Eddy covariance flux measurements of gaseous elemental mercury over a grassland

Direct measurements of the net ecosystem exchange (NEE) of gaseous elemental mercury (Hg-0) are important to improve our understanding of global Hg cycling and, ultimately, human and wildlife Hg exposure. The lack of long-term, ecosystem-scale measurements causes large uncertainties in Hg-0 flux estimates. It currently remains unclear whether terrestrial ecosystems are net sinks or sources of atmospheric Hg-0. Here, we show a detailed validation of direct Hg-0 flux measurements based on the eddy covariance technique (Eddy Mercury) using a Lumex RA-915 AM mercury monitor. The flux detection limit derived from a zero-flux experiment in the laboratory was 0.22 ng m(-2) h(-1) (maximum) with a 50% cutoff at 0.074 ng m(-2) h(-1). We present eddy covariance NEE measurements of Hg-0 over a low-Hg soil (41-75 ng Hg g(-1) in the topsoil, referring to a depth of 0-10 cm), conducted in summer 2018 at a managed grassland at the Swiss FluxNet site in Chamau, Switzerland (CH-Cha). The statistical estimate of the Hg-0 flux detection limit under outdoor conditions at the site was 5.9 ng m(-2) h(-1') (50% cutoff). We measured a net summertime emission over a period of 34 d with a median Hg-0 flux of 2.5 ng m(-2) h(-1) (with a -0.6 to 7.4 ng m(-2) h(-1) range between the 25th and 75th percentiles). We observed a distinct diel cycle with higher median daytime fluxes (8.4 ng m(-2) h(-1)) than night-time fluxes (1.0 ng m(-2) h(-1)). Drought stress during the measurement campaign in summer 2018 induced partial stomata closure of vegetation. Partial stomata closure led to a midday depression in CO2 uptake, which did not recover during the afternoon. The median CO2 flux was only 24% of the median CO2 flux measured during the same period in the previous year (2017). We suggest that partial stomata closure also dampened Hg-0 uptake by vegetation, resulting in a NEE of Hg-0 that was dominated by soil emission. Finally, we provide suggestions to further improve the precision and handling of the "Eddy Mercury" system in order to assure its suitability for long-term NEE measurements of Hg-0 over natural background surfaces with low soil Hg concentrations (< 100 ng g(-1)). With these improvements, Eddy Mercury has the potential to be integrated into global networks of micrometeorological tower sites (FluxNet) and to provide the long-term observations on terrestrial atmosphere Hg-0 exchange necessary to validate regional and global mercury models

Large inter-annual variation in carbon sink strength of a permanent grassland over 16 years: Impacts of management practices and climate

Permanent grasslands cover one third of the European agricultural area and are known to store large amounts of carbon (C) in their soils. However, long-term assessments of their C sink strength are still scarce. Thus, we investigated the C budget of an intensively managed, permanent grassland in Switzerland over 16 years, compared the results to changes in soil C stocks, and determined the most important drivers of the net ecosystem CO2 exchange (NEE). Combining NEE fluxes with C imports and C exports, we quantified the grassland C budget, i.e., net biome production (NBP). We observed a large inter-annual variation in NBP, with 9 of the 16 years indicating a C sink, and 7 years indicating a C source. On average, the grassland was a small C sink to C neutral, with a NBP of -70±106 g C m$^{−2}$ yr$^{−1}$ (mean±95% confidence interval). Mean NEE fluxes were -284±115 g C m$^{−2}$ yr$^{−1}$, C exports via harvest 335±73 g C m$^{−2}$ yr$^{−1}$, and organic C imports via slurry -121±43 g C m$^{−2}$ yr$^{−1}$. Soil C stocks from 0 to 0.7 m did not change significantly (decrease of 27.5 g C m$^{−2}$ yr$^{−1}$ over 13 years). Inter-annual variation in NBP was affected by management practices and environmental conditions. In the last five years, NBP was positive (C source), most likely due to decreasing C imports in combination with extreme weather conditions. Our study demonstrated the importance of covering multiple years with different management events when assessing the C sink strength of a site. Maintaining even a small grassland C sink in the future will be challenging and will require continuous organic C imports

Evaluation of the ECOSSE Model for Estimating Soil Respiration from Eight European Permanent Grassland Sites

Acknowledgments We are grateful to Lukas Hörtnagl, Keller Sabina, Shiva Ghiasi and people from other investigated sites for providing us with the data. Funding This work is funded by the Super-G project (funded under EU Horizon 2020 programme: project number 774124).Peer reviewedPublisher PD

Climate scenarios for Switzerland CH2018 - approach and implications

To make sound decisions in the face of climate change, government agencies, policymakers and private stakeholders require suitable climate information on local to regional scales. In Switzerland, the development of climate change scenarios is strongly linked to the climate adaptation strategy of the Confederation. The current climate scenarios for Switzerland CH2018 - released in form of six user-oriented products - were the result of an intensive collaboration between academia and administration under the umbrella of the National Centre for Climate Services (NCCS), accounting for user needs and stakeholder dialogues from the beginning. A rigorous scientific concept ensured consistency throughout the various analysis steps of the EURO-CORDEX projections and a common procedure on how to extract robust results and deal with associated uncertainties. The main results show that Switzerland?s climate will face dry summers, heavy precipitation, more hot days and snow-scarce winters. Approximately half of these changes could be alleviated by mid-century through strong global mitigation efforts. A comprehensive communication concept ensured that the results were rolled out and distilled in specific user-oriented communication measures to increase their uptake and to make them actionable. A narrative approach with four fictitious persons was used to communicate the key messages to the general public. Three years after the release, the climate scenarios have proven to be an indispensable information basis for users in climate adaptation and for downstream applications. Potential for extensions and updates has been identified since then and will shape the concept and planning of the next scenario generation in Switzerland

Permanent grasslands in Europe : Land use change and intensification decrease their multifunctionality

Acknowledgments We acknowledge the financial support of this work by European Union Horizon 2020 research and innovation programme, under grant agreement 774124, project SUPER-G (Developing Sustainable Permanent Grassland Systems and Policies).Peer reviewedPublisher PD

The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data

The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.Peer reviewe