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.

A Correction to this paper has been published: https://doi.org/10.1038/s41597-021-00851-9.

Large weighable lysimeters allow a precise determination of the soil water balance and the quantification of both water exchange at the soil-atmosphere interface and the flux below the root zone toward the groundwater. If well embedded into an equally-vegetated environment, they reach a hitherto unprecedented accuracy in estimating precipitation (P) by rain, dew, fog, rime and snow, and evapotranspiration (ET). Lysimeters largely avoid errors made by traditional measurement systems, such as the wind error of Hellmann rain gauges, the island error of class-A pans, or errors from soil-water measurements that are subject to subsurface heterogeneity. If the amount of seepage water is added to the lysimeter mass, temporal changes of the lysimeter mass can be used to solve the water balance equation for atmospheric fluxes. Increasing mass indicates P, decreasing mass ET. The determination of the net water balance (sum of P and ET) is accurate and robust. A problem arises in the separate estimation of the underlying P and ET fluxes, because weight differences in specified time intervals are affected by stochastic fluctuations due to mechani- cal vibration, which may be caused by wind or other factors. The aim of this study is to evaluate algorithms that aim on eliminating the effects of these fluctuations and to estimate actual fluxes across the soil-atmosphere boundary and the soil water balance from lysimeter measurements. We use synthetic and real measured data from large lysimeters to test which strategies of data evaluation can be applied, and which degree of accuracy can be reached

We conduct the first 4D‐Var inversion of NH₃ accounting for NH₃ bi‐directional flux, using CrIS satellite NH₃ observations over Europe in 2016. We find posterior NH₃ emissions peak more in springtime than prior emissions at continental to national scales, and annually they are generally smaller than the prior emissions over central Europe, but larger over most of the rest of Europe. Annual posterior anthropogenic NH₃ emissions for 25 European Union members (EU25) are 25% higher than the prior emissions and very close (<2% difference) to other inventories. Our posterior annual anthropogenic emissions for EU25, the UK, the Netherlands, and Switzerland are generally 10%–20% smaller than when treating NH₃ fluxes as uni‐directional emissions, while the monthly regional difference can be up to 34% (Switzerland in July). Compared to monthly mean in‐situ observations, our posterior NH₃ emissions from both schemes generally improve the magnitude and seasonality of simulated surface NH₃ and bulk NH_x wet deposition throughout most of Europe, whereas evaluation against hourly measurements at a background site shows the bi‐directional scheme better captures observed diurnal variability of surface NH₃. This contrast highlights the need for accurately simulating diurnal variability of NH₃ in assimilation of sun‐synchronous observations and also the potential value of future geostationary satellite observations. Overall, our top‐down ammonia emissions can help to examine the effectiveness of air pollution control policies to facilitate future air pollution management, as well as helping us understand the uncertainty in top‐down NH₃ emissions estimates associated with treatment of NH₃ surface exchange.

Drought and heat events, such as the 2018 European drought, interact with the exchange of energy between the land surface and the atmosphere, potentially affecting albedo, sensible and latent heat fluxes, as well as CO ₂ exchange. Each of these quantities may aggravate or mitigate the drought, heat, their side effects on productivity, water scarcity and global warming. We used measurements of 56 eddy covariance sites across Europe to examine the response of fluxes to extreme drought prevailing most of the year 2018 and how the response differed across various ecosystem types (forests, grasslands, croplands and peatlands). Each component of the surface radiation and energy balance observed in 2018 was compared to available data per site during a reference period 2004–2017. Based on anomalies in precipitation and reference evapotranspiration, we classified 46 sites as drought affected. These received on average 9% more solar radiation and released 32% more sensible heat to the atmosphere compared to the mean of the reference period. In general, drought decreased net CO ₂ uptake by 17.8%, but did not significantly change net evapotranspiration. The response of these fluxes differed characteristically between ecosystems; in particular, the general increase in the evaporative index was strongest in peatlands and weakest in croplands. This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.