CO2 Flux Measurements Research Articles

Effects of natural non-volcanic CO2 leakage on soil microbial community composition and diversity

Geological carbon capture and storage (CCS) can reduce anthropogenic CO2 emissions, but questions exist about impacts at the surface if CO2 leaks from deep storage reservoirs. To examine potential impacts on soils, previous studies have investigated the geochemistry and microbiology of volcanic soils hosting high fluxes of CO2 rich gas. This study builds on those previous investigations by considering impacts of CO2 leakage at a non-volcanic site, where deep geogenic CO2 leaks from a cracked well casing. At the site, we collected 26 soil cores adjacent to soil gas monitoring wells. Based on measured CO2 fluxes, the soil samples fall into two groups 1) high CO2 (flux = 304.6 ± 272.1 g m−2 d−1, conc. = 29.1 ± 34 %) and 2) low CO2 (flux = 15.8 ± 6.1 g m−2 d−1, conc. = 0.8 ± 0.9 %). Soil pH was significantly lower (p < 0.05) in high flux group samples (4.6 ± 0.3) than the low flux ones (5.3 ± 0.7). Beta diversity calculations using 16S rRNA gene sequences and redundancy analysis (RDA) revealed clear clustering of microbial communities relative to CO2 flux and significant correlations of community composition with pH and organic carbon content. In the high flux soils, abundant microbial groups included Acidobacteriota, Ktedonobacteria, and SC-I-84 in the phylum Proteobacteria, as well as Nitrososphaeria, a genus of ammonia oxidizing archaea. Compared to volcanic sites described previously, our non-volcanic site had slight differences in soil geochemical properties and gradual shifts in community compositions between CO2 hotspots and background locations. Moreover, the elevated abundance of SC-I-84 has not been reported in studies of volcanic sites. This study improves our ability to predict potential environmental impacts of geological CCS by expanding the range of conditions over which existing CO2 leakage has been observed.

Assessment of Greenhouse Gas Emissions into the Atmosphere from the Northern Peatlands Using the Wetland-DNDC Simulation Model: A Case Study of the Great Vasyugan Mire, Western Siberia

The peatlands of Western Siberia occupy an area of about 1 million km2 and act as important regulator of carbon exchange between the earth and the atmosphere. Extrapolation of the results of discrete field measurements of CO2 fluxes in bog ecosystems to such a territory is a difficult task, and one of the ways to overcome it is to use a simulation model such as DNDC. However, using this model with a specific territory requires ground verification to confirm its effectiveness. Here, we tested the DNDC model on the largest pristine bog ecosystem of the world, the Great Vasyugan Mire (GVM). The GVM of western Siberia is virtually undisturbed by anthropogenic activity and is the largest bog of Northern Eurasia (53,000 km2). Based on various ground-based observations, the performance of the Wetland-DNDC model was demonstrated (Thale coefficient 0.085 and R2 = 0.675 for CO2). Model input parameters specific to the GVM were constrained and model sensitivity to a wide range of input parameters was analyzed. The estimated annual terrestrial carbon fluxes in 2019 from the GVM test site are mainly controlled by plant respiration (61%) and forest floor degradation (38%). The net CO2 emission flux was 8600 kg C ha−1 year−1, which is in line with estimates from other independent studies.

Geochemistry and fluxes of gases from hydrothermal features at Newberry Volcano, Oregon, USA

We present the chemical and isotopic compositions of gases and fluxes of CO2 from the hydrothermal features of Newberry Volcano, a large composite volcano located in Oregon's Cascade Range with a summit caldera that hosts two lakes, Paulina and East Lakes. Gas samples were collected from 1982 to 2021 from Paulina Hot Springs (PHS) on the shore of Paulina Lake, East Lake Hot Springs (ELHS) on the shore of East Lake, and Obsidian Flow Gas Seep (OFGS), an area of diffuse gas emissions. Surveys of CO2 flux were conducted in 2020 at OFGS (1400 m2) and East Lake (4.1 km2). Gases from all three sites were CO2-rich (≥79 mol% in dry gas) but showed considerable compositional variability over time due to interaction with ground and surface water. An increase in H2S concentrations and decline in CO2/H2S ratios in ELHS gases coincided with a drop in East Lake water level from 1999 to 2021. ELHS and OFGS gases were high in CH4 relative to PHS and the δ13C of CH4 values for ELHS gases (−72.2 and − 63.6 ‰) reflected a predominantly biogenic origin. The dominant source of N2 and Ar in PHS, ELHS, and OFGS samples was likely groundwater. Helium isotopic ratios (6.47 to 8.02 Rc/Ra) support a persistent source of magmatic He beneath Newberry caldera and consistently high values measured at OFGS and PHS relative to ELHS suggest distinct fluid flow paths from depth to the surface features. The δ13C of CO2 and CO2/3He values (−8.9 to −5.35 ‰ and 1.3 × 109 to 4.6 × 1010, respectively) measured in gases reflect contributions of CO2 from both mantle and crustal sources. Measured CO2 fluxes at OFGS and East Lake ranged from 1 to 8808 and < 1 to 364 g m−2 d−1, respectively. A CO2 emission rate of 0.5 t d−1 was calculated for OFGS. The CO2 emission rate estimated for East Lake was 30 t d−1 and when compared to prior estimates, reflects steady-state lake degassing. An enhanced geochemical monitoring plan, including annual sampling of gases at ELHS, OFGS, and PHS for geochemical analysis, installation of a continuous lake-level monitoring station at East Lake, and annual CO2 flux surveys at OFGS, would provide valuable background data and insights into any precursor volcanic activity. Integrating geochemical data with data from the real-time seismic and GPS network at Newberry Volcano could better resolve and interpret potential changes in its magma-hydrothermal system.

Shallow portion of an active geothermal system revealed by multidisciplinary studies: The case of Le Biancane (Larderello, Italy)

The natural park of Le Biancane is located in the southern sector of the Larderello-Travale geothermal field (LTGF). It extends over an approximately 100,000 m2 area where the impermeable caprock is locally absent and deep fluids may directly reach the surface. Through a multidisciplinary approach including measurements of soil CO2 flux (total output of 11.5 t day−1), soil temperature (average 34.4 °C), stable isotope and chemical data on fluids from fumaroles (dominated by a mixture of geothermal gases and air or gases from air-saturated meteoric water), and structural analysis of the formation outcropping, we found that anomalous CO2 emissions are positively correlated with shallow temperature anomalies. These are in restricted locations adjacent to vents and fumaroles, where a network of well-connected fractures (preferentially NW-SE and NE-SW orientated and with steep dips) drains efficiently allowing upward migration of the deep fluids and the energy toward the surface.

Open-path anti-pollution multi-pass cell-based TDLAS sensor for the online measurement of atmospheric H2O and CO2 fluxes.

We report an open-path and anti-pollution multi-pass cell based tunable diode laser absorption spectroscopy (TDLAS) sensor, which was designed for online measurement of atmospheric H2O and CO2 fluxes. It is mainly composed of two plano-convex mirrors coated on a convex surface, which makes it different from traditional multi-pass cells. This design does not allow a direct contact between the coating layer of the lens and air, thereby realizing the anti-pollution effect of the coating layer. Two DFB lasers operating at 1392 nm and 2004 nm were employed to target H2O and CO2 absorption lines, respectively. Allan analysis of variance indicated that detection limits of H2O and CO2 were 5.98 ppm and 0.68 ppm, respectively, at an average time of 0.1 s. The sensor performance was demonstrated by measuring CO2 and H2O flux emissions at Jiangdu Agricultural Monitoring Station in Jiangsu Province. The results were compared with those obtained using the commercial instrument LI-7500, which is based on non-dispersive infrared technology. The developed gas analysis instrument exhibited good consistency with commercial instruments, and its accuracy was comparable; thus, it has strong application prospects for flux measurements in any ecosystem.

Tracking vegetation phenology of pristine northern boreal peatlands by combining digital photography with CO2 flux and remote sensing data

Abstract. Vegetation phenology, which refers to the seasonal changes in plant physiology, biomass and plant cover, is affected by many abiotic factors, such as precipitation, temperature and water availability. Phenology is also associated with the carbon dioxide (CO2) exchange between ecosystems and the atmosphere. We employed digital cameras to monitor the vegetation phenology of three northern boreal peatlands during five growing seasons. We derived a greenness index (green chromatic coordinate, GCC) from the images and combined the results with measurements of CO2 flux, air temperature and high-resolution satellite data (Sentinel-2). From the digital camera images it was possible to extract greenness dynamics on the vegetation community and even species level. The highest GCC and daily maximum gross photosynthetic production (GPPmax) were observed at the site with the highest nutrient availability and richest vegetation. The short-term temperature response of GCC depended on temperature and varied among the sites and months. Although the seasonal development and year-to-year variation in GCC and GPPmax showed consistent patterns, the short-term variation in GPPmax was explained by GCC only during limited periods. GCC clearly indicated the main phases of the growing season, and peatland vegetation showed capability to fully compensate for the impaired growth resulting from a late growing season start. The GCC data derived from Sentinel-2 and digital cameras showed similar seasonal courses, but a reliable timing of different phenological phases depended upon the temporal coverage of satellite data.

Air-water interactions regulating water temperature of lakes: Direct observations (Agamon Hula, Israel) and analytical solutions

Air-water interactions regulate lake-water temperature by balancing the rate of change of water temperature (stored heat) with the incoming and outgoing heat fluxes, which are functions of water temperature and external forcing. Yet, there is a large knowledge gap in quantifying the thermoregulation of a lake, and especially managed lakes, which is hypothesized to be related to both external environmental forcing and management decisions on lake depth and water inflow. Here we explore the thermoregulation of a restored and managed Mediterranean lake (Agamon Hula, Israel), by direct measurements of all major heat fluxes. We interpret the results with a rigorous analysis of the thermoregulation equation, obtained by formulating the energy balance equation in terms of water temperature. The equation shows how water temperature of a lake shifts toward steady state, scaled by the lake’s response time. Where the steady-state temperature was reached, it was controlled by external environmental forcing and not by lake depth. Whereas the thermal response time to re-attain steady state, following an abrupt change in various environmental conditions, is controlled by water depth. The lake-temperature response to pre-defined oscillations of the environmental forcing showed that the amplitude of water temperature fluctuations and the time delay from steady-state are controlled by the ratio between the environmental forcing’s time period (diurnal, seasonal or other cycles) and the thermal response time of the lake. The summertime measured CO2 fluxes of Agamon Hula revealed the lake acts as a CO2 source to the atmosphere, overpassing similar water bodies from different climates.

Trace gas fluxes from tidal salt marsh soils: implications for carbon–sulfur biogeochemistry

Abstract. Tidal salt marsh soils can be a dynamic source of greenhouse gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), as well as sulfur-based trace gases such as carbon disulfide (CS2) and dimethylsulfide (DMS) which play roles in global climate and carbon–sulfur biogeochemistry. Due to the difficulty in measuring trace gases in coastal ecosystems (e.g., flooding, salinity), our current understanding is based on snapshot instantaneous measurements (e.g., performed during daytime low tide) which complicates our ability to assess the role of these ecosystems for natural climate solutions. We performed continuous, automated measurements of soil trace gas fluxes throughout the growing season to obtain high-temporal frequency data and to provide insights into magnitudes and temporal variability across rapidly changing conditions such as tidal cycles. We found that soil CO2 fluxes did not show a consistent diel pattern, CH4, N2O, and CS2 fluxes were highly variable with frequent pulse emissions (&gt; 2500 %, &gt; 10 000 %, and &gt; 4500 % change, respectively), and DMS fluxes only occurred midday with changes &gt; 185 000 %. When we compared continuous measurements with discrete temporal measurements (during daytime, at low tide), discrete measurements of soil CO2 fluxes were comparable with those from continuous measurements but misrepresent the temporal variability and magnitudes of CH4, N2O, DMS, and CS2. Discrepancies between the continuous and discrete measurement data result in differences for calculating the sustained global warming potential (SGWP), mainly by an overestimation of CH4 fluxes when using discrete measurements. The high temporal variability of trace gas fluxes complicates the accurate calculation of budgets for use in blue carbon accounting and earth system models.

Carbon flux variation and associated biomass energy storage economic value implications in the Dinghushan Biosphere Reserve

Subtropical forests are a major carbon (C) storage pool and are therefore rich in biomass energy reservoirs. However, there have been no studies that quantify the C storage economic value, especially in the region of the Dinghushan Biosphere Reserve. This study is the first attempt to apply the eddy covariance technique to measure biomass energy storage and the associated economic value and demonstrates that CO2 flux measurement is a viable method for quantifying the ecosystem biomass energy storage. CO2 flux and micro-meteorological data were collected and analyzed to calculate the daily C sequestration and establish their inter-relationship. In addition, we used the C price estimates of Guangdong Province in China to determine the capital value of the C sequestered. The measured average annual precipitation was 1459.4 mm, the average temperature was 22.9 °C and the highest photosynthetic photon flux density (PPFD) was 334.92 μmol m−2s−1. CO2 flux and biomass energy storage varied seasonally. Photosynthetic photon flux density (PPFD) was highly correlated with gross primary production (GPP) (r2 = 0.99, p < 0.0001). Soil temperature was correlated with net ecosystem exchange (NEE) (r2 = 0.84, p < 0.0001). There was a positive correlation (r2 = 0.49, p < 0.0001) between soil moisture and NEE. The Dinghushan Biosphere Reserve sequestered an average of 5300 tCy−1 equivalent to RMB 3.3 million (USD 530,000). We conclude that economically, the Dinghushan Biosphere Reserve is highly significant to China's Guangdong Province because of its rich biomass energy reserves via C sequestration and our findings will stimulate the formulation of more aggressive conservation policies that will inevitably lead to better management of the reserve.

The effects of glucose addition and water table manipulation on peat quality of drained peatland forests with different management practices

AbstractPeatlands are globally significant C storage because waterlogged conditions slow down organic matter (OM) decomposition. Changes in the water table (WT) because of global warming or drainage, consecutive vegetation succession, and enhanced root exudation causing priming may transform peatlands from C sinks to sources. We studied how glucose addition, WT, and forest harvesting affect the chemical composition of peat and decomposition rate by incubating peat columns collected from drained clear‐cut (CC) and forested (FD) peatlands. Columns were divided into high or low WT, and half were labeled with 13C to study the priming effect on peat decomposition and peat chemical quality. We measured CO2 fluxes, peat OM, and water quality. There was no detectable priming effect after glucose addition. Lowering of the WT led to increased CO2 efflux, which during the measurements averaged between 39 and 291 μg m–2 s–1. Low WT also decreased the proportion of water‐soluble OM in CC areas but not in FD areas. The proportion of recalcitrant OM in surface peat was higher in forest than in clear cut. Forest management also affected the quality of dissolved OM in soil water, with CC showing higher concentrations of recalcitrant compounds. Decomposition and OM quality were governed by forest management practices and WT manipulation. In the future, the C sink capacity of forested peatlands will be regulated by changes in WT level, forest management, and quality of OM.

Soil water depletion induces discrepancies between in situ measured vegetation indices and photosynthesis in a temperate heathland

Vegetation indices (VIs) derived from optical sensors have been used as proxies for monitoring gross primary productivity (GPP). In contrast to satellite-based VIs, whose temporal resolution is typically limited, especially in cloudy areas, in situ derived VIs may have higher temporal resolution. This fine temporal frequency implies much larger sample sizes to test the performance of VIs by comparing with eddy covariance-based in situ GPP estimates. Here, we tested the potential of in situ measured VIs to estimate GPP in a temperate heathland ecosystem. We compared half-hourly GPP values derived from an eddy covariance CO2 flux measurements with several greenness-, structure- and chlorophyll-sensitive VIs (e.g., the Terrestrial Chlorophyll Index [TCI] and the Inverted Red-Edge Chlorophyll Index [IRECI], among others) obtained by multiband sensors mounted at the tower. Results showed that tested VIs differed in their ability to capture the temporal variability of GPP during non-drought condition, and that all VIs failed in describing GPP during an extreme drought event. After integrating a drought indicator (e.g., soil water content, precipitation/potential evapotranspiration ratio or actual/potential evapotranspiration ratio) in the regression model, the performances of VIs drastically improved. Among all tested indices, IRECI and TCI were the most promising VIs, capturing best the temporal variation in GPP (R2 = 0.73 and RMSE = 1.85, and R2 = 0.77, RMSE = 1.69, respectively), provided that drought stress was properly accounted for. Our findings have implications for the development and improvement of global ecological models for drought monitoring based on proximal and remote sensing data. Our results have also a strong impact on our ability to upscale CO2 fluxes using satellite sensors (e.g. Sentinel-2 and Sentinel-3) of heathland ecosystem characterized by heather vegetation which is relatively resistant to the structural changes in the canopy and strongly affects the interpretation of the remote sensing signals.

Air-Sea Trace Gas Fluxes: Direct and Indirect Measurements

The past decade has seen significant technological advance in the observation of trace gas fluxes over the open ocean, most notably CO2, but also an impressive list of other gases. Here we will emphasize flux observations from the air-side of the interface including both turbulent covariance (direct) and surface-layer similarity-based (indirect) bulk transfer velocity methods. Most applications of direct covariance observations have been from ships but recently work has intensified on buoy-based implementation. The principal use of direct methods is to quantify empirical coefficients in bulk estimates of the gas transfer velocity. Advances in direct measurements and some recent field programs that capture a considerable range of conditions with wind speeds exceeding 20 ms-1 are discussed. We use coincident direct flux measurements of CO2 and dimethylsulfide (DMS) to infer the scaling of interfacial viscous and bubble-mediated (whitecap driven) gas transfer mechanisms. This analysis suggests modest chemical enhancement of CO2 flux at low wind speed. We include some updates to the theoretical structure of bulk parameterizations (including chemical enhancement) as framed in the COAREG gas transfer algorithm.

Source decomposition of eddy-covariance CO2 flux measurements for evaluating a high-resolution urban CO2 emissions inventory

We present the comparison of source-partitioned CO2 flux measurements with a high-resolution urban CO2 emissions inventory (Hestia). Tower-based measurements of CO and 14C are used to partition net CO2 flux measurements into fossil and biogenic components. A flux footprint model is used to quantify spatial variation in flux measurements. We compare the daily cycle and spatial structure of Hestia and eddy-covariance derived fossil fuel CO2 emissions on a seasonal basis. Hestia inventory emissions exceed the eddy-covariance measured emissions by 0.36 µmol m−2 s−1 (3.2%) in the cold season and 0.62 µmol m−2 s−1 (9.1%) in the warm season. The daily cycle of fluxes in both products matches closely, with correlations in the hourly mean fluxes of 0.86 (cold season) and 0.93 (warm season). The spatially averaged fluxes also agree in each season and a persistent spatial pattern in the differences during both seasons that may suggest a bias related to residential heating emissions. In addition, in the cold season, the magnitudes of average daytime biological uptake and nighttime respiration at this flux site are approximately 15% and 27% of the mean fossil fuel CO2 emissions over the same time period, contradicting common assumptions of no significant biological CO2 exchange in northern cities during winter. This work demonstrates the effectiveness of using trace gas ratios to adapt eddy-covariance flux measurements in urban environments for disaggregating anthropogenic CO2 emissions and urban ecosystem fluxes at high spatial and temporal resolution.

Radon signature of CO2 flux constrains the depth of degassing: Furnas volcano (Azores, Portugal) versus Syabru-Bensi (Nepal Himalayas)

Substantial terrestrial gas emissions, such as carbon dioxide (CO2), are associated with active volcanoes and hydrothermal systems. However, while fundamental for the prediction of future activity, it remains difficult so far to determine the depth of the gas sources. Here we show how the combined measurement of CO2 and radon-222 fluxes at the surface constrains the depth of degassing at two hydrothermal systems in geodynamically active contexts: Furnas Lake Fumarolic Field (FLFF, Azores, Portugal) with mantellic and volcano-magmatic CO2, and Syabru-Bensi Hydrothermal System (SBHS, Central Nepal) with metamorphic CO2. At both sites, radon fluxes reach exceptionally high values (> 10 Bq m−2 s−1) systematically associated with large CO2 fluxes (> 10 kg m−2 day−1). The significant radon‒CO2 fluxes correlation is well reproduced by an advective–diffusive model of radon transport, constrained by a thorough characterisation of radon sources. Estimates of degassing depth, 2580 ± 180 m at FLFF and 380 ± 20 m at SBHS, are compatible with known structures of both systems. Our approach demonstrates that radon‒CO2 coupling is a powerful tool to ascertain gas sources and monitor active sites. The exceptionally high radon discharge from FLFF during quiescence (≈ 9 GBq day−1) suggests significant radon output from volcanoes worldwide, potentially affecting atmosphere ionisation and climate.

Rates of atmospheric CO2 capture using magnesium oxide powder

Magnesium oxide (MgO) looping is a proposed technology for gigatonne-scale carbon dioxide (CO2) removal from the atmosphere. Here, we determined CO2 removal rates by measuring CO2 fluxes and total inorganic carbon (TIC) using a deposit of MgO powder (10 cm thickness; 76 kg MgO/m2). The majority of the MgO powder (∼87 wt%) was transformed to brucite [Mg(OH)2] after the first two wetting and drying cycles. CO2 fluxes varied depending on water content and porosity: −3.9 (saturated/wet), −12.5 (optimal), and −2.2 (dry) kg CO2/m2/yr. At these rates, only 3–18% of the MgO would react in 1 yr and require areas of 80,000–455,000 km2 to sequester 1 Gt CO2/yr. Gas-filled porosity of ∼39–58% and water contents of 7–17 wt% were optimal for CO2 ingress and carbonation. Dypingite [Mg5(CO3)4(OH)2·∼5H2O] was the sink of atmospheric CO2 and TIC increased from 0.2% to 7.5% CO2. The δ13C values of the solids (avg. −14.9‰; VPDB) were well below those expected for dypingite precipitated in equilibrium with laboratory CO2, demonstrating that CO2 supply was rate-limiting, despite daily tilling of the deposit. A linear extrapolation of the CO2 fluxes shows that 90% carbonation of MgO will require 5–27 yr. CO2 removal rates slowed down with time over 1 yr, in agreement with the expectation that the progressive depletion of reactants (MgO/brucite) is likely to substantially slow CO2 removal rates and greatly extend the time needed to achieve complete carbonation.

Effects of chamber characteristics on CO2 and CH4 flux at the water-air interface measured by the chamber method

The chamber method is widely used to measure CO2 and CH4 flux in inland water. However, the designs of chamber used in various studies are different and lack unified standards, which would affect the observation results. To clarify the impacts of chamber characteristics, including light transmittance, air pressure difference inside and outside the chamber, and gas mixing degree in the chamber, on CO2 and CH4 flux measurements at the water-air interface, we compared the effects of transparent/opaque chamber, the chamber with/without air pressure equalizing device and fan on CO2 and CH4 flux measurements in the aquaculture pond, based on the multi-channel closed dynamic chamber system. The results showed that, during the daytime in summer, compared with the transparent chamber which could measure the actual CO2 flux, when CO2 was emitted from the pond, the opaque chamber overestimated the CO2 flux by 90%; when CO2 was absorbed by the pond, the opaque chamber underestimated the CO2 flux by 50%. The CH4 diffusion flux measured by the opaque chamber was 40% lower than that measured by the transparent chamber. There was no significant difference between CO2 and CH4 flux measured by the chamber with and without air pressure equalizing device. CO2 flux observed by the chamber without fan had poor representativeness, being 20% higher than that observed by the chamber with fan. Moreover, CH4 flux emitted through different pathways could not be distinguished using the chamber without fan. Therefore, when the chamber method was used to observe the CO2 and CH4 flux at the water-air interface, the chamber shall be transparent and be installed with fan.

Comparison of Radiocarbon‐ and Background Location‐Corrections on Soil‐Gas CO2 Flux‐Based NSZD Rate Measurements at Petroleum Impacted Sites

AbstractThe measurement of contaminant natural source zone depletion (NSZD) rates has become an important tool to manage petroleum contaminated sites. Most NSZD rate measurement methods rely on a balance on the biodegradation by‐products (either carbon or heat). Carbon balance‐based methods stoichiometrically convert measured soil‐gas CO2 flux related to contaminant degradation to equivalent contaminant mass loses. CO2 flux‐based methods require separating the fraction of the total CO2 flux produced by NSZD from the fraction of CO2 flux produced by natural soil processes (due to modern carbon turnover). Two method corrections are available to distinguish the nature of the measured CO2 flux: (1) The background location correction subtracts a measurement from an unimpacted location from measurements collected at impacted locations, and (2) a location‐specific radiocarbon (14C) analysis that differentiates the modern and old carbon fractions of each total CO2 measurement. This work evaluates both correction approaches using 36 measurements at impacted locations from five sites. The 14C‐corrected data shows that the magnitude and variability of the modern carbon‐related fraction (noise) and the NSZD related fossil fuel signal are similar, suggesting a location‐specific correction is more valid. Only one sparsely vegetated arid site showed close agreement between both corrections (14C‐ and background location). These results highlight the strong effect of the correction method used on NSZD rate measurements, and the importance of considering data quality on the subsequent data management process.

Dormant Season Vegetation Phenology and Eddy Fluxes in Native Tallgrass Prairies of the U.S. Southern Plains

Carbon dioxide (CO2) fluxes and evapotranspiration (ET) during the non-growing season can contribute significantly to the annual carbon and water budgets of agroecosystems. Comparative studies of vegetation phenology and the dynamics of CO2 fluxes and ET during the dormant season of native tallgrass prairies from different landscape positions under the same climatic regime are scarce. Thus, this study compared the dynamics of satellite-derived vegetation phenology (as captured by the enhanced vegetation index (EVI) and the normalized difference vegetation index (NDVI)) and eddy covariance (EC)-measured CO2 fluxes and ET in six differently managed native tallgrass prairie pastures during dormant seasons (November through March). During December–February, vegetation phenology (EVI and NDVI) and the dynamics of eddy fluxes were comparable across all pastures in most years. Large discrepancies in fluxes were observed during March (the time of the initiation of growth of dominant warm-season grasses) across years and pastures due to the influence of weather conditions and management practices. The results illustrated the interactive effects between prescribed spring burns and rainfall on vegetation phenology (i.e., positive and negative impacts of prescribed spring burns under non-drought and drought conditions, respectively). The EVI better tracked the phenology of tallgrass prairie during the dormant season than did NDVI. Similar EVI and NDVI values for the periods when flux magnitudes were different among pastures and years, most likely due to the satellite sensors’ inability to fully observe the presence of some cool-season C3 species under residues, necessitated a multi-level validation approach of using ground-truth observations of species composition, EC measurements, PhenoCam (digital) images, and finer-resolution satellite data to further validate the vegetation phenology derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) during dormant seasons. This study provides novel insights into the dynamics of vegetation phenology, CO2 fluxes, and ET of tallgrass prairie during the dormant season in the U.S. Southern Great Plains.

Carbon–water coupling and its relationship with environmental and biological factors in a planted Caragana liouana shrub community in desert steppe, northwest China

Abstract The carbon and water cycle, an important biophysical process of terrestrial ecosystems, is changed by anthropogenic revegetation in arid and semiarid areas. However, there is still a lack of understanding of the mechanisms of carbon and water coupling in intrinsic ecosystems in the context of human activities. Based on the CO2 and H2O flux measurements of the desert steppe with the planted shrub Caragana liouana, this study explored the carbon and water flux coupling of the ecosystem by analyzing the variations in gross primary productivity (GPP), evapotranspiration (ET) and water use efficiency (WUE) and discussing the driving mechanisms of biological factors. The seasonal variation in climate factors induced a periodic variation pattern of biophysical traits and carbon and water fluxes. The GPP and ET fluctuated in seasons, but the WUE was relatively stable in the growing season. The GPP, ET and WUE were significantly driven by global radiation (Rg), temperature (Ta and Ts), water vapor pressure deficit, leaf area index and plant water stress index (PWSI). However, Rg, temperature and PWSI were the most important factors regulating WUE. Rg and temperature directly affected WUE with a positive effect but indirectly inhibited WUE by rising PWSI. Plant water stress inhibited photosynthesis and transpiration of the planted shrub community in the desert steppe. When the plant water stress exceeded a threshold (PWSI &amp;gt;0.54), the WUE would decrease since the GPP responded more quickly to the plant water stress than ET. Our findings suggest that policies related to large-scale carbon sequestration initiatives under afforestation must first fully consider the status of water consumption and WUE.

Metrology for low-cost CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; sensors applications: the case of a steady-state through-flow (SS-TF) chamber for CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes observations

Abstract. Soil CO2 emissions are one of the largest contributions to the global carbon cycle, and a full understanding of processes generating them and how climate change may modify them is needed and still uncertain. Thus, a dense spatial and temporal network of CO2 flux measurements from soil could help reduce uncertainty in the global carbon budgets. In the present study, the design, assembly, and calibration of low-cost air enquirer kits, including CO2 and environmental parameters sensors, is presented. Different types of calibrations for the CO2 sensors and their associated errors are calculated. In addition, for the first time, this type of sensor has been applied to design, develop, and test a new steady-state through-flow (SS-TF) chamber for simultaneous measurements of CO2 fluxes in soil and CO2 concentrations in air. The sensors' responses were corrected for temperature, relative humidity, and pressure conditions in order to reduce the uncertainty in the measured CO2 values and of the following calculated CO2 fluxes based on SS-TF. CO2 soil fluxes measured by the proposed SS-TF and by a standard closed non-steady-state non-through-flow (NSS-NTF) chamber were briefly compared to ensure the reliability of the results. The use of a multiparametric fitting reduced the total uncertainty of the CO2 concentration measurements by 62 %, compared with the uncertainty that occurred when a simple CO2 calibration was applied, and by 90 %, when compared to the uncertainty declared by the manufacturer. The new SS-TF system allows the continuous measurement of CO2 fluxes and CO2 ambient air with low cost (EUR ∼1200), low energy demand (&lt;5 W), and low maintenance (twice per year due to sensor calibration requirements).