CO<sub>2</sub> and CH<sub>4</sub> fluxes are decoupled from organic carbon loss in drying reservoir sediments

Abstract

<p><strong>Abstract.</strong> Reservoirs are a prominent feature of the current global hydrological landscape, and their sediments are the site of extensive organic carbon burial. Meanwhile, reservoirs frequently go dry due to drought and/or water management decisions. Nonetheless, the fate of organic carbon buried in reservoir sediments upon drying is largely unknown. Here, we conducted a 45-day-long laboratory incubation of sediment cores collected from a western Mediterranean reservoir to investigate carbon dynamics in drying sediment. Drying sediment cores emitted more CO<sub>2</sub> over the course of the incubation than sediment cores incubated with overlaying water (206.7 ± 47.9 vs. 69.2 ± 18.1 mmol CO2 m<sup>−2</sup> day<sup>−1</sup>, mean ± SE). Organic carbon content at the end of the incubation was lower in drying cores, which suggests that this higher CO<sub>2</sub> efflux was due to organic carbon mineralization. However, the apparent rate of organic C reduction in the drying sediments (568.6 ± 247.2 mmol CO<sub>2</sub> m<sup>−2</sup> day<sup>−1</sup>, mean ± SE) was higher than C emission. Meanwhile, sediment cores collected from a reservoir area that had already been exposed for 2+ years displayed net CO<sub>2</sub> influx from the atmosphere to the sediment (−136.0 ± 27.5 mmol CO<sub>2</sub> m<sup>−2</sup> day<sup>−1</sup>, mean ± SE) during the incubation period. Sediment mineralogy suggests that this CO<sub>2</sub> influx was caused by a relative increase in calcium carbonate chemical weathering. Thus, we found that while organic carbon decomposition in newly dry reservoir sediment causes measurable organic carbon loss and carbon gas emissions to the atmosphere, other processes can offset these emissions on short time frames and compromise the use of carbon emissions as a proxy for organic carbon mineralization in drying sediments.</p>

Spatial upscaling of CO2 emissions from exposed river sediments of the Elbe River during an extreme drought

“…Note that for areal share calculation, the classes Tree/Bush and Water shown in Figure 2 are not considered since we focus on exposed sediments only. chemical weathering (Light, Catalán, Giralt, & Marcé, 2019). The general CO 2 flux picture displays the highest fluxes for land cover types such as dark sediment, reed and grass, which cover only 9% of the total area, whereas land cover types with lower fluxes cover 91% of the study area ( Figure 2).…”