CO2 Exchange Fluxes Research Articles

Seasonal variations in carbon dioxide exchange fluxes at a taiga–tundra boundary ecosystem in Northeastern Siberia

Arctic and boreal permafrost ecosystems in Eastern Siberia, considered crucial to the climate system and global carbon cycle, are particularly vulnerable to climate change. This study investigates carbon dioxide (CO2) exchange fluxes over northeastern Siberia from 2013 to 2015 in a taiga–tundra boundary ecosystem for which such measurements are scarce. The growing season (May–September) net CO2 exchange flux (NEE) was −39.4 (−60.1 to −20.2) gCm−2, with ecosystem respiration (RE) = 306.2 (288.1–317.9) gCm−2 and gross primary production (GPP) = −345.5 (−372.5 to −317.7) gCm−2. Microclimatic factors determining these CO2 exchange fluxes change seasonally. These fluxes are significantly affected by the timing of the onset of C uptake, which is reflected by changes in the soil temperature in spring and early summer, following which fluxes respond well to the photosynthetic photon flux density, especially for NEE. These CO2 exchange fluxes at the northeastern Siberian taiga–tundra boundary ecosystem are significantly smaller than those previously reported at southern-taiga forest sites. Spring snow meltwater-rich soil moisture conditions render southern-taiga sites as stronger CO2 sinks in June than taiga–tundra boundary ecosystem, which may be largely responsible for the pronounced north–south gradient in growing season NEE.

Phenology and plant functional type dominance drive CO2 exchange in seminatural grasslands in the Pyrenees

AbstractUnderstanding the mechanisms underlying net ecosystem CO2 exchange (NEE) in mountain grasslands is important to quantify their relevance in the global carbon budget. However, complex interactions between environmental variables and vegetation on NEE remain unclear; and there is a lack of empirical data, especially from the high elevations and the Mediterranean region. A chamber-based survey of CO2 exchange measurements was carried out in two climatically contrasted grasslands (montane v. subalpine) of the Pyrenees; assessing the relative contribution of phenology and environmental variables on CO2 exchange at the seasonal scale, and the influence of plant functional type dominance (grasses, forbs and legumes) on the NEE light response. Results show that phenology plays a crucial role as a CO2 exchange driver, suggesting a differential behaviour of the vegetation community depending on the environment. The subalpine grassland had a more delayed phenology compared to the montane, being more temperature than water constrained. However, temperature increased net CO2 uptake at a higher rate in the subalpine than in the montane grassland. During the peak biomass, productivity (+74%) and net CO2 uptake (NEE +48%) were higher in the subalpine grassland than in the montane grassland. The delayed phenology at the subalpine grassland reduced vegetation's sensitivity to summer dryness, and CO2 exchange fluxes were less constrained by low soil water content. The NEE light response suggested that legume dominated plots had higher net CO2 uptake per unit of biomass than grasses. Detailed information on phenology and vegetation composition is essential to understand elevation and climatic differences in CO2 exchange.

Five‐Year Measurements of Net Ecosystem CO2 Exchange at a Fen in the Zoige Peatlands on the Qinghai‐Tibetan Plateau

AbstractLittle is known about the relationship between carbon sequestration and environment in some alpine peatland ecosystems, especially in remote areas such as the Qinghai‐Tibetan Plateau. In the present study, net ecosystem CO2 exchange (NEE) flux was measured from 2013 to 2017 at the Riganqiao fen in the Zoige peatlands on the eastern Qinghai‐Tibetan Plateau using the eddy covariance technique. In the growing season, diurnal CO2 uptake peaks occurred between 12:00 and 14:00, while emission peaks occurred between 21:00 and 22:00. Carbon uptake peak occurred mainly from June to July in the growing season, and the maximum daily net CO2 uptake of −8.4 g C m−2 d−1 was observed in mid‐June 2016. The Riganqiao peatland was a net CO2 sink, with uptake (g C m−2 yr−1) of average −171.4 ± 38.1 during the five years. Photosynthetically active radiation (PAR) and soil temperature (Ts) were significantly correlated with diurnal net CO2 uptake. In growing seasons, PAR was significantly correlated with daytime CO2 uptake, and Ts was a major determinant of ecosystem respiration during growing and nongrowing seasons, although the fitting functions differed between the two seasons. Rainfall influenced interannual variability of cumulative NEE and water table depth (WTD) and volumetric water content (VWC) were both significantly correlated with interannual NEE. Our results suggest that in the long term, peatland NEE is affected mainly by the water regime, which is influenced by soil temperature and water condition.

Contribution of Biological Effects to the Carbon Sources/Sinks and the Trophic Status of the Ecosystem in the Changjiang (Yangtze) River Estuary Plume in Summer as Indicated by Net Ecosystem Production Variations

We conducted 24-h real-time monitoring of temperature, salinity, dissolved oxygen, and nutrients in the near-shore (M4-1), front (M4-8), and offshore (M4-13) regions of the 31° N section of the Changjiang (Yangtze) River estuary plume in summer. Carbon dioxide partial pressure changes caused by biological processes (pCO2bio) and net ecosystem production (NEP) were calculated using a mass balance model and used to determine the relative contribution of biological processes (including the release of CO2 from organic matter degradation by microbes and CO2 uptake by phytoplankton) to the CO2 flux in the Changjiang River estuary plume. Results show that seawater in the near-shore region is a source of atmospheric CO2, and the front and offshore regions generally serve as atmospheric CO2 sinks. In the mixed layer of the three regions, pCO2bio has an overall positive feedback effect on the air–sea CO2 exchange flux. The contribution of biological processes to the air–sea CO2 exchange flux (Cont) in the three regions changes to varying extents. From west to east, the daily means (±standard deviation) of the Cont are 32% (±40%), 34% (±216%), and 9% (±13%), respectively. In the front region, the Cont reaches values as high as 360%. Under the mixed layer, the daily means of potential Conts in the near-shore, front, and offshore regions are 34% (±43%), 8% (±13%), and 19% (±24%), respectively. The daily 24-hour means of NEP show that the near-shore region is a heterotrophic system, the front and offshore regions are autotrophic systems in the mixed layer, and all three regions are heterotrophic under the mixed layer.

Diffusive CO2 Flux Across the Water-air Interface of Reclaimed Shrimp Ponds in the Minjiang River Estuary Based on the TBL Model

Freshwater aquatic ecosystems are important sources of greenhouse gases, such as CO2. However, few studies have presented data on the greenhouse gas flux from coastal aquaculture ponds. Diffusion models are important tools for estimating the CO2 exchange flux across the water-air interface of aquatic ecosystems. Several different parameterized means were selected to estimate the CO2 gas exchange rate (kx) and CO2 diffusive flux across the water-air interface of shrimp ponds in the Minjiang River Estuary. The results indicated that:① the CO2 gas exchange rate and diffusive flux over the culture period all presented significant temporal variation. This variation showed a dynamic trend:October > September > November > July > August and November > July > August > September > October. ② Wind speed, kx, CO2 concentration, pH, DOC concentration, and Chl-a concentration were important factors affecting the temporal variation of CO2 diffusive flux. ③ There were differences in the estimated value of CO2 diffusive flux across the water-air interface of the culture ponds in the Minjiang River Estuary among different parameterized approaches (P<0.01). This indicates that the model method has some uncertainties in estimating the CO2 diffusive flux in culture ponds. Our results suggest that the models RC01 and CW03 are more suitable methods for estimating the CO2 diffusive flux at the water-air interface of estuarine reclaimed aquaculture ponds in the Minjiang River Estuary, after comprehensive analysis of the water environment and the different estimation results.

Water-Air Interface CO2 Exchange Flux of Typical Lakes in a Mountainous Area of the Western Chongqing and Their Influencing Factors

To examine the mountainous lake CO2 evasion in Southwest China, partial pressures of carbon dioxide[p(CO2)] and the CO2 exchange flux[F(CO2)] via the water-air interface of nine mountainous lakes in Chongqing, China, have been studied in summer using the thin boundary layer model (TBL) and floating chambers. Key water quality parameters were concomitantly measured. The results indicate that the pCO2 in the mountainous lakes in western Chongqing ranges from 2.1 to 45.0 Pa, with a mean value of (18.1±12.1) Pa. The mean CO2 fluxes calculated by the TBL model and chamber method are (-8.0±2.9), (-3.4±3.6), and (-7.1±22.3) mmol·(m2·d)-1, respectively. The p(CO2) and F(CO2) have positive correlations with the wind speed and ORP but negative correlations with the pH. Our study indicates that mountainous lakes are atmospheric sinks of CO2 and the TBL model should be cautiously adopted.

Carbon sinks/sources in the Yellow and East China Seas—Air-sea interface exchange, dissolution in seawater, and burial in sediments

The sinks/sources of carbon in the Yellow Sea (YS) and East China Sea (ECS), which are important continental shelf seas in China, could exert a great influence on coastal ecosystem dynamics and the regional climate change process. The CO2 exchange process across the seawater-air interface, dissolved and particulate carbon in seawater, and carbon burial in sediments were studied to understand the sinks/sources of carbon in the continental shelf seas of China. The YS and the ECS generally have different patterns of seasonal air-sea CO2 exchange. In the YS, regions west of 124°E can absorb CO2 from the atmosphere during spring and winter, and release CO2 to the atmosphere during summer and autumn. The entire YS is considered as a CO2 source throughout the year with respect to the atmosphere, but there are still uncertainties regarding the exact air-sea CO2 exchange flux. Surface temperature and phytoplankton production were the key controlling factors of the air-sea CO2 exchange flux in the offshore region and nearshore region of the YS, respectively. The ECS can absorb CO2 during spring, summer, and winter and release CO2 to the atmosphere during autumn. The annual average exchange rate in the ECS was −4.2±3.2 mmol m−2 d−1 and it served as an obvious sink for atmospheric CO2 with an air-sea exchange flux of 13.7×106 t. The controlling factors of the air-sea CO2 exchange in the ECS varied significantly seasonally. Storage of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the YS and the ECS were 425×106 t and 1364×106 t, and 28.2×106 t and 54.1×106 t, respectively. Long-term observation showed that the DOC content in the YS had a decreasing trend, indicating that the “practical carbon sink” in the YS was decreasing. The total amount of particulate organic carbon (POC) stored in the YS and ECS was 10.6×106 t, which was comparable to the air-sea CO2 flux in these two continental shelf seas. The amounts of carbon sequestered by phytoplankton in the YS and the ECS were 60.42×106 t and 153.41×106 t, respectively. Artificial breeding of macroalgae could effectively enhance blue carbon sequestration, which could fix 0.36×106–0.45×106 t of carbon annually. Organic carbon (OC) buried in the sediments of the YS was estimated to be 4.75×106 t, and OC of marine origin was 3.03×106 t, accounting for 5.0% of the TOC fixed by phytoplankton primary production. In the ECS, the corresponding depositional flux of OC in the sediment was estimated to be 7.4×106 t yr−1 , and the marine-origin OC was 5.5×106 t, accounting for 5.4% of the phytoplankton primary production. Due to the relatively high average depositional flux of OC in the sediment, the YS and ECS have considerable potential to store a vast amount of “blue carbon.”

The CarbonTracker Data Assimilation System for CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C (CTDAS-C13 v1.0): retrieving information on land–atmosphere exchange processes

Abstract. To improve our understanding of the global carbon balance and its representation in terrestrial biosphere models, we present here a first dual-species application of the CarbonTracker Data Assimilation System (CTDAS). The system's modular design allows for assimilating multiple atmospheric trace gases simultaneously to infer exchange fluxes at the Earth surface. In the prototype discussed here, we interpret signals recorded in observed carbon dioxide (CO2) along with observed ratios of its stable isotopologues 13CO2∕12CO2 (δ13C). The latter is in particular a valuable tracer to untangle CO2 exchange from land and oceans. Potentially, it can also be used as a proxy for continent-wide drought stress in plants, largely because the ratio of 13CO2 and 12CO2 molecules removed from the atmosphere by plants is dependent on moisture conditions.The dual-species CTDAS system varies the net exchange fluxes of both 13CO2 and CO2 in ocean and terrestrial biosphere models to create an ensemble of 13CO2 and CO2 fluxes that propagates through an atmospheric transport model. Based on differences between observed and simulated 13CO2 and CO2 mole fractions (and thus δ13C) our Bayesian minimization approach solves for weekly adjustments to both net fluxes and isotopic terrestrial discrimination that minimizes the difference between observed and estimated mole fractions.With this system, we are able to estimate changes in terrestrial δ13C exchange on seasonal and continental scales in the Northern Hemisphere where the observational network is most dense. Our results indicate a decrease in stomatal conductance on a continent-wide scale during a severe drought. These changes could only be detected after applying combined atmospheric CO2 and δ13C constraints as done in this work. The additional constraints on surface CO2 exchange from δ13C observations neither affected the estimated carbon fluxes nor compromised our ability to match observed CO2 variations. The prototype presented here can be of great benefit not only to study the global carbon balance but also to potentially function as a data-driven diagnostic to assess multiple leaf-level exchange parameterizations in carbon-climate models that influence the CO2, water, isotope, and energy balance.

Carbon sequestration by mangrove forest: One approach for managing carbon dioxide emission from coal-based power plant

Mangroves are known as natural carbon sinks, taking CO2 out of the atmosphere and store it in their biomass for many years. This study aimed to investigate the capacity of world's largest mangrove, the Sundarbans (Indian part) to sequester anthropogenic CO2 emitted from the proximate coal-based thermal power plant in Kolaghat (∼100 km away from mangrove site). Study also includes Kolkata, one of the largest metropolises of India (∼150 km away from mangrove site) for comparing micrometeorological parameters, biosphere-atmosphere CO2 exchange fluxes and atmospheric pollutants between three distinct environments: mangrove-power plant-metropolis. Hourly sampling of atmospheric CO2 in all three sites (late December 2011 and early January 2012) revealed that CO2 concentrations and emission fluxes were maximum around the power plant (360–621 ppmv, 5.6–56.7 mg m−2s−1 respectively) followed by the metropolis (383–459 ppmv, 3.8–20.4 mg m−2s−1 respectively) and mangroves (277–408 ppmv, −8.9–11.4 mg m−2s−1, respectively). Monthly coal consumption rates (41–57, in 104 ton month−1) were converted to CO2 suggesting that 2.83 Tg C was added to the atmosphere in 2011 for the generation of 7469732 MW energy from the power plant. Indian Sundarbans (4264 km2) sequestered total of 2.79 Tg C which was 0.64% of the annual fossil fuel emission from India in the same time period. Based on these data from 2010 to 2011, it is calculated that about 4328 km2 mangrove forest coverage is needed to sequester all CO2 emitted from the Kolaghat power plant.

Dissolved carbon biogeochemistry and export in mangrove-dominated rivers of the Florida Everglades

Abstract. The Shark and Harney rivers, located on the southwest coast of Florida, USA, originate in the freshwater, karstic marshes of the Everglades and flow through the largest contiguous mangrove forest in North America. In November 2010 and 2011, dissolved carbon source–sink dynamics was examined in these rivers during SF6 tracer release experiments. Approximately 80 % of the total dissolved carbon flux out of the Shark and Harney rivers during these experiments was in the form of inorganic carbon, either via air–water CO2 exchange or longitudinal flux of dissolved inorganic carbon (DIC) to the coastal ocean. Between 42 and 48 % of the total mangrove-derived DIC flux into the rivers was emitted to the atmosphere, with the remaining being discharged to the coastal ocean. Dissolved organic carbon (DOC) represented ca. 10 % of the total mangrove-derived dissolved carbon flux from the forests to the rivers. The sum of mangrove-derived DIC and DOC export from the forest to these rivers was estimated to be at least 18.9 to 24.5 mmol m−2 d−1, a rate lower than other independent estimates from Shark River and from other mangrove forests. Results from these experiments also suggest that in Shark and Harney rivers, mangrove contribution to the estuarine flux of dissolved carbon to the ocean is less than 10 %.

Diel cycling and flux of HCO3− in a typical karst spring-fed stream of southwestern China

We investigated the diel variations of the dissolved inorganic carbon, isotopic composition, and partial CO2 pressure from a karst spring (Guangcun Village, Guangxi, Southwest China) to the 1,350 m downstream profile of the stream. In addition, the carbon loss and CO2 exchange flux at the water-gas interface were also estimated. The results showed that the pH value and DO in the stream varied regularly on a daily basis with the temperature of stream water, suggesting that the photosynthesis of aquatic plants and algae is the controlling factor for the diel variations of the pH and DO. During the monitoring period, while the DIC (mainly in HCO3−) input (at spring) was relatively stable at about 4.46 mmol L−1, the concentrations of HCO3− and Ca2+ at downstream showed a diel cycle of daytime decrease and nighttime increase, with an amplitude of 22.4 %. We also found out that the CO2 degassing mainly occurred in the upper reach of the surface stream right after groundwater is exposed to the surface. The total CO2 exchange flux of the entire monitoring stream section was calculated to be 29.83 kg d−1, accounting for 17.8 % of the DIC loss, which means that approximately 4/5 of the loss was converted into organic carbon or calcite precipitation. Compared with the total carbon input at spring, this carbon loss only accounts for 6.5 % of the total carbon amount (1.4 % of which was converted into organic carbon and 1.1 % of which was degassed to the atmosphere), indicating that the DIC of karst groundwater in low order surface stream of Guancun is stable in general, with 1 % being lost to the atmosphere. This suggests that on a daily timescale, carbon loss in the form of CO2 of low order karst streams with lower gradient is much less pronounced. Key words: inorganic carbon cycle, spring-fed stream, aquatic vegetation photosynthesis, CO2 degassing, inorganic carbon flux, karst.

Variations of CO2 Exchange Fluxes Across Water-air Interface and Environmental Meaning in a Surface Stream in Subtropical Karst Area, SW China

CO2 cycle process or sources/sinks are not only the basis of understanding and responding to global climate change, but also the core of the current global climate change research. Gas exchange across water-air interface in terrestrial surface water is an important way of nutrient elements (carbon, nitrogen) exchange between aquatic ecosystems and ambient air. Escaping CO2 gas from surface water is also actively involved in the modern carbon cycle. In the material cycle in karst regions, CO2 plays a key role in karst processes, driving the formation of karst features. Karst groundwater with high water CO2 partial pressure (pCO2) often shows highly positive CO2 concentration gradient to atmosphere after it is discharged to surface, so the evaluation of CO2 exchange fluxes across karst water-air interface is important for karst carbon cycle research. This paper researched CO2 exchange fluxes across water-air interface in the karst surface stream in detail which was fed by Guancun subterranean stream in Liuzhou city, Guangxi province. Closed static chamber method and portable hand-holding CO2 sensor (GM70) were both employed in CO2 exchange fluxes monitoring. The results showed that CO2 degassing was the mainly form of CO2 exchange across the steam water-air interface. CO2 degassing flux in subterranean stream outlet (G1 site) ranged from 139.48 to 890.84 mg·(m2·h)-1 with an average of 445.72 mg·(m2·h)-1. CO2 degassing flux in stream downstream site (G2 site) ranged from 16.54 to 844.18 mg·(m2·h)-1 with an average of 159.81 mg·(m2·h)-1. The CO2 degassing flux in G1site was higher than that in G2 site. CO2 degassing fluxes in rainy season in both G1 and G2 site were higher than those in dry season. Stable carbon analysis of CO2 gas (δ13C-CO2) found that CO2 degassing from karst stream might influence air CO2 carbon isotope near water surface, which resulted in the more negative δ13C-CO2 value with the increase of CO2 degassing flux. Significant spatio-temporal variations of δ13C-CO2 were found, and the δ13C-CO2 in the rainy season was more negative than that in dry season and δ13C-CO2 in G1 site was more negative than that in G2 site. As a result of stream CO2 degassing, the hydrochemical characteristics of steam varied along stream running, which resulted in decrease of HCO3-, EC and pCO2 and increase of pH, SIc and δ13C-DIC in the stream.

Very high CO2 exchange fluxes at the peak of the rainy season in a West African grazed semi-arid savanna ecosystem

Africa is a sink of carbon, but there are large gaps in our knowledge regarding the CO2 exchange fluxes for many African ecosystems. Here, we analyse multi-annual eddy covariance data of CO2 exchange fluxes for a grazed Sahelian semi-arid savanna ecosystem in Senegal, West Africa. The aim of the study is to investigate the high CO2 exchange fluxes measured at the peak of the rainy season at the Dahra field site: gross primary productivity and ecosystem respiration peaked at values up to −48 μmol CO2 m−2 s−1 and 20 μmol CO2 m−2 s−1, respectively. Possible explanations for such high fluxes include a combination of moderately dense herbaceous C4 ground vegetation, high soil nutrient availability and a grazing pressure increasing the fluxes. Even though the peak net CO2 uptake was high, the annual budget of −229 ± 7 ± 49 g C m−2 y−1 (±random errors ± systematic errors) is comparable to that of other semi-arid savanna sites due the short length of the rainy season. An inter-comparison between the open-path and a closed-path infrared sensor indicated no systematic errors related to the instrumentation. An uncertainty analysis of long-term NEE budgets indicated that corrections for air density fluctuations were the largest error source (11.3% out of 24.3% uncertainty). Soil organic carbon data indicated a substantial increase in the soil organic carbon pool for the uppermost .20 m. These findings have large implications for the perception of the carbon sink/source of Sahelian ecosystems and its response to climate change.

Separation of biospheric and fossil fuel fluxes of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; by atmospheric inversion of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; measurements: Observation System Simulations

Abstract. National annual total CO2 emissions from combustion of fossil fuels are likely known to within 5–10 % for most developed countries. However, uncertainties are inevitably larger (by unknown amounts) for emission estimates at regional and monthly scales, or for developing countries. Given recent international efforts to establish emission reduction targets, independent determination and verification of regional and national scale fossil fuel CO2 emissions are likely to become increasingly important. Here, we take advantage of the fact that precise measurements of 14C in CO2 provide a largely unbiased tracer for recently added fossil-fuel-derived CO2 in the atmosphere and present an atmospheric inversion technique to jointly assimilate observations of CO2 and 14CO2 in order to simultaneously estimate fossil fuel emissions and biospheric exchange fluxes of CO2. Using this method in a set of Observation System Simulation Experiments (OSSEs), we show that given the coverage of 14CO2 measurements available in 2010 (969 over North America, 1063 globally), we can recover the US national total fossil fuel emission to better than 1 % for the year and to within 5 % for most months. Increasing the number of 14CO2 observations to ∼ 5000 per year over North America, as recently recommended by the National Academy of Science (NAS) (Pacala et al., 2010), we recover monthly emissions to within 5 % for all months for the US as a whole and also for smaller, highly emissive regions over which the specified data coverage is relatively dense, such as for the New England states or the NY-NJ-PA tri-state area. This result suggests that, given continued improvement in state-of-the art transport models, a measurement program similar in scale to that recommended by the NAS can provide for independent verification of bottom-up inventories of fossil fuel CO2 at the regional and national scale. In addition, we show that the dual tracer inversion framework can detect and minimize biases in estimates of the biospheric flux that would otherwise arise in a traditional CO2-only inversion when prescribing fixed but inaccurate fossil fuel fluxes.

Simultaneous assimilation of SMOS soil moisture and atmospheric CO2 in-situ observations to constrain the global terrestrial carbon cycle

Abstract Carbon dioxide (CO 2) is the most important anthropogenic greenhouse gas contributing to about half of the total anthropogenic change in the Earth's radiation budget. And about half of the anthropogenic CO2 emissions stay in the atmosphere, the remainder is taken up by the biosphere. It is of paramount importance to better understand CO2 sources and sinks and their spatio-temporal distribution. In the context of climate change this information is needed to improve the projections of future trends in carbon sinks and sources. Since the terrestrial carbon and water cycles are tightly coupled by biological plant processes, i.e. through the stomatal gas exchange with the atmosphere, it is expected that information on the soil moisture state will help to constrain terrestrial carbon fluxes. In the present feasibility study we employ the Carbon Cycle Data Assimilation System CCDAS to pioneer the assimilation of the SMOS L3 soil moisture product together with another biophysical data set — in this case atmospheric CO2 flask samples. The two data streams are assimilated into a process-based model of the terrestrial carbon cycle over two years. CCDAS aims to optimise model process parameters and subsequently land surface CO2 exchange fluxes. We find that the assimilation of SMOS data improves the agreement with independent soil moisture data from the active ASCAT instrument. In both cases the assimilation also improves the fit of modelled atmospheric CO2 to the observations at flask sampling sites which have not been used in the assimilation. Reduction of uncertainty relative to the prior is generally high for both regional net ecosystem productivity and net primary productivity and considerably higher than for assimilating CO2 only, which quantifies the added value of SMOS observations as a constraint on the terrestrial carbon cycle. The study demonstrates a high potential for a SMOS L4 carbon flux product.

Soil moisture, species composition interact to regulate CO2 and CH4 fluxes in dry meadows on the Tibetan Plateau

Alpine meadows on the Tibetan Plateau are important sinks for atmosphere carbon (C). Environmental degradation is transforming the continuous sedge (e.g., Kobresia humilis) meadows into a mosaic of sedge- and forb-dominated (e.g., Potentilla leuconota) patches, and this provides an ideal system in which to examine how the change in species composition (from sedge- to forb-dominated) affects the CO2 and CH4 fluxes and their response to environmental change. Net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), and methane (CH4) fluxes were measured monthly both in sedge- and forb-dominated patches during the growing seasons of 2011 and 2012. The seasonal variations of NEE (negative values represent net uptake of CO2) were negatively affected by soil water content (SWC) both in the sedge (R=−0.46, P=0.02) and in the forb (R=−0.71, P<0.001) sites, and the SWC regulated the NEE through exerting influence on gross ecosystem photosynthesis (GEP) (P<0.001) rather than on Reco (P>0.1). The CH4 fluxes were positively correlated with SWC for both sedge (R=0.48, P=0.016) and forb sites (R=0.70, P<0.001). Nevertheless, the CH4 emission can offset only 0.05% of the net C uptake as a result of the increase in SWC. Change in species composition did not affect the rates in NEE, Reco, and GEP (P>0.05); however, it led to the NEE of the meadow being more sensitive to SWC change (P=0.042). Change in species composition significantly reduced the CH4 flux (P=0.05), which were attributed to the decrease in soil labile carbon (e.g., microbial biomass carbon) and soil enzymatic activity (e.g., protease and urease). However, it increased the soil moisture sensitivity of CH4 flux (P=0.033). We conclude that the C sink strength of the alpine dry meadow is subject to the change in soil water availability on the Plateau, and the alpine meadow will become more vulnerable to drought with the ongoing spread of forb-dominated patches. Our results highlight the role of species composition in regulating the response of the alpine meadow C cycle to environmental change.

Carbon sequestration and decreased CO2 emission caused by terrestrial aquatic photosynthesis: Insights from diel hydrochemical variations in an epikarst spring and two spring-fed ponds in different seasons

Whether carbonate weathering could produce a stable carbon sink depends primarily on the utilization of dissolved inorganic carbon (DIC) by aquatic phototrophs (the so-called Biological Carbon Pump-BCP effect). On this basis, water temperature (T), pH, electrical conductivity (EC) and dissolved oxygen (DO) were synchronously monitored at 15-min resolution for one and two days respectively in January and October 2013 in Maolan Spring and the spring-fed midstream and downstream ponds in Maolan Nature Reserve, China. A thermodynamic model was used to link the continuous data to allow calculation of CO2 partial pressures (pCO2) and calcite saturation indexes (SIC). A floating static chamber was placed on the water surface successively at all sites to quantify CO2 exchange flux between atmosphere and water so as to evaluate the BCP effect. Results show that, in both winter and autumn, remarkable diel variations of hydrochemical parameters were present in the midstream pond where DO, pH, and SIC increased in the day and decreased during the night while EC, [HCO3−], [Ca2+] and pCO2 showed inverse changes mainly due to the metabolic processes of the flourishing submerged plants, with photosynthesis dominating in the day and respiration dominating at night. However, hydrochemical parameters in the spring and downstream pond show less change since few submerged plants developed there. It was determined that the BCP effect in the midstream pond was 285 ± 193 t C km−2 a−1 in winter and 892 ± 300 t C km−2 a−1 in autumn, indicating a potential significant role of terrestrial aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink.

Dynamics in carbon exchange fluxes for a grazed semi-arid savanna ecosystem in West Africa

The main aim of this paper is to study land–atmosphere exchange of carbon dioxide (CO2) for semi-arid savanna ecosystems of the Sahel region and its response to climatic and environmental change. A subsidiary aim is to study and quantify the seasonal dynamics in light use efficiency (ϵ) being a key variable in scaling carbon fluxes from ground observations using earth observation data. The net ecosystem exchange of carbon dioxide (NEE) 2010–2013 was measured using the eddy covariance technique at a grazed semi-arid savanna site in Senegal, West Africa. Night-time NEE was not related to temperature, confirming that care should be taken before applying temperature response curves for hot dry semi-arid regions when partitioning NEE into gross primary productivity (GPP) and ecosystem respiration (Reco). Partitioning was instead done using light response curves. The values of ϵ ranged between 0.02g carbon (C) MJ−1 for the dry season and 2.27gCMJ−1 for the peak of the rainy season, and its seasonal dynamics was governed by vegetation phenology, photosynthetically active radiation, soil moisture and vapor pressure deficit (VPD). The CO2 exchange fluxes were very high in comparison to other semi-arid savanna sites; half-hourly GPP and Reco peaked at −43μmol CO2m−2s−1 and 20μmol CO2m−2s−1, and daily GPP and Reco peaked at −15gCm−2 and 12gCm−2, respectively. Possible explanations for the high CO2 fluxes are a high fraction of C4 species, alleviated water stress conditions, and a strong grazing pressure that results in compensatory growth and fertilization effects. We also conclude that vegetation phenology, soil moisture, radiation, VPD and temperature were major components in determining the seasonal dynamics of CO2 fluxes. Despite the height of the peak of the growing season CO2 fluxes, the annual C budget (average NEE: −271gCm−2) were similar to that in other semi-arid ecosystems because the short rainy season resulted in a short growing season. Global circulation models project a decrease in rainfall, an increase in temperature and a shorter growing season for the western Sahel region, and the productivity and the sink function of this semi-arid ecosystem may thus be lower in the future.

First combined flux chamber survey of mercury and CO2 emissions from soil diffuse degassing at Solfatara of Pozzuoli crater, Campi Flegrei (Italy): Mapping and quantification of gas release

There have been limited studies to date targeting gaseous elemental mercury (GEM) flux from soil emission in enriched volcanic substrates and its relation with CO2 release and tectonic structures. In order to evaluate and understand the processes of soil–air exchanges involved at Solfatara of Pozzuoli volcano, the most active zone of Campi Flegrei caldera (Italy), an intensive field measurement survey has been achieved in September 2013 by using high-time resolution techniques. Soil–air exchange fluxes of GEM and CO2 have been measured simultaneously at 116 points, widely distributed within the crater. Quantification of gas flux has been assessed by using field accumulation chamber method in conjunction with a Lumex®-RA 915+ portable mercury vapor analyzer and a LICOR for CO2 determination, respectively. The spatial distribution of GEM and CO2 emissions correlated quite closely with the hydrothermal and geological features of the studied area. The highest GEM fluxes (from 4.04 to 5.9×10−5gm−2d−1) were encountered close to the southern part of the crater interested by an intense fumarolic activity and along the SE–SW tectonic fracture (1.26×10−6–6.91×10−5gGEMm−2d−1). Conversely, the lowest values have been detected all along the western rim of the crater, characterized by a weak gas flux and a lush vegetation on a very sealed clay soil, which likely inhibited mercury emission (range: 1.5×10−7–7.18×10−6gGEMm−2d−1). Results indicate that the GEM exchange between soil and air inside the Solfatara crater is about 2–3 orders of magnitude stronger than that in the background areas (10−8–10−7gm−2d−1). CO2 soil diffuse degassing exhibited an analogous spatial pattern to the GEM fluxes, with emission rates ranging from about 15 to ~20,000gCO2m−2d−1, from the outermost western zones to the south-eastern sector of the crater. The observed significant correlation between GEM and CO2 suggested that in volcanic system GEM volatilizes from substrate in a similar manner to the release of CO2. The quantitative estimation of the total amount of CO2 and GEM released from the Solfatara crater gave values of about 304±13 and 3.7±0.2×10−6td−1, respectively. Finally, based on our dataset and previous work, we propose that an average GEM/CO2 molar ratio of ~2×10−8 (n=9) is best representative of hydrothermal degassing. Taking into account the uncertainty in global hydrothermal CO2 emissions from sub-aerial environments (~1012Molyr−1), we infer a global volcanic GEM flux from hydrothermal environments of ~about 8.5tyr−1. Although this value has to be considered as a lower limit for the global emission of GEM from these sources, we suggest that on a local scale hydrothermal activity can be regarded as a significant source of GEM than previously recognized to the atmospheric pool.

Air-sea CO2 exchange process in the southern Yellow Sea in April of 2011, and June, July, October of 2012

The partial pressure of CO2 (pCO2) and air-sea CO2 exchange flux (FCO2) in the southern Yellow Sea (SYS, 120–125°E, 31.5–37°N) were investigated basing on the field surveys conducted in April of 2011, and June, July, October of 2012. With significant spatial variations, surface pCO2 ranged from 243 to 574μatm, 206 to 620μatm, 102 to 655μatm and 328 to 557μatm in April, June, July and October, respectively. Nearshore area of Shandong Peninsula and Jiangsu Shallow (depth<50m) were pCO2-supersaturated (pCO2=400–600μatm), as the result of intensive water mixing which brought the bottom CO2-rich water to the surface layer. Conversely, offshore area of SYS center (depth>50m) was pCO2-undersaturated (pCO2<390μatm) in April, June and October, but supersaturated in July. Phytoplankton production sustained by abundant nutrient and suitable hydrodynamic conditions was of great importance for this undersaturated pCO2. Moreover, extreme low pCO2 (pCO2<300μatm) was observed in the Changjiang plume (32.5–33.5°N, 123–125°E) in July, which was also related with the biological uptake of CO2. Average air-sea CO2 exchange flux of the SYS in April, June, July and October was −3.16±0.40mmolm−2d−1,−4.56±0.34mmolm−2d−1, −0.36±0.51mmolm−2d−1, and 6.67±0.57mmolm−2d−1, respectively. As a whole, the SYS behaved as a weak CO2 sink during April to October, with an average flux for about −0.35mmolm−2d−1. As for the controlling factors for pCO2 variation, temperature played the dominant role in October, whereas the non-temperature factors, such as vertical mixing, Changjiang plume and biological activity, were considered as the primary controlling factors in June and July. Spatially, the control of temperature on pCO2 was predominant in the offshore SYS; the non-temperature factors were predominant in the shallow nearshore area, especially in coast of Shandong Peninsula and the Jiangsu Shallow.