Temporal patterns and drivers of CO 2 emission from dry sediments in a groyne field of a large river

[摘要] River sediments falling dry at low water levels aresources of CO 2 to the atmosphere. While the general relevance ofCO 2 emissions from dry sediments has been acknowledged and someregulatory mechanisms have been identified, knowledge on mechanisms and temporaldynamics is still sparse. Using a combination of high-frequency measurementsand two field campaigns we thus aimed to identify processes responsible forCO 2 emissions and to assess temporal dynamics of CO 2 emissionsfrom dry sediments at a large German river. CO 2 emissions were largely driven by microbial respiration in thesediment. Observed CO 2 fluxes could be explained by patterns andresponses of sediment respiration rates measured in laboratory incubations.We exclude groundwater as a significant source of CO 2 because theCO 2 concentration in the groundwater was too low to explain CO 2 fluxes. Furthermore, CO 2 fluxes were not related to radon fluxes, whichwe used to trace groundwater-derived degassing of CO 2 . CO 2 emissions were strongly regulated by temperature resulting in largediurnal fluctuations of CO 2 emissions with emissions peaking during theday. The diurnal temperature–CO 2 flux relation exhibited ahysteresis which highlights the effect of transport processes in thesediment and makes it difficult to identify temperature dependence fromsimple linear regressions. The temperature response of CO 2 flux andsediment respiration rates in laboratory incubations was identical. Alsodeeper sediment layers apparently contributed to CO 2 emissions becausethe CO 2 flux was correlated with the thickness of the unsaturated zone,resulting in CO 2 fluxes increasing with distance to the localgroundwater level and with distance to the river. Rain events loweredCO 2 emissions from dry river sediments probably by blocking CO 2 transport from deeper sediment layers to the atmosphere. Terrestrialvegetation growing on exposed sediments greatly increased respiratorysediment CO 2 emissions. We conclude that the regulation of CO 2 emissions from dry river sediments is complex. Diurnal measurements aremandatory and even CO 2 uptake in the dark by phototrophicmicro-organisms has to be considered when assessing the impact of drysediments on CO 2 emissions from rivers.

[学科分类] 大气科学