River sediments falling dry at low water levels are sources of CO₂ to the atmosphere. While the general relevance of CO₂ emissions from dry sediments has been acknowledged and some regulatory mechanisms have been identified, knowledge on mechanisms and temporal dynamics is still sparse. Using a combination of high-frequency measurements and two field campaigns we thus aimed to identify processes responsible for CO₂ emissions and to assess temporal dynamics of CO₂ emissions from dry sediments at a large German river.

CO₂ emissions were largely driven by microbial respiration in the sediment. Observed CO₂ fluxes could be explained by patterns and responses of sediment respiration rates measured in laboratory incubations. We exclude groundwater as a significant source of CO₂ because the CO₂ concentration in the groundwater was too low to explain CO₂ fluxes. Furthermore, CO₂ fluxes were not related to radon fluxes, which we used to trace groundwater-derived degassing of CO₂.

CO₂ emissions were strongly regulated by temperature resulting in large diurnal fluctuations of CO₂ emissions with emissions peaking during the day. The diurnal temperature–CO₂ flux relation exhibited a hysteresis which highlights the effect of transport processes in the sediment and makes it difficult to identify temperature dependence from simple linear regressions. The temperature response of CO₂ flux and sediment respiration rates in laboratory incubations was identical. Also deeper sediment layers apparently contributed to CO₂ emissions because the CO₂ flux was correlated with the thickness of the unsaturated zone, resulting in CO₂ fluxes increasing with distance to the local groundwater level and with distance to the river. Rain events lowered CO₂ emissions from dry river sediments probably by blocking CO₂ transport from deeper sediment layers to the atmosphere. Terrestrial vegetation growing on exposed sediments greatly increased respiratory sediment CO₂ emissions. We conclude that the regulation of CO₂ emissions from dry river sediments is complex. Diurnal measurements are mandatory and even CO₂ uptake in the dark by phototrophic micro-organisms has to be considered when assessing the impact of dry sediments on CO₂ emissions from rivers.