Net Source Of CO2 Research Articles

The initial three years of carbon dioxide exchange between the atmosphere and a reclaimed oil sand wetland

Northern peatlands contain up to 20% of the ∼3000 Pg of global soil organic carbon. Carbon-rich peatlands cover upwards of 65% of the landscape in northern Canada where resource extraction activities disturb both the carbon pools and the future carbon sequestration capacity of the landscape. Previous estimates of the carbon losses from this disturbance predict a complete loss of the region’s peatland carbon pool. Mining industries operating in these sensitive environments have recently begun constructing closure landscapes which are intended to develop carbon cycle processes similar to undisturbed northern peatlands. This study investigates eddy covariance fluxes of carbon dioxide (CO2) at one of Canada’s first fully constructed boreal plains watersheds, the Sandhill Fen Watershed. During the first three years since inception, only the lowland region had an annual net ecosystem exchange of CO2 (NEE) indicative of increasing CO2 sink potential. The lowland region was characterized by saturated salvaged peat soils, standing water, thriving communities of Typha and Carex spp. and was a net CO2 sink of 77 g C m−2 in the third year. At the same time the upland and the midland regions characterized by moist salvaged peat soils and a mix of herbaceous, shrub and planted Picea glauca and Pinus banksiana remained net sources of CO2. Despite similar rates of gross primary production, ecosystem and plot-level respiration rates in the lowland were significantly lower than in the midland region, likely due to very low reduction potentials within the lowland’s saturated soils. With no other significant outflows of carbon, the lowland of the Sandhill Fen Watershed may be in the early stages of organic matter accumulation. Due to limited oxidation of the salvaged peat substrate in the lowland region, wetland reclamation employing these techniques may reduce the disturbance loss of the carbon pool in the boreal plains.

The role of porewater exchange as a driver of CO2 flux to the atmosphere in a temperate estuary (Squamish, Canada)

Porewater exchange is an important yet poorly understood component of the coastal carbon cycle. Here, a high-resolution automated radon (222Rn, a natural porewater tracer) and CO2 time series was conducted in the Squamish Central Estuary (Canada) over eight consecutive tidal cycles to assess the relative importance of porewater exchange on estuarine carbon dynamics. Radon and CO2 observations revealed a clear tidal trend which is indicative of porewater exchange driven by tidal pumping. A radon mass balance indicated an average porewater exchange rate of 14.9 cm day−1 (4.3% of the tidal prism). The estuary was a net source of CO2 to the atmosphere (average 212 ± 19 mmol m−2 day−1). Porewater exchange accounted for 9%, 5% and 30% of net dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and CO2 exported out of the Squamish Central Estuary, respectively, while porewater inputs of free CO2 accounted for 38% of the atmospheric evasion. These flux estimates as well as strong correlations between pCO2 and 222Rn suggest that porewater exchange has a strong influence on CO2 concentrations in the estuary even though they are a small contributor to overall DIC fluxes.

Hawaii Coastal Seawater CO2 Network: A Statistical Evaluation of a Decade of Observations on Tropical Coral Reefs

A statistical evaluation of nearly ten years of high-resolution surface seawater carbon dioxide partial pressure (pCO2) time-series data collected from coastal moorings around O’ahu, Hawai’i suggest that these coral reef ecosystems were largely a net source of CO2 to the atmosphere between 2008 and 2016. The largest air-sea flux (1.24 ± 0.33 mol m-2 yr-1) and the largest variability in seawater pCO2 (950 μatm overall range or 8x the open ocean range) were observed at the CRIMP-2 site, near a shallow barrier coral reef system in Kaneohe Bay O’ahu. Two south shore sites, Kilo Nalu and Ala Wai, also exhibited about twice the surface water pCO2 variability of the open ocean, but had net fluxes that were much closer to the open ocean than the strongly calcifying system at CRIMP-2. All mooring sites showed the opposite seasonal cycle from the atmosphere, with the highest values in the summer and lower values in the winter. Average coastal diurnal variabilities ranged from a high of 192 µatm/day to a low of 32 µatm/day at the CRIMP-2 and Kilo Nalu sites, respectively, which is one to two orders of magnitude greater than observed at the open ocean site. Here we examine the modes and drivers of variability at the different coastal sites. Although daily to seasonal variations in pCO2 and air-sea CO2 fluxes are strongly affected by localized processes, basin-scale climate oscillations also affect the variability on interannual time scales.

Automated measurements of greenhouse gases fluxes from tree stems and soils: magnitudes, patterns and drivers

Tree stems exchange CO2, CH4 and N2O with the atmosphere but the magnitudes, patterns and drivers of these greenhouse gas (GHG) fluxes remain poorly understood. Our understanding mainly comes from static-manual measurements, which provide limited information on the temporal variability and magnitude of these fluxes. We measured hourly CO2, CH4 and N2O fluxes at two stem heights and adjacent soils within an upland temperate forest. We analyzed diurnal and seasonal variability of fluxes and biophysical drivers (i.e., temperature, soil moisture, sap flux). Tree stems were a net source of CO2 (3.80 ± 0.18 µmol m−2 s−1; mean ± 95% CI) and CH4 (0.37 ± 0.18 nmol m−2 s−1), but a sink for N2O (−0.016 ± 0.008 nmol m−2 s−1). Time series analysis showed diurnal temporal correlations between these gases with temperature or sap flux for certain days. CO2 and CH4 showed a clear seasonal pattern explained by temperature, soil water content and sap flux. Relationships between stem, soil fluxes and their drivers suggest that CH4 for stem emissions could be partially produced belowground. High-frequency measurements demonstrate that: a) tree stems exchange GHGs with the atmosphere at multiple time scales; and b) are needed to better estimate fluxes magnitudes and understand underlying mechanisms of GHG stem emissions.

Assessing the impact of peat erosion on growing season CO2 fluxes by comparing erosional peat pans and surrounding vegetated haggs

Peatlands are recognised as an important but vulnerable ecological resource. Understanding the effects of existing damage, in this case erosion, enables more informed land management decisions to be made. Over the growing seasons of 2013 and 2014 photosynthesis and ecosystem respiration were measured using closed chamber techniques within vegetated haggs and erosional peat pans in Dartmoor National Park, southwest England. Below-ground total and heterotrophic respiration were measured and autotrophic respiration estimated from the vegetated haggs. The mean water table was significantly higher in the peat pans than in the vegetated haggs; because of this, and the switching from submerged to dry peat, there were differences in vegetation composition, photosynthesis and ecosystem respiration. In the peat pans photosynthetic CO2 uptake and ecosystem respiration were greater than in the vegetated haggs and strongly dependent on the depth to water table (r2 > 0.78, p < 0.001). Whilst in the vegetated haggs, photosynthesis and ecosystem respiration had the strongest relationships with normalised difference vegetation index (NDVI) (r2 = 0.82, p < 0.001) and soil temperature at 15 cm depth (r2 = 0.77, p = 0.001). Autotrophic and total below-ground respiration in the vegetated haggs varied with soil temperature; heterotrophic respiration increased as water tables fell. An empirically derived net ecosystem model estimated that over the two growing seasons both the vegetated haggs (29 and 20 gC m−2; 95% confidence intervals of − 570 to 762 and − 873 to 1105 gC m−2) and the peat pans (7 and 8 gC m−2; 95% confidence intervals of − 147 to 465 and − 136 to 436 gC m−2) were most likely net CO2 sources. This study suggests that not only the visibly degraded bare peat pans but also the surrounding vegetated haggs are losing carbon to the atmosphere, particularly during warmer and drier conditions, highlighting a need for ecohydrological restoration.

The Great Barrier Reef: A source of CO2 to the atmosphere

The Great Barrier Reef (GBR) is the largest contiguous coral reef system in the world. Carbonate chemistry studies and flux quantification within the GBR have largely focused on reef calcification and dissolution, with relatively little work on shelf-scale CO2 dynamics. In this manuscript, we describe the shelf-scale seasonal variability in inorganic carbon and air-sea CO2 fluxes over the main seasons (wet summer, early dry and late dry seasons) in the GBR.Our large-scale dataset reveals that despite spatial-temporal variations, the GBR as a whole is a net source of CO2 to the atmosphere, with calculated air–sea fluxes varying between −6.19 and 12.17 mmol m−2 d−1 (average ± standard error: 1.44 ± 0.15 mmol m−2 d−1), with the strongest release of CO2 occurring during the wet season. The release of CO2 to the atmosphere is likely controlled by mixing of Coral Sea surface water, typically oversaturated in CO2, with the warm shelf waters of the GBR. This leads to oversaturation of the GBR system relative to the atmosphere and a consequent net CO2 release.

CO2 emissions from hydroelectric reservoirs in the Tigris River basin, a semi-arid region of southeastern Turkey

Three hydroelectric reservoirs (Kralkızı, Dicle and Batman) in the Tigris River basin (Turkey) were sampled monthly during one year in order to reveal spatial and seasonal changes in aqueous partial pressure of CO2 (pCO2) and to estimate diffusive fluxes of CO2 from the reservoirs’ surface water. pCO2 concentrations did not show significant spatial differences, while they showed significant seasonal variations. Temperature, precipitation and biological CO2 uptake through photosynthesis controlled pCO2 seasonality in the reservoirs, with maximal concentrations in the winter (ranging from 516.9 µatm in Kralkızı to 1299.2 µatm in Dicle) and minimal concentrations in the spring (ranging from 47.7 µatm in Batman to 140.7 µatm in Kralkızı). Most studies reported that reservoirs worldwide are net sources of CO2 to the atmosphere. However, the reservoirs in this study were sinks for atmospheric CO2 during the spring, summer and autumn seasons, while they were CO2 sources to the atmosphere during the winter. Air-water CO2 fluxes in Kralkızı, Dicle and Batman dam reservoirs were 2.39, 32.88 and 8.12 mmol/m2/day in the winter, respectively. On an annual basis, all three reservoirs acted as a sink for atmospheric CO2. These estimated CO2 fluxes were in the lower range for temperate reservoirs, despite the potential for winter conditions that shifted the reservoirs from sink to net source for atmospheric CO2.

Seasonality of pCO2 in a hard‐water lake of the northern Great Plains: The legacy effects of climate and limnological conditions over 36 years

AbstractBiogeochemical processes are active year‐round in ice‐covered lakes, such that processes in one season can affect limnological conditions in subsequent seasons. However, the extent and nature of these legacy effects are poorly understood, particularly for the CO2 content of lakes and when considering gas exchange with the atmosphere. Here, we used a unique 36‐yr dataset of weekly limnological measurements of Buffalo Pound Lake in the northern Great Plains to assess seasonal changes in CO2 concentration and flux and determine how dependent lake pCO2 is on limnological conditions of previous seasons. We found that the lake was a net source of CO2 to the atmosphere (mean 18.5 ± 7.4 mol CO2 m−2 yr−1), with spring potentially accounting for the majority (~ 64% ± 20%) of CO2 efflux, assuming ice in spring was permeable to gas exchange (32.9% ± 19.8% if not). Analysis with generalized additive models (GAMs) demonstrated that current and antecedent seasonal conditions combined to explain 72.6% of deviance in spring pCO2, but that the strength of model predictions and the importance of antecedent conditions diminished in GAMs of summer (43.6%) and fall (23.3%) CO2 levels. This research suggests that pCO2 is regulated by a combination of coeval and historical environmental conditions and shows that quantification of seasonal and annual fluxes requires a mechanistic understanding of the legacy effects of preceding time intervals.

Modelling seawater carbonate chemistry in shellfish aquaculture regions: Insights into CO2 release associated with shell formation and growth

Mollusc aquaculture is a high-value industry that is increasing production rapidly in Europe and across the globe. In recent years, there has been discussion of the potential wide-ranging environmental benefits of this form of food production. One aspect of mollusc aquaculture that has received scrutiny is the production of calcareous shells (CaCO3). Mollusc shell growth has sometimes been described as a sink for atmospheric CO2, as it locks away carbon in solid mineral form. However, more rigorous carbonate chemistry modelling, including concurrent changes in seawater pCO2, pH, dissolved inorganic carbon, and total alkalinity, shows that calcification is a net CO2 source to the atmosphere. Combined with discussions about whether mollusc respiration should be included in carbon footprint modelling, this suggests that greater in-depth understanding is required before shellfish aquaculture can be included in carbon trading schemes and footprint calculations. Here, we show that regional differences in the marine carbonate system can alter the amount of CO2 released per unit CaCO3 formation. Our carbonate chemistry modelling shows that a coastal mussel farm in southern Portugal releases up to ~0.290 g of CO2 per g of CaCO3 shell formed. In comparison, an identical farm in the coastal Baltic Sea would produce up to 33% more CO2 per g of CaCO3 (~0.385 g-CO2·(g-CaCO3)−1). This spatial variability should therefore also be considered if mollusc aquaculture is to be included in future carbon trading schemes, and in planning future expansion of production across the industry.

Greenhouse Gas Production and Transport in Desert Soils of the Southwestern United States

AbstractDeserts comprise a large portion of the Earth's land area, yet their role in the fluxes and cycles of greenhouse gases is poorly known and their likely response to climate change largely unexplored. We report a reconnaissance investigation of the concentrations and fluxes of CO2, CH4, and N2O along two elevation (climate) gradients in the southwestern United States. In‐soil concentrations of CO2 increased with elevation (up to 5,000 ppm). Concentrations of CH4 declined with depth in all soils (to less than 1 ppm), but the rates of decrease with depth increased with elevation. In contrast, concentrations and depth trends of N2O varied erratically. Soils were net CO2 sources (0 to &gt;1,500 kg CO2·ha−1·year−1), and net CH4 sinks (0.2 to &gt;3 kg CH4·ha−1·year−1). The small and variable N2O fluxes were inconsistent with the trends in soil N δ15N values, which decreased by 5‰ to 6‰ over about 1,000 m of elevation. The high soil N δ15N values (up to nearly 17‰ at the lowest elevation) indicate that there is a soil N loss mechanism that is highly depleted in 15N, and gaseous losses—either NH3 or N2O/N2—are suspected of driving these values. In summary, there appears to be a strong climate control on both soil CO2 and CH4 concentrations and to a lesser degree on calculated fluxes. The soil N trace gas concentrations indicate that deserts can be either small sources or sinks of N2O and that there may be significant consumption of arid soil N2O.

Seasonal Trends in Surface pCO2 and Air‐Sea CO2 Fluxes in Apalachicola Bay, Florida, From VIIRS Ocean Color

AbstractEstuaries have been recognized as important sources of carbon dioxide (CO2) to the atmosphere; however, contributions of these systems to regional and global carbon budgets are not well constrained due to limited information on seasonal and spatial variability. In this study, we use satellite remote sensing to obtain seasonal pCO2 distribution and air‐sea CO2 fluxes in Apalachicola Bay, a national estuarine research reserve located in the northern Gulf of Mexico that receives seasonally varying dissolved organic matter‐rich waters from the Apalachicola River. A combination of time series (2005–2016) and seasonal field observations (2015–2016) of pH and biophysical variables were used to develop seasonal pH‐pCO2 relationships for obtaining surface pCO2 estimates and air‐sea CO2 fluxes from Visible and Infrared Imaging Radiometer Suite (VIIRS) ocean color data. Monthly and seasonal maps of pCO2 and air‐sea CO2 fluxes showed a general trend of higher fluxes in winter and summer corresponding to high river flow and warm water temperatures. However, CO2 fixation via photosynthesis and low water temperatures contributed to lower fluxes to the atmosphere in spring and fall, respectively. Throughout the study period, Apalachicola Bay was a net source of CO2 with large seasonal and spatial variability and a mean annual CO2 flux to the atmosphere of 3.4 ± 3.1 mol·m−2·year−1 (9.4 ± 8.5 mmol·m−2·day−1), consistent with fluxes reported for other estuaries. This study demonstrates the critical role that satellite observations can play to improve the estuarine contributions to the global carbon flux estimates.

The impact of rain events on CO2 emissions from contrasting land use systems in semi-arid West African savannas

In the future the Sudanian savanna – one of West Africa's high-potential “bread baskets” – will likely face shorter rainy seasons with more extreme rains and droughts. That could have serious impacts on the vegetation and its carbon dioxide (CO2) exchange with potentially increasing CO2 emissions accelerating climate warming. Understanding how the CO2 fluxes in this area respond to environmental variables, in particular rain events, is essential, but available data are scarce. In this study, we monitored net ecosystem exchange (NEE) of CO2, rainfall and other environmental parameters during four years at three savannas. Savannas were characterized by different vegetation due to different land use: i) woody and nearly pristine, ii) mixture of cropland and grassland and iii) intensive grazing. The impact of rain events on CO2 exchange for these contrasting ecosystems were analyzed for single rain events (short-term) and on a yearly time scale (long-term) using three eddy covariance towers. We found that the woody pristine savanna site was a prominent sink of CO2 (−864 to −1299 g CO2 m−2 y−1) while the degraded sites were net CO2 sources (118 to 605 g CO2 m−2 y−1) with a complicated relation with annual rainfall amounts. The NEE responses to single rain events revealed that daytime rain systematically decreased the sink strengths at all sites, which might be associated with decreased light availability. At the degraded sites, additional factors increasing CO2 losses were rain duration and dry spell length. The observed patterns of immediate CO2 flux responses to rainfall at differently used savannas indicate strong internal feedbacks between vegetation and land use changes and raise the question whether the CO2 sink strengths might be overestimated with possible implications for global CO2 budgets. Sustainable adaptation strategies need to be developed for West Africa.

Factors controlling evaporation and the CO2 flux over an open water lake in southwest of China on multiple temporal scales

Interactions between lakes and the atmosphere are poorly understood, especially for lakes in highland areas. A better understanding of the physical processes controlling turbulent exchange dynamics over lakes is crucial to improve the numerical modelling of the lake–climate interaction. Four years of continuous measurements of energy and CO2 fluxes are conducted over a shallow highland lake (Lake Erhai) in China using eddy covariance (EC) technique from 2012 to 2015. It is found that the latent heat flux (LE) and sensible heat flux (H) has distinct diurnal and seasonal variations. The diurnal H and LE ranges from −31 to 21 W/m2 and from 31 to 171 W/m2, respectively. The heat is stored in the lake from January to June and released from July to the end of the year. The annual total evaporation of Lake Erhai varies from 1120.8 to 1228.5 mm/a, which is always higher than the annual total precipitation. On diurnal scale, the CO2 uptake is observed during most of the midday period. On an annual scale, the Lake Erhai behaves as a net CO2 source, with an annual CO2 budget ranging from 117.5 to 161.7 g C m−2 a−1. The temperature difference (ΔT) between the water surface (T s) and the air temperature (T a), and the product of ΔT and wind speed (U) are the major drivers of H from half‐hourly to monthly scale. The main factors controlling LE are U and the product of U and the vapour pressure difference (VPD) on half‐hourly and daily scales, the total cloud cover, and net radiation (R n) on the monthly scale. Photosynthetic active radiation (PAR) and U has a close relationship with half‐hourly and daily CO2 fluxes, respectively, while annual precipitation and T s has a larger effect on annual CO2 fluxes.

The role of Eurasian beaver (Castor fiber) in the storage, emission and deposition of carbon in lakes and rivers of the River Ob flood plain, western Siberia

Several studies have reported significant emission of greenhouse gasses (GHG) from beaver dams, suggesting that ponds created by beavers are a net source of CO2 and CH4. However, most evidence come from studies conducted in North America (on Castor canadensis) without a parallel comparison with the Eurasian beaver's (Castor fiber) impacts and a critical consideration of the importance of the carbon deposition in dam sediments. The most abundant population of the Eurasian beaver lives in Russia, notably within the River Ob watershed in Western Siberia which is the second largest floodplain on Earth. Consequently, we assessed the holistic impact of Eurasian beavers on the multiple carbon pools in water and on other related biogeochemical parameters of the Ob's floodplain streams. We compared dammed and flowing streams in a floodplain of the middle course of the river. We found that beavers in western Siberia increase the stream emission of methane by about 15 times by building their dams. This is similar to what has been documented in North America. A new finding from the present study is that Siberian beavers facilitate 1) nutrient recycling by speeding up the nutrient release from particulate organic matter; and 2) carbon sequestration by increasing the amount of dissolved organic carbon. This carbon becomes in part recalcitrant when buried in sediments and is, therefore, removed from the short-term carbon cycle. These new results should be taken into consideration in river management and provide a further reason for the conservation and management of Eurasian Beavers.

Response of hydrology and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux to experimentally altered rainfall frequency in a temperate poor fen, southern Ontario, Canada

Abstract. Predicted changes to the precipitation regime in many parts of the world include intensifying the distribution into lower frequency, large magnitude events. The corresponding alterations to the soil moisture regime may affect plant growth and soil respiration, particularly in peatlands, where large stores of organic carbon are due to gross ecosystem productivity (GEP) exceeding ecosystem respiration (ER). This study uses lab monoliths corroborated with field measurements to examine the effect of changing rainfall frequency on peatland moisture controls on CO2 uptake in an undisturbed cool temperate poor fen. Lab monoliths and field plots containing mosses, sedges, or shrubs received either 2.3, 1, or 0.5 precipitation events per week, with total rainfall held constant. Decreasing rain frequency led to lower near-surface volumetric moisture content (VMC), water table (WT), and soil tension for all vegetation types, with minimal effect on evapotranspiration. The presence of sedges in particular led to soil tensions of ≥100 cm of water for a sizeable duration (37 %) of the experiment. Altered rainfall frequencies affected GEP but had little effect on ER; overall, low-frequency rain led to a reduced net CO2 uptake for all three vegetation types. VMC had a strong control on GEP and net ecosystem exchange (NEE) of the Sphagnum capillifolium monoliths, and decreasing rainfall frequency influenced these relationships. Overall, communities dominated by mosses became net sources of CO2 after 3 days without rain, whereas sedge communities remained net sinks for up to 14 days without rain. The results of this study demonstrate the hydrological controls of peatland CO2 exchange dynamics influenced by changing precipitation frequency; furthermore, they suggest these predicted changes in frequency will lead to increased sedge GEP but limit the carbon-sink function of peatlands.

Seasonal controls on ecosystem-scale CO2 and energy exchange in a Sonoran Desert characterized by the saguaro cactus (Carnegiea gigantea).

Episodic precipitation pulses are important for driving biological activity in desert ecosystems. The pattern of precipitation, including the size of rain events and the duration of time between events, can influence ecosystem net CO2 exchange (NEE) by shifting the balance between ecosystem photosynthesis and respiration. Our objective was to measure the response of NEE and its major components, to seasonal variation in precipitation and other environmental conditions. The study was conducted at a site, where 40-60% of annual precipitation comes from the North American Monsoon that typically brings rain in July-September, a time period when temperatures are near the seasonal peak. The results were compared to a model of the expected responses of NEE to seasonal changes in precipitation and temperature. We measured NEE using the eddy covariance technique during September 2015-August 2016. The ecosystem showed large (fivefold) seasonal variation in maximum photosynthesis and ecosystem respiration rate at 10°C that corresponded to seasonal variation in precipitation and temperature. Ecosystem respiratory activity exceeded photosynthetic activity, so the ecosystem was a net source of CO2 to the atmosphere during June-October, a period that included monsoon rain inputs. Only during the winter months (November-March) did photosynthesis exceed respiration, resulting in net ecosystem carbon sequestration. The ecosystem recorded a net loss of 10gCm-2year-1, which was likely caused by below normal annual precipitation during the study. Our results illustrated the important interaction between seasonal variation in precipitation and temperature in controlling the ecosystem carbon budget.

Air-sea CO2 fluxes and spatial distribution of seawater pCO2 in Yongle Atoll, northern-central South China Sea

Yongle Atoll is a complex atoll composed of 13 separated small atolls/reefs in the Xisha Islands, northern-central South China Sea. Wide, deep channels connect the Yongle Atoll lagoon with the open ocean. The sea surface partial pressure of CO2 (pCO2) and sea–air CO2 flux was measured at different geomorphological locations on the atoll. Atmospheric pCO2 generally showed small diurnal variations, while seawater pCO2 showed large diurnal variations. Lingyang Reef lagoon, an atoll within Yongle Atoll, was used to check the spatial difference in seawater pCO2 and air–sea CO2 flux. The diurnal variation in seawater pCO2 was greater in the lagoon (258–748 μatm) than outside the lagoon area (354–476 μatm). Differences were also observed in the air–sea CO2 fluxes depending on geomorphological zone. The mean seawater pCO2 at Yongle Atoll is in the order of Lingyang Reef lagoon > inner reef slope of the Yongle Atoll > Yongle Atoll channel slope ≈ outer reef slope of the Yongle Atoll. The degree mixing between the lagoon and open sea through channels may have a key role in the horizontal distribution of seawater pCO2 in the Yongle Atoll. Additional analyses indicated that the daily variations in surface seawater pCO2 are significantly influenced by biological metabolic processes. The mean air–sea CO2 flux in the Yongle Atoll was ~ 3.2 mmol CO2 m−2 d−1, indicating that the Yongle Atoll acts as a net source of CO2 to the atmosphere.

CO2 oversaturation and degassing using chambers and a new gas transfer velocity model from the Three Gorges Reservoir surface

Reservoirs are considered as important carbon source of the atmosphere, whilst, regional and global reservoir CO2 quantification is hampered by data limitation and bias in spatial and temporal sampling. By deploying chamber measurements and employing the newly developed model of gas transfer velocity, CO2 partial pressure (pCO2) and evasion in the main stem of the Three Gorges Reservoir (TGR) were investigated. The pCO2 ranged from 429 to 8668 μatm with an average of 2511.6 ± 1721.3 μatm, 6.1-fold higher than the ambient air pCO2 (mean: 410 μatm). All the samples were net CO2 sources via water-air interface, displaying pronounced spatial and monthly variability. The CO2 areal flux averaged 212.5 ± 120.1 mmol/m2/d in June, 123.3 ± 78.5 mmol/m2/d in July in the lotic TGR main stream, much higher than its lentic system, i.e., 79.6 ± 41.3 mmol/m2/d in November, and 76.3 ± 88.1 mmol/m2/d in March. Much lower k levels in the lentic reservoir surface resulted in lower CO2 evasion rates. Furthermore, dam impoundment considerably altered the riverine carbon cycle, as reflected by the changing magnitude of CO2 efflux and environmental controls of dissolved CO2. Precipitation and concurrent soil CO2 influx exhibited a central role in controlling riverine pCO2, and respiration of allochthonous organic carbon was a secondary factor in the TGR lotic system, whilst, both in-stream metabolism and terrestrial inputs played crucial roles in controlling aqueous CO2 in the TGR lentic system. In comparison, we provided key findings of k model and more reliable CO2 quantification with a consideration of water level shifts and a complete coverage of spatial sampling. Our higher CO2 emission (1.47 (1.16–2.13) Tg CO2/y) than previous studies called more field measurements to assess the resulting changes in CO2 flux owing to dam operation and changing environment, and their implications for regional carbon budgets should be warranted.

Water geochemistry of rivers draining karst-dominated regions, Guangxi province, South China: Implications for chemical weathering and role of sulfuric acid

Guangxi province, located in South China, is a typical karst region affected by acid depositions for a couple of decades. Water samples from the major river basins in this region were collected and the chemical compositions were measured with a main purpose to quantify chemical weathering rates and associated CO2 budget. The average total dissolved solids (TDS) of the river waters (232 mg/l) are comparable with other karstic rivers. The major ion compositions of the river waters are characterized by the dominance of Ca2+, Mg2+, HCO3−, and are significantly rich in SO42−. The chemical compositions of the river waters indicate that four major reservoirs (atmospheric, anthropogenic, carbonate and silicate inputs) contribute to the dissolved loads. The carbonate chemical weathering rates range from 61.4 t/km2/yr for the Qingshuihe river basin to 93.3 t/km2/yr for the Zuojiang river basin. The silicate chemical weathering rates range from 2.9 t/km2/yr for the Qingshuihe river basin to 7.9 t/km2/yr for the Liujiang river basin. The CO2 consumption rates by carbonate weathering range from 559 × 103 mol/km2/yr for the Qingshuihe river basin to 817 × 103 mol/km2/yr for the Zuojiang river basin. The CO2 consumption rates by silicate weathering range from 19 × 103 mol/km2/yr for the Zuojiang river basin to 85 × 103 mol/km2/yr for the Liujiang river basin. Sulfuric acid is involved as a proton donor in weathering reactions, and about 9–15% of the chemical weathering rates of the river basins are sulfuric acid-induced. The CO2 consumption rates are only 85–91% of the value when assuming carbonic acid provides all the protons in the weathering reactions. The CO2 released from sulfuric acid-induced carbonate weathering would exceed the CO2 consumed by silicate weathering, making the river basins in Guangxi net CO2 sources on geological timescales. The total net CO2 releasing flux is approximately 15.7 × 109 mol/yr (0.19 × 1012 gC/yr). This result quantitatively highlights the fact that CO2 budget by chemical weathering will be largely modified by sulfuric acid, especially in seriously acid deposition affected region.

Carbon biogeochemistry of a flooded Pantanal forest over three annual flood cycles

The Pantanal is the largest wetland in the world and yet little is known about the variability in carbon (C) dynamics across its flood seasons. We examined the effect of inundation on the C cycle in the 2013–2015 flood cycles illustrated by dissolved CO2, CH4, organic C (DOC) concentration measurements, and optical properties of dissolved organic matter (DOM) evaluated by absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC). During the 2015 flood cycle, pCO2 varied between 5973 and 14,292 μatm, with pCH4 concentrations ranging between 2956 and 51,675 μatm respectively, with high temporal variability for both gases. The supersaturation of CO2 and CH4 in relation to the atmospheric equilibrium caused the system to behave as a net source of CO2 and CH4 to the atmosphere with evasion rates of 320 mg CO2 m−2 d−1 and 20 mg CH4 m−2 d−1, respectively. Mean DOC concentration was 7.0 ± 0.4 mg L−1 and did not differ between flood cycles. Higher concentrations of DOC were measured at the start (rising floodwaters) and at the end (receding floodwaters) of flood cycles, while lower DOC concentrations were observed during the peak flood. The PARAFAC analysis indicated the presence of five DOM components: humic (C1 and C2) and fulvic type material (C3) showed the highest relative abundance (68.5% of the total PARAFAC component fluorescence), as well as protein-like material (C4 and C5) derived from microorganisms. Our measured diffusive flux levels were below the range of emissions found for wetlands and floodplains for CO2, but were slightly higher for CH4 relative to other studies in lakes and seasonally flooded areas of the Pantanal. The large variations in concentrations of CO2, CH4 and DOC and the optical properties of DOM during the course of each flood cycle suggest a close relationship between carbon and water cycles in this tropical wetland.