Freshwater reservoir drawdown areas are an important yet not well understood source of atmospheric CO2. Quantifying CO2 emissions from drawdown areas is challenged by considerable spatial and temporal variability. In this study we aimed to assess temporal variability to understand the drivers of drawdown area CO2 emissions and to improve reservoir CO2 emission budgets. We measured CO2 emissions from the drawdown area of a German reservoir for three months with hourly resolution using automatic flux chambers. To analyze drivers of CO2 emissions we monitored sediment temperature and moisture as well as meteorological variables. For comparison we continuously measured CO2 emissions from the water surface by Eddy Covariance. CO2 fluxes showed pronounced diurnal and seasonal variability driven by temperature. Sediment moisture had a negative effect on CO2 fluxes – low fluxes were observed close to the waterline and after rain events. Inhibition of CO2 transport by water blocking sediment pores was indicated by a hysteresis between sediment temperature and CO2 flux, which was amplified under moist conditions. We show that daily CO2 emissions are overestimated by more than 25% if fluxes are measured only during midday. A comparison with CO2 emissions from the water surface measured by eddy covariance revealed that drawdown area and water surface contributed equally to the reservoirs CO2 emissions, despite the much small surface area of the drawdown area. Our study clearly shows that CO2 fluxes between reservoir drawdown area and atmosphere have a large diurnal variability in the same range as seasonal variability and within system variability. High temporal resolution measurements of CO2 emissions from the drawdown area are essential for precise quantification of greenhouse gas emissions from reservoirs.

Abstract Purpose In recent years, intercropping in olive groves has become a sustainable practice for mitigating climate change. Therefore, the objective of this study was to determine whether crop diversification in rainfed olive groves modifies carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ) emissions and key chemical and enzymatic properties of the soil in the short term. Materials and methods A field experiment was conducted under semi-arid Mediterranean conditions using four treatments: a conventional olive monocrop (CP) and three crop diversification systems: saffron ( Crocus sativus ) (D-S), oats and vetch ( Vicia sativa and Avena sativa ) (D-O) and lavandin ( Lavandula × intermedia ) (D-L). CO 2 , N 2 O, and CH 4 emissions were measured periodically using closed static chamber techniques. Simultaneously, soil samples were analyzed to determine physicochemical parameters (pH, total nitrogen (TN), total organic carbon (TOC), P available, K, and Fe) and enzymatic activities (β-1,4-glucosidase (BG), leucine aminopeptidase (LA), N-acetylglucosaminidase (NAG), Arylsulfatase (AS) and phosphatase (AP) and dehydrogenase (DH)) in order to evaluate the interactions between soil properties and gas fluxes. Results and discussion The results showed that CO 2 emissions were significantly higher in D-S (385.2 mg CO 2 m -2 h -1 ) and D-O (335.2 mg CO 2 m -2 h -1 ) compared to CP, which was attributed to increased microbial and root activity promoted by the vegetation cover. In contrast, N 2 O fluxes (0.002 mg N 2 O m -2 h -1 in CP, 0.007 in D-S, 0.001 in D-O, and − 0.002 in D-L) and CH 4 (–0.2 mg CH 4 m -2 h -1 in CP, 0.1 in D-S, 0.2 in D-O, and 0.2 in D-L) did not show significant differences between all treatments ( p > 0.05 ), probably due to the prevailing semi-arid conditions. D-O had higher values of P available, Fe, and TN, while D-S and D-L stood out for higher values of K and TOC, both compared to CP. Enzymatic activities were particularly higher in D-S and D-O, specifically BG, LA, and AS. In addition, positive and significant correlations were observed between CO₂ accumulation and several soil parameters: in D-S, with POC, total Zn, exchangeable Na, total Cu, and BG, LA, and AS activities; and in D-L, with POC, exchangeable Na, total Mn, P available, TN, and LA and AS activities, highlighting that these indicators contribute to the regulation of greenhouse gas (GHG) emissions. Conclusion It was concluded that crop diversification in rainfed olive groves increases CO 2 emissions in the short term due to intensified biological activity, but at the same time improves soil fertility and biochemical functionality. However, long-term monitoring is required to determine whether these systems contribute to carbon stabilization and net sequestration, ensuring their potential as sustainable strategies for climate change mitigation in Mediterranean agroecosystems.