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1. Magnitudes and Drivers of Greenhouse Gas Fluxes in Floodplain Ponds During Drawdown and Inundation by the Three Gorges Reservoir

   https://doi.org/10.1029/2018JG004701  

   Miller, B. L. ; Chen, H. ; He, Y. ; Yuan, X. ; Holtgrieve, G. W. ( August 2019 , Journal of Geophysical Research: Biogeosciences)

   Hydropower reservoirs are well‐known emitters of greenhouse gases to the atmosphere. This is due in part to seasonal water level fluctuations that transfer terrestrial C and N from floodplains to reservoirs. Partial pressures and fluxes of the greenhouse gases CH₄, CO₂, and N₂O are also a function of in situ biological C and N cycling and overall ecosystem metabolism, which varies on a diel basis within inland waters. Thus, greenhouse gas emissions in hydropower reservoirs likely vary over seasonal and diel time scales with local hydrology and ecosystem metabolism. China's Three Gorges Reservoir is among the largest and newest in the world, with a floodplain that encompasses approximately one third of the reservoir area. We measured diel partial pressures and fluxes of greenhouse gases in ponds on the Three Gorges Floodplain. We repeated these measurements on the submerged floodplain following inundation by the Three Gorges Reservoir. During reservoir drawdown, CH₄ebullition comprised 60–68% of emissions from floodplain ponds to the atmosphere. Using linear mixed effects modeling, we show that partial pressures of CH₄and CO₂and diffusive CO₂fluxes in floodplain ponds varied on a diel basis with in situ respiration. Floodplain inundation by the Three Gorges Reservoir significantly moderated areal CH₄diffusion and ebullition. DielpCO₂,pCH₄,pN₂O, and diffusive fluxes of CO₂on the submerged floodplain were also driven by in situ respiration. The drawdown/inundation cycle of the Three Gorges Reservoir therefore changes the magnitudes of aquatic greenhouse gas fluxes on its floodplain.

    

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2. Uncertainties in Turbulent Statistics and Fluxes of CO 2 Associated With Density Effect Corrections

   https://doi.org/10.1029/2020GL088859  

   Gao, Zhongming ; Liu, Heping ; Arntzen, Evan ; Mcfarland, Douglas P. ; Chen, Xingyuan ; Huang, Maoyi ( August 2020 , Geophysical Research Letters)

   Density effect corrections (DECs) are applied to adjust raw CO₂fluxes measured by eddy covariance (EC) systems with open‐path gas analyzers. DEC is also required for adjusting the measured CO₂concentration fluctuations to obtain the adjusted CO₂concentrations for analyzing turbulent statistics or quantifying fluxes. However, our data show that the power spectra of the DEC‐adjusted CO₂concentrations are distorted in the high‐frequency range, as compared with the corresponding spectra of temperature and water vapor density. This contradicts the similarity behavior of scalars, as suggested by Monin‐Obukhov similarity theory. It is demonstrated that such a distortion is caused by the DEC‐induced spikes in the DEC‐adjusted CO₂, altering turbulent statistics of CO₂and scalar similarity between CO₂and other scalars. Our results suggest that CO₂fluxes are overestimated by applying DEC especially under high Bowen ratio conditions, potentially leading to substantial uncertainties in long‐term ecosystem carbon exchange in dry regions.

    

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3. Automated measurements of greenhouse gases fluxes from tree stems and soils: magnitudes, patterns and drivers

   https://doi.org/10.1038/s41598-019-39663-8  

   Barba, Josep ; Poyatos, Rafael ; Vargas, Rodrigo ( March 2019 , Scientific Reports)

   Tree stems exchange CO₂, CH₄and N₂O with the atmosphere but the magnitudes, patterns and drivers of these greenhouse gas (GHG) fluxes remain poorly understood. Our understanding mainly comes from static-manual measurements, which provide limited information on the temporal variability and magnitude of these fluxes. We measured hourly CO₂, CH₄and N₂O fluxes at two stem heights and adjacent soils within an upland temperate forest. We analyzed diurnal and seasonal variability of fluxes and biophysical drivers (i.e., temperature, soil moisture, sap flux). Tree stems were a net source of CO₂(3.80 ± 0.18 µmol m⁻²s⁻¹; mean ± 95% CI) and CH₄(0.37 ± 0.18 nmol m⁻²s⁻¹), but a sink for N₂O (−0.016 ± 0.008 nmol m⁻²s⁻¹). Time series analysis showed diurnal temporal correlations between these gases with temperature or sap flux for certain days. CO₂and CH₄showed a clear seasonal pattern explained by temperature, soil water content and sap flux. Relationships between stem, soil fluxes and their drivers suggest that CH₄for stem emissions could be partially produced belowground. High-frequency measurements demonstrate that: a) tree stems exchange GHGs with the atmosphere at multiple time scales; and b) are needed to better estimate fluxes magnitudes and understand underlying mechanisms of GHG stem emissions.

    

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4. Variability and drivers of CO 2 , CH 4 , and N 2 O concentrations in streams across the United States

   https://doi.org/10.1002/lno.12281  

   DelVecchia, Amanda G. ; Rhea, Spencer ; Aho, Kelly S. ; Stanley, Emily H. ; Hotchkiss, Erin R. ; Carter, Alice ; Bernhardt, Emily S. ( December 2022 , Limnology and Oceanography)

   Streams and rivers are major sources of greenhouse gases (GHGs) to the atmosphere, as carbon and nitrogen are converted and outgassed during transport. Although our understanding of drivers of individual GHG fluxes has improved with numerous site‐specific studies and global‐scale compilations, our ability to parse out interrelated physical and biogeochemical drivers of gas concentrations is limited by a lack of consistently collected, temporally continuous samples of GHGs and their associated drivers. We present a first analysis of such a dataset collected by the National Ecological Observatory Network across 27 streams and rivers across ecoclimatic domains of the United States. Average concentrations of CO₂ranged from 36.9 ± 0.88 to 404 ± 33 μmol L⁻¹, CH₄from 0.003 ± 0.0003 to 4.99 ± 0.72 μmol L⁻¹, and N₂O from 0.015 to 0.04 μmol L⁻¹and spanned ranges of previous global compilations. Both CO₂and CH₄were strongly affected by physical drivers including mean air temperature and stream slope, as well as by dissolved oxygen and total nitrogen concentrations. N₂O was exclusively correlated with total nitrogen concentrations. Results suggested that potential for gas exchange dominated patterns in gas concentrations at the site level, but contributions of in‐stream aerobic and anaerobic metabolism, and groundwater also likely varied across sites. The highest gas concentrations as well as highest variability occurred in low‐gradient, warmer, and nonperennial systems. These results are a first step in providing unprecedented, continuous estimates of GHG flux constrained by temporally variable physical and biogeochemical drivers of GHG production.

    

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5. Dried, closed-path eddy covariance method for measuring carbon dioxide flux over sea ice

   https://doi.org/10.5194/amt-11-6075-2018  

   Butterworth, Brian J. ; Else, Brent G. ( January 2018 , Atmospheric Measurement Techniques)

   Abstract. The Arctic marine environment plays an important role inthe global carbon cycle. However, there remain large uncertainties in howsea ice affects air–sea fluxes of carbon dioxide (CO₂), partially dueto disagreement between the two main methods (enclosure and eddy covariance)for measuring CO₂ flux ($F_{\mathrm{CO}{}_{\mathrm{2}}}$). The enclosure method has appearedto produce more credible $F_{\mathrm{CO}{}_{\mathrm{2}}}$ than eddy covariance (EC), but is notsuited for collecting long-term, ecosystem-scale flux datasets in suchremote regions. Here we describe the design and performance of an EC systemto measure $F_{\mathrm{CO}{}_{\mathrm{2}}}$ over landfast sea ice that addresses the shortcomingsof previous EC systems. The system was installed on a 10m tower onQikirtaarjuk Island – a small rock outcrop in Dease Strait located roughly35km west of Cambridge Bay, Nunavut, in the Canadian Arctic Archipelago. Thesystem incorporates recent developments in the field of air–sea gasexchange by measuring atmospheric CO₂ using a closed-path infrared gasanalyzer (IRGA) with a dried sample airstream, thus avoiding the known watervapor issues associated with using open-path IRGAs in low-flux environments.A description of the methods and the results from 4 months of continuousflux measurements from May through August 2017 are presented, highlightingthe winter to summer transition from ice cover to open water. We show thatthe dried, closed-path EC system greatly reduces the magnitude of measured$F_{\mathrm{CO}{}_{\mathrm{2}}}$ compared to simultaneous open-path EC measurements, and for thefirst time reconciles EC and enclosure flux measurements over sea ice. Thisnovel EC installation is capable of operating year-round on solar and windpower, and therefore promises to deliver new insights into the magnitude ofCO₂ fluxes and their driving processes through the annual sea icecycle.

    

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