamber method. Based on the response of soil CO₂ efflux to soil moisture and temperature, annual efflux rates were determined, which were then related to soil properties, such as pH, texture, soil organic carbon (SOC) and nitrogen (N) content. Soil CO₂ efflux ranged between 1006 (±99) and 2214 (±118) gC m⁻² year⁻¹. Maximum efflux occurred in a former floodplain forest that was disconnected from Elbe River water table fluctuations. SOC‐specific soil CO₂ efflux (gC gSOC⁻¹ year⁻¹) was smallest in low‐lying forests of the active flooding zone and reflected by the appearance of redoximorphic mottling close to the soil surface. Fine texture (<6.3 μm), SOC and N were related positively and electric conductivity, C/N and pH negatively to total soil CO₂ efflux. Soil pH and fine texture were the strongest univariate predictors for total soil CO₂ efflux (both R² = 0.59). Fine texture, pH and C/N ratio explained 66% of the variance in total soil CO₂ efflux according to multiple linear regression. We conclude that, in hardwood floodplain forests, soil CO₂ efflux is mainly controlled by fine texture and soil pH. Fine texture can be related to soil moisture and nutrient availability and may have a positive effect on the activity of microorganisms.Highlights Soil CO₂ efflux ranged between 1006 (±99) and 2214 (±118) gC m² year⁻¹, whereby the maximum was measured in a disconnected floodplain forest Maximum soil CO₂ efflux was measured in a disconnected floodplain forest SOC‐specific soil CO₂ efflux was smallest in forests where redoximorphic mottling occurred close to the surface Fine texture (<6.3 μm) and pH were the strongest predictors for annual soil CO₂ efflux