Commercially available fast-response analysers for methane (CH4) and nitrous oxide (N2O) have recently become more sensitive, more robust and easier to operate. This has made their application for long-term flux measurements with the eddy-covariance method more feasible. Unlike for carbon dioxide (CO2) and water vapour (H2O), there have so far been no guidelines on how to optimise and standardise the measurements. This paper reviews the state-of-the-art of the various steps of the measurements and discusses aspects such as instrument selection, setup and maintenance, data processing as well as the additional measurements needed to aid interpretation and gap-filling. It presents the methodological protocol for eddy covariance measurements of CH4 and N2O fluxes as agreed for the ecosystem station network of the pan-European Research Infrastructure Integrated Carbon Observation System and provides a first international standard that is suggested to be adopted more widely. Fluxes can be episodic and the processes controlling the fluxes are complex, preventing simple mechanistic gap-filling strategies. Fluxes are often near or below the detection limit, requiring additional care during data processing. The protocol sets out the best practice for these conditions to avoid biasing the results and long-term budgets. It summarises the current approach to gap-filling.

Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, spatial upscaling and modelling of observed ecosystem carbon and GHG dynamics. The applied sampling design was developed and formulated in protocols by the scientific community, representing a trade-off between an ideal dataset and practical feasibility. The use of open-access, high-quality and multi-level data products by different user communities is crucial for the Integrated Carbon Observation System in order to achieve its scientific potential and societal value.

The Okavango Delta is a huge alluvial megafan in northwestern Botswana. Despite numerous geologic, geochemical, geophysical, and hydrologic investigations over the past half-century, the sedimentary units underlying the delta are largely unknown. To address this issue, helicopter transient electromagnetic data (HTEM) have been collected across the entire delta and coincident ground-based electrical resistance tomographic (ERT) and transient electromagnetic (TEM) data have been acquired at two locations, one along the delta's western margin and one in its north-central region. Inversions of the HTEM data have yielded three-layer resistivity models in which a relatively homogeneous conductive layer is sandwiched between two resistive layers. The three-layer HTEM model is reproduced in models obtained from independently and jointly inverting the ground-based data. The conductive layer's low resistivities and depths to its upper and lower boundaries are practically equal in the HTEM and ground-based models. Resistivities of the upper resistive layer are similar in the various models, with the ground-based estimates being somewhat higher than those of the HTEM model at one site and somewhat lower at the other site. For the basal resistive layer, it can only be concluded that its resistivity must be substantially higher than that of the overlying conductive layer. An interpretation of the HTEM and ground-based resistivity models in the delta's north-central region, appropriately constrained by the surface geology, high-resolution seismic refraction-reflection models, and borehole logs suggests the following structure: basement overlain at progressively shallower depths by freshwater-saturated sand and gravel that represent the remnants of a Paleo Okavango Megafan, saline-water-saturated sand, and lacustrine clay originally deposited in Paleo Lake Makgadikgadi, and freshwater-saturated megafan and fluvial sediments of the current Okavango Delta.

Biophysical and biochemical traits of foliage and twigs at the top of tree canopies provide essential information on trees' ecophysiology. Conventional methods used for canopy sampling are typically time consuming and costly, while the very canopy tops are still out of reach unless a canopy crane is used. Thus, we developed a novel twig sampling method using a device attached to a drone that allows to grasp and cut small twigs and bring them to the ground for immediate analysis. This "Flying Tree Top Sampler" (FTTS) complements existing methods used in tree canopy research. This paper describes the method to make it openly available to interested users, provides the necessary technical details, and reports on a proof-of-concept application by probing a single tree during senescence. In this demonstration example we show that the ability to investigate the top part of the tree leads to the clear finding that chlorophyll content of the top leaves is significantly lower than that of leaves in the lower canopy. Without the samples collected by the FTTS, the decrease of chlorophyll content with height would not have been of statistical significance using a p = 0.05 significance threshold. Besides its advantages, the existing limitations of the FTTS are discussed, and suggestions for future developments are provided.

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