Hot-Moments of Soil CO2 Efflux in a Water-Limited Grassland

The metabolic activity of water-limited ecosystems is strongly linked to the timing and magnitude of precipitation pulses that can trigger disproportionately high (i.e., hot-moments) ecosystem CO2 fluxes. We analyzed over 2-years of continuous measurements of soil CO2 efflux (Fs) under vegetation (Fsveg) and at bare soil (Fsbare) in a water-limited grassland. The continuous wavelet transform was used to: (a) describe the temporal variability of Fs; (b) test the performance of empirical models ranging in complexity; and (c) identify hot-moments of Fs. We used partial wavelet coherence (PWC) analysis to test the temporal correlation between Fs with temperature and soil moisture. The PWC analysis provided evidence that soil moisture overshadows the influence of soil temperature for Fs in this water limited ecosystem. Precipitation pulses triggered hot-moments that increased Fsveg (up to 9000%) and Fsbare (up to 17,000%) with respect to pre-pulse rates. Highly parameterized empirical models (using support vector machine (SVM) or an 8-day moving window) are good approaches for representing the daily temporal variability of Fs, but SVM is a promising approach to represent high temporal variability of Fs (i.e., hourly estimates). Our results have implications for the representation of hot-moments of ecosystem CO2 fluxes in these globally distributed ecosystems

Winds induce CO2 exchange with the atmosphere and vadose zone transport in a karstic ecosystem

Research on the subterranean CO dynamics has focused individually on either surface soils or bedrock cavities, neglecting the interaction of both systems as a whole. In this regard, the vadose zone contains CO-enriched air (ca. 5% by volume) in the first meters, and its exchange with the atmosphere can represent from 10 to 90% of total ecosystem CO emissions. Despite its importance, to date still lacking are reliable and robust databases of vadose zone CO contents that would improve knowledge of seasonal-annual aboveground-belowground CO balances. Here we study 2.5 years of vadose zone CO dynamics in a semiarid ecosystem. The experimental design includes an integrative approach to continuously measure CO in vertical and horizontal soil profiles, following gradients from surface to deep horizons and from areas of net biological CO production (under plants) to areas of lowest CO production (bare soil), as well as a bedrock borehole representing karst cavities and ecosystem-scale exchanges. We found that CO followed similar seasonal patterns for the different layers, with the maximum seasonal values of CO delayed with depth (deeper more delayed). However, the behavior of CO transport differed markedly among layers. Advective transport driven by wind induced CO emission both in surface soil and bedrock, but with negligible effect on subsurface soil, which appears to act as a buffer impeding rapid CO exchanges. Our study provides the first evidence of enrichment of CO under plant, hypothesizing that CO-rich air could come from root zone or by transport from deepest layers through cracks and fissures.These data were funded by the Andalusian regional government project GEOCARBO (P08-RNM-3721), including European Union ERDF funds, with support from Spanish Ministry of Science and Innovation projects SOILPROF (CGL2011-15276-E), CARBORAD (CGL2011-27493), and GEISpain (CGL2014-52838-C2-1-R). This research was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme, DIESEL project (625988).Peer Reviewe

Ecosystem CO2 release driven by wind occurs in drylands at global scale

Subterranean ventilation is a non-diffusive transport process that provokes the abrupt transfer of CO2-rich air (previously stored) through water-free soil pores and cracks from the vadose zone to the atmosphere, under high-turbulence conditions. In dryland ecosystems, whose biological carbon exchanges are poorly characterized, it can strongly determine eddy-covariance CO2 fluxes that are used to validate remote sensing products and constrain models of gross primary productivity. Although subterranean ventilation episodes (VE) may occur in arid and semi-arid regions, which are unsung players in the global carbon cycle, little research has focused on the role of VE CO2 emissions in land–atmosphere CO2 exchange. This study shows clear empirical evidence of globally occurring VE. To identify VE, we used in situ quality-controlled eddy-covariance open data of carbon fluxes and ancillary variables from 145 sites in different open land covers (grassland, cropland, shrubland, savanna, and barren) across the globe. We selected the analyzed database from the FLUXNET2015, AmeriFlux, OzFlux, and AsiaFlux networks. To standardize the analysis, we designed an algorithm to detect CO2 emissions produced by VE at all sites considered in this study. Its main requirement is the presence of considerable and non-spurious correlation between the friction velocity (i.e., turbulence) and CO2 emissions. Of the sites analyzed, 34% exhibited the occurrence of VE. This vented CO2 emerged mainly from arid ecosystems (84%) and sites with hot and dry periods. Despite some limitations in data availability, this research demonstrates that VE-driven CO2 emissions occur globally. Future research should seek a better understanding of its drivers and the improvement of partitioning models, to reduce uncertainties in estimated biological CO2 exchanges and infer their contribution to the global net ecosystem carbon balance.Publishe

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

CO2 exchange between terrestrial ecosystems and the atmosphere is key to understanding the feedbacks between climate change and the land surface. In regions with carbonaceous parent material, CO2 exchange patterns occur that cannot be explained by biological processes, such as disproportionate outgassing during the daytime or nighttime CO2 uptake during periods when all vegetation is senescent. Neither of these phenomena can be attributed to carbonate weathering reactions, since their CO2 exchange rates are too small. Soil ventilation induced by high atmospheric turbulence is found to explain atypical CO2 exchange between carbonaceous systems and the atmosphere. However, by strongly altering subsurface CO2 concentrations, ventilation can be expected to influence carbonate weathering rates. By imposing ventilation-driven CO2 outgassing in a carbonate weathering model, we show here that carbonate geochemistry is accelerated and does play a surprisingly large role in the observed CO2 exchange pattern of a semi-arid ecosystem. We found that by rapidly depleting soil CO2 during the daytime, ventilation disturbs soil carbonate equilibria and therefore strongly magnifies daytime carbonate precipitation and associated CO2 production. At night, ventilation ceases and the depleted CO2 concentrations increase steadily. Dissolution of carbonate is now enhanced, which consumes CO2 and largely compensates for the enhanced daytime carbonate precipitation. This is why only a relatively small effect on global carbonate weathering rates is to be expected. On the short term, however, ventilation has a drastic effect on synoptic carbonate weathering rates, resulting in a pronounced diel pattern that exacerbates the non-biological behavior of soil–atmosphere CO2 exchanges in dry regions \mbox{with carbonate soils}.M. Roland was granted by the Institute for Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen). I. A. Janssens and R. Van Grieken acknowledge the Research Foundation – Flanders (FWO). P. Serrano-Ortiz is funded by a postdoctoral fellowship from the Spanish Ministry of Science and Innovation. S. Cuezva was funded by a postdoctoral fellowship from the Spanish Ministry of Science and Innovation, research programme Juan de la Cierva

Ground-Based Optical Measurements at European Flux Sites: A Review of Methods, Instruments and Current Controversies

This paper reviews the currently available optical sensors, their limitations and opportunities for deployment at Eddy Covariance (EC) sites in Europe. This review is based on the results obtained from an online survey designed and disseminated by the Co-cooperation in Science and Technology (COST) Action ESO903—“Spectral Sampling Tools for Vegetation Biophysical Parameters and Flux Measurements in Europe” that provided a complete view on spectral sampling activities carried out within the different research teams in European countries. The results have highlighted that a wide variety of optical sensors are in use at flux sites across Europe, and responses further demonstrated that users were not always fully aware of the key issues underpinning repeatability and the reproducibility of their spectral measurements. The key findings of this survey point towards the need for greater awareness of the need for standardisation and development of a common protocol of optical sampling at the European EC sites

COSORE: A community database for continuous soil respiration and other soil‐atmosphere greenhouse gas flux data

Globally, soils store two to three times as much carbon as currently resides in the atmosphere, and it is critical to understand how soil greenhouse gas (GHG) emissions and uptake will respond to ongoing climate change. In particular, the soil‐to‐atmosphere CO2 flux, commonly though imprecisely termed soil respiration (RS), is one of the largest carbon fluxes in the Earth system. An increasing number of high‐frequency RS measurements (typically, from an automated system with hourly sampling) have been made over the last two decades; an increasing number of methane measurements are being made with such systems as well. Such high frequency data are an invaluable resource for understanding GHG fluxes, but lack a central database or repository. Here we describe the lightweight, open‐source COSORE (COntinuous SOil REspiration) database and software, that focuses on automated, continuous and long‐term GHG flux datasets, and is intended to serve as a community resource for earth sciences, climate change syntheses and model evaluation. Contributed datasets are mapped to a single, consistent standard, with metadata on contributors, geographic location, measurement conditions and ancillary data. The design emphasizes the importance of reproducibility, scientific transparency and open access to data. While being oriented towards continuously measured RS, the database design accommodates other soil‐atmosphere measurements (e.g. ecosystem respiration, chamber‐measured net ecosystem exchange, methane fluxes) as well as experimental treatments (heterotrophic only, etc.). We give brief examples of the types of analyses possible using this new community resource and describe its accompanying R software package

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

Heterogeneous contributions of change in population distribution of body mass index to change in obesity and underweight NCD Risk Factor Collaboration (NCD-RisC)

From 1985 to 2016, the prevalence of underweight decreased, and that of obesity and severe obesity increased, in most regions, with significant variation in the magnitude of these changes across regions. We investigated how much change in mean body mass index (BMI) explains changes in the prevalence of underweight, obesity, and severe obesity in different regions using data from 2896 population-based studies with 187 million participants. Changes in the prevalence of underweight and total obesity, and to a lesser extent severe obesity, are largely driven by shifts in the distribution of BMI, with smaller contributions from changes in the shape of the distribution. In East and Southeast Asia and sub-Saharan Africa, the underweight tail of the BMI distribution was left behind as the distribution shifted. There is a need for policies that address all forms of malnutrition by making healthy foods accessible and affordable, while restricting unhealthy foods through fiscal and regulatory restrictions

Cave ventilation is influenced by variations in the CO\u3csub\u3e2\u3c/sub\u3e-dependent virtual temperature

Dynamics and drivers of ventilation in caves are of growing interest for different fields of science. Accumulated CO2 in caves can be exchanged with the atmosphere, modifying the internal CO2 content, affecting stalagmite growth rates, deteriorating rupestrian paintings or creating new minerals. Current estimates of cave ventilation neglect the role of high CO2 concentrations in determining air density – approximated via the virtual temperature (Tv) –, affecting buoyancy and therefore the release or storage of CO2. Here we try to improve knowledge and understanding of cave ventilation through the use of Tv in CO2-rich air to explain buoyancy for different values of temperature (T) and CO2 content. Also, we show differences between T and Tv for 14 different experimental sites in the vadose zone, demonstrating the importance of using the correct definition of Tv to determine air buoyancy in caves. The calculation of Tv (including CO2 effects) is currently available via internet using an excel template, requiring the input of CO2 (%), air temperature (ºC) and relative humidity (%)