CO2 Dynamics Research Articles

Dynamics of Picea abies mortality and CO2 and CH4 fluxes from spruce trees decomposition in the southwest of the Valdai Upland, Russia

A mass decline of Picea abies (hereinafter – spruce), often associated with outbreaks of Ips typographus, is one of the main reasons for the reduction of spruce forests. In turn, dry and fallen trees can be both stock and source of greenhouse gases at various stages of decomposition. In our study, using an unmanned aerial vehicle, we evaluated the dynamics of spruce decline in two forest types in the southwest of the Valdai Upland (Central Forest State Nature Reserve, Russia), namely Sphagnum-bilberry forests and nemoral spruce forests. It was found that the rate of decline in Sphagnum-bilberry spruce forest was much higher than in nemoral spruce forest. By the fourth year after a windfall on 0.13 km2, 913 spruce individuals had withered in Sphagnum-bilberry forest and 66 ones in the nemoral spruce forest. Based on direct measurements of greenhouse gas fluxes by chamber method on dead trunks and coarse woody debris, it was found that in relative values the highest amount of CO2 is emitted by coarse woody debris of the decay classes 3–4 (800–1800 mg CO2 × m-2 × h-1). Deadwood and coarse woody debris from the first decay classes are assumed to be a source of CH4 (0.0008–0.0070 mg CO2 × m-2 × h-1), and from classes 3–5 they are a stock (from -0.0070 mg CO2 × m-2 × h-1 to -0.0009 mg CO2 × m-2 × h-1). When converted to the total surface areas of deadwood and coarse woody debris of the study sites, it was found that coarse woody debris of the decay classes 3–5 (2.3–13.6 kg CO2 × h-1) made the highest contribution to the integral CO2 emission, and deadwood (67 mg CH4 × h-1) made the highest contribution to the CH4 emission. Significant differences in greenhouse gas fluxes were found both between deadwood and decay classes of coarse woody debris, and between fluxes from deadwood and coarse woody debris of individual decay classes in various forest types. The results have shown the importance of considering deadwood and all available decay classes of coarse woody debris when estimating greenhouse gas fluxes from dead timber and the contribution of debris to the carbon cycle in forest ecosystems.

Carbon Dioxide and Methane Distribution in Peat Deposits of an Oligotrophic Forest Swamp in Western Siberia and Their Emission

Swamps and wetlands of forested areas, as well as the non-forest swamps are jointly involved in the global carbon cycle. They play an important role in depositing the greenhouse gases. This article analyses the long-term dynamics of the greenhouse gases (CO2 and CH4) concentration in a peat deposit and their emission in a natural and reclaimed oligotrophic bog in the southern taiga zone of Western Siberia. It was found that a significant role in the CO2 and CH4 concentration dynamics in the peat deposits belongs to the weather conditions of each month of the warm period, the botanical composition and the activity of biochemical processes, as well as the structure and physical and mechanical properties of a peat deposit. Also identified were the intra-deposit, spatial and temporal dynamics of the greenhouse gases concentrations’ indicators. It has been proven that there are multicomponent dependences of CO2 and CH4 concentrations on the parameters of external and intra-deposit conditions. The concentrations’ parameters in the oligotrophic bog’s peat deposit from the swampy catch basin of the Klyuch river were determined as follows: extreme concentrations of CO2 were 0.002–3.64 mmol/dm3, extreme concentrations of CH4 were 0.003–2.03 mmol/dm3; the CO2 normalised fluxes varied from /–22.2/ to 157.8 mg C/(m2 · h), the CH4 normalised fluxes’ extreme values were equal /–3.0/–5.3 mg C/(m2 · h), the CO2 fluxes for the warm period were 20–110 g C/(m2 · year), the CH4 fluxes for the warm period were 0.8–3.7 C/(m2 · year). At the forest reclamation site, a slight increase in the CO2 normalised fluxes activity was revealed compared to a natural swamp, while the greenhouse gases’ concentrations in the peat deposit were found to be comparable in both cases. It was shown that in the absence of an operating drainage system, the process of re-bogging occurs. Due to the vast areas of the West Siberian Plain’s swamps and the aggressive nature of the bogging process, moderate and selective forest reclamation is suggested for its taiga zone.

Riverine CO2 variations in permafrost catchments of the Yangtze River source region: Hot spots and hot moments

Rivers and streams are pivotal modulators in regional and global carbon cycles, but riverine CO2 flux is still uncertain for permafrost watersheds. Here we present the seasonal CO2 partial pressure (pCO2) and CO2 emission flux (FCO2) of 8 rivers and streams in the Yangtze River source region (YRSR), which have high permafrost coverage and seasonally thawed active layer. The YRSR rivers and streams are generally supersaturated with CO2, although there are a few sites with CO2 undersaturation during spring. The small headwater streams are CO2 hot spots that show significantly higher pCO2 (52 % higher) and FCO2 (792 % higher) than larger rivers. Both pCO2 and FCO2 show distinct seasonality across the study sites. pCO2 and FCO2 peak in summer and exhibit much lower levels in autumn and spring, indicating that hot moments of riverine CO2 occur in summer. Seasonal pCO2 and FCO2 variations are jointly controlled by hydrology, active layer dynamics and associated processes. The warm summer causes active layer thaw and highly active soil respiration, which release a large quantity of soil carbon and increase the CO2 sources via strengthened hydrologic connectivity. The high rainfall and more thaw-released water in summer bring high discharge, which can increase the water velocity and gas exchange rate and thus CO2 emission flux. Most of the variances of seasonal FCO2 (95 %) can be explained by hydrology and active layer thaw depth. Nevertheless, the hydrological process and seasonally thawed active layer over Qinghai-Tibet Plateau (QTP) play crucial roles in riverine carbon export due to the summer monsoon-dominated climate in QTP. Our results suggest that full seasonal coverage of CO2 dynamics is essential to quantify the annual CO2 flux accurately. Changing climate and warming permafrost may alter the annual CO2 emission due to deeper flow paths, hydrology changes, and longer emission windows throughout the year.

A synthesis of ecosystem metabolism of China's major rivers and coastal zones (2000–2020)

AbstractEcosystem metabolism is a vital measurement for understanding the dynamics of aquatic environments. China has a range of aquatic resources and in the past two decades, there has been an increasing interest in ecosystem metabolism research. However, no studies have evaluated overall variation and environmental patterns which affect metabolism in China's vast range of aquatic systems. For this paper, we searched for articles with metabolism data for China's major rivers and coastal zones from 2000 to 2020. We identified 127 studies documenting ecosystem metabolism functions including primary production (PP), community respiration (CR) and net ecosystem production (NEP). Using path analysis, we documented relationships across a range of environmental parameters. Light availability affected riverine and coastal PP (<0.1–10.7 gC m−2 day−1), while riverine PP was also linked to precipitation, photosynthesis‐related biomass, and land‐use variables. Riverine and coastal CR ranged from 0.01 to 24.3 gC m−2 day−1 and was affected strongly by nutrient concentrations. Average NEP ranged from 2.9 to 9.2 gC m−2 day−1, suggesting the potential of some of the evaluated ecosystems to be CO2 sinks. Within this review, the large spatiotemporal range allows for a wide‐ranging understanding of the variation of metabolic processes which contribute to CO2 dynamics and the role of aquatic ecosystems in regional climate change. Additionally, by evaluating aquatic ecosystem metabolism across such a diverse range of environmental conditions this work provides a scientific basis for future assessments of ecological risks under the complex conditions of climate change.This article is categorized under: Science of Water > Water Quality Science of Water > Water and Environmental Change Water and Life > Nature of Freshwater Ecosystems

Opportunities beyond CO2 for climate mitigation

Opportunities beyond CO2 for climate mitigation

Correlation between structure and dynamics of CO2 confined in Mg-MOF-74 and the role of inter-crystalline space: A molecular dynamics simulation study

Mg-MOF-74 is a metal–organic framework (MOF) with a high capacity for CO2 adsorption. Given the importance of CO2 confinement in Mg-MOF-74 for capture and storage applications, it is important to understand CO2 behavior in Mg-MOF-74. While most molecular simulation studies use ideal single crystal models of nano-porous substrates, the existence of inter-crystalline space is shown to have profound effects on the adsorption, structure and dynamics of the adsorbed fluid. Here we report a molecular dynamics simulation study at 300 K, of CO2 confined in Mg-MOF-74 with inter-crystalline spacing of different widths. Six strong sites of CO2 adsorption are found at the periphery of the Mg-MOF-74 pores in addition to a relatively weak adsorption at the pore center. On insertion of inter-crystalline spacing, additional sites of strong adsorption are seen close to the pore opening, which delocalize as the inter-crystalline space is widened while adsorption in the pore center grows. This redistribution has important implications for the dynamics of CO2. With wider inter-crystalline spacing, anomalous loading dependence of translational diffusivity is observed. In general, inter-crystalline spacing suppresses the rotational motion of CO2. Translational motion, however, exhibits an enhancement on the introduction of inter-crystalline space but gets suppressed when this space is widened.

Net Ecosystem CO2 Exchange in Mountain Grasslands Is Seriously Endangered by the Temperature Increase in the Eastern Pyrenees

Mediterranean mountain grasslands, including the Pyrenees, are highly vulnerable to climate change, due to the increasing temperatures and heat weaves frequency, among other factors. However, the effects of the increased temperatures on CO2 fluxes in those ecosystems have been barley explored. To address this gap of knowledge, we established the FLUXPYR-ECOFUN micrometeorological flux network, which included three eddy covariance flux stations in grasslands along a management and a climatic gradient (montane to subalpine) at the Pyrenees; we aimed at assessing interactions among environmental and phenological drivers on CO2 fluxes, with special attention at the role of temperature as CO2 flux driver under the different climatic and management conditions across the studied gradient. Our results showed that temperature drove CO2 dynamics along the studied gradient in different ways. At the subalpine grassland net CO2 uptake was linearly enhanced by temperature and CO2 fluxes had not reached a temperature shifting point yet (according to the segmented linear models) at which the net uptake would become CO2 emissions. This suggests that in the short term, and under the incoming enhanced temperatures, sub-alpine grasslands in the Pyrenees might increase their net CO2 uptake, although the mid long-term uptake may be compromised. On the contrary, the montane grasslands already presented CO2 emissions at the highest temperatures, most likely driven by a decrease in the greenness and photosynthesis, which suggests that montane grasslands are expected to reduce their CO2 sink capacity under the increasing temperatures. Overall, mountain grasslands in the mid- to long-term in the Pyrenees may experience a reduction in their net CO2 uptake capacity under the current climate change scenario.

COVID-19 pandemic underscores role of green space in urban carbon dynamics

For Detroit Michigan the arrival of COVID-19 led to intensive measures to prevent further spread of the virus resulting in consequent changes in traffic and energy use. We take advantage of these different emission scenarios to explore CO2 dynamics in a postindustrial city with a declining population and increasing green space. We present atmospheric CO2 concentration and net urban ecosystem exchange of CO2 (NUE) from a typical eddy covariance system and canopy greenness from a field camera on the Wayne State University campus in midtown Detroit. We categorized our study period (January 18, 2020–July 31, 2020) into three subperiods associated with the state-wide shelter-in-place order. Our results support that the city was a net carbon source throughout the period, particularly during the shelter-in-place period, although reduced traffic lowered CO2 concentrations and NUE. However, during the post-order period when traffic was highest, atmospheric CO2 concentrations and NUE were lowest, suggesting that the greening of urban vegetation may have greater carbon mitigation potential than lowering anthropogenic carbon emissions through traffic reductions.

Atmospheric CO2, soil carbon stock and control variables in managed and degraded pastures in central Brazil

Brazil has 166 million hectares of pastures, 58.8% (97.6 million hectares) of which are in some degree of degradation. Besides the low animal support capacity and vegetative yield, degraded pastures also contribute significantly to soil carbon losses, through CO2 emission. This study aimed to understand the temporal dynamics of atmospheric CO2 in contrast between degraded pastures (DP) and managed pastures (MP) in the Cerrado Brazil biome. Using the Pasture Atlas and 6-year time series, the variables were collected under the pasture classes: column-average concentration of carbon dioxide in the atmosphere (xCO2), Soil Carbon Stock (SCS), as well as Solar-induced Chlorophyll Fluorescence (SIF), Normalized Difference Vegetation Index (NDVI) and Leaf Area Index (LAI) as vegetative characteristics, and Land Surface Temperature Amplitude (LST Amplitude) and Precipitation as climatic aspects. Except for xCO2, MP was significantly different from DP with higher SIF, NDVI, LAI, Precipitation, and SCS, but smaller LST Amplitude. The hypothesis test (Student t-test p < 0.05) results indicate higher Precipitation and SCS in MP when compared to DP due to its positive effect of higher vegetative values. Linear regression and Pearson's correlation analysis indicate that xCO2 negatively relates to the precipitation and vegetative variables (NDVI, LAI, and SIF), but positively with LST Amplitude and SCS. Soil carbon stock obtained a significant xCO2 relationship (p < 0.001) only in the DP, suggesting that the environmental stress experienced by degraded pastures impacts soil carbon losses as a source of CO2. Therefore, this study concludes that DP with carbon stored in the soil is the main source of CO2 for the atmosphere when compared to MP. So, if the necessary care is not taken, the organic carbon from these degraded pastures will be lost to the atmosphere as CO2. These findings may favor nature-based solutions indicating that proper pasture management is an important aspect to mitigate emissions and improving soil carbon content helping to mitigate atmospheric CO2.

Interaction between seawater carbon dioxide dynamics and stratification in shallow coastal waters: A preliminary study based on a weekly validated three-dimensional ecological model

Shallow coastal waters (SCWs) have attracted wide attention in recent years due to their strong carbon sequestration capacity. However, the complex carbon dioxide (CO2) dynamics in the water column makes it difficult to estimate the air–water CO2 fluxes (FCO2) accurately. We developed a numerical model of CO2 dynamics in water based on field measurements for a typical stratified semi-enclosed shallow bay: the Yatsushiro Sea, Japan. The developed model showed an excellent ability to reproduce the stratification and CO2 dynamics of the Yatsushiro Sea. Through numerical model simulations, we analyzed the annual CO2 dynamics in the Yatsushiro Sea in 2018. The results show that the effect of stratification on the CO2 dynamics in seawater varies greatly depending on the distance from the estuary and the period. In the estuarine region, stratification manifests itself throughout the year by promoting the maintenance of a high partial pressure of CO2 (pCO2) in surface waters, resulting in surface pCO2 being higher than atmospheric pCO2 for up to 40 days during the flood period (average surface pCO2 of 539.94 µatm). In contrast, in areas farther from the estuary, stratification mainly acts to promote the maintenance of high pCO2 in surface waters during periods of high freshwater influence. Then changes to a lower surface pCO2 before the freshwater influence leads towards complete dissipation. Finally, we estimated the FCO2 of the Yatsushiro Sea in 2018, and the results showed that the Yatsushiro Sea was a sink area for atmospheric CO2 in 2018 (−1.70 mmol/m2/day).

CO2 dynamics in a small and old subtropical reservoir in East Asia: Environmental controls driving seasonal and spatial variability

Inland waters have been increasingly viewed as hotspots for greenhouse gas (GHG) emissions owing to their strong capability to intercept and mineralize carbon from the terrestrial environment. Although small waterbodies in humid subtropical climates have the potential to emit considerable amounts of GHG, their emission patterns have remained understudied. This study involved intensive measurements of carbon dioxide (CO2) emissions from a small reservoir and its upstream and downstream reaches located in subtropical Hong Kong. Our results revealed that a variety of metabolic, hydrological, and hydrochemical processes play a critical role in regulating its CO2 dynamics. The reservoir was an overall source of CO2 to the atmosphere with an average areal flux of 24.6 mmol m−2 d−1, and it occasionally functioned as a sink for atmospheric CO2 under intense solar radiation when primary productivity was high. This flux is on the low side relative to that of global (sub)tropical reservoirs, which was likely attributable to the prolonged history of the reservoir (>150 years) and the occasional undersaturation of CO2 in the water column. We also noticed pronounced differences in the underlying controls of CO2 dynamics between the reservoir and its upstream and downstream reaches, emphasizing the importance of taking into account the distinct characteristics of both lentic and lotic waters when evaluating catchment-scale CO2 fluxes.

Air Quality Monitoring for Preventive Conservation of the Built Heritage Deteriorated by Salt Crystallization

The Santo Domingo Diocesan School (Orihuela, SE Spain) is a singular case study that relates air quality monitoring and stone conservation. The monument suffers severe damage due to salt crystallization by rising damp, which is related to the indoor environment dynamics, high groundwater salt content, and microstructure, the building’s porous stone (biocalcarenite). Results revealed that 222Rn concentration in indoor air is low due to the building’s high ventilation rate, despite the medium-low geogenic radon potential in the area. Wavelets analysis showed that ventilation is caused by outdoor and indoor temperature variations and directly affects the 222Rn and CO2 dynamics inside the church. CO2, as well as relative humidity (RH), presented periodicities with intermediate frequencies (5–8 days) related to visits. These RH and temperature (T) variations intensified the salt damage by increasing the frequency of dissolution-crystallization cycles. The mineralogical characterization and geochemical calculations concluded that chlorides (halite and sylvite), sulfates (thenardite, epsomite, hexahydrite and aphthitalite) and nitrates (niter) present from dissolution forms to whiskers and hopper-type morphologies. This indicates high saturation values and, therefore, higher crystallization pressure values within the porous media of the biocalcarenite. On the contrary, the near-equilibrium crystal shape and incongruent precipitation reaction for humberstonite suggest a lower alteration for this salt on the building stone.

Rotationally-Resolved Two-Dimensional Infrared Spectroscopy of CO2(g): Rotational Wavepackets and Angular Momentum Transfer.

Angular momentum transfer and wavepacket dynamics of CO2(g) were measured on the picosecond time scale using polarization-resolved two-dimensional infrared (2D-IR) spectroscopy. The dynamics of rotational levels up to Jmax ≈ 50 are observed simultaneously at room temperature. Rotational wavepackets launched by the pump pulses cause oscillations in the intensity of individual peaks and beating patterns in the 2D-IR spectra. The structure of the rotationally resolved 2D-IR spectrum is explained using nonlinear response function theory. Spectral diffusion of the rotationally resolved 2D-IR peaks reveals information about angular momentum transfer. We demonstrate the ability to directly measure inelastic angular momentum dynamics simultaneously across the ∼50 thermally excited rotational levels over several hundred picoseconds.

Patterns and Regulation of Hypolimnetic CO2 and CH4 in a Tropical Reservoir Using a Process‐Based Modeling Approach

AbstractHypolimnetic waters and sediments of lakes and reservoirs are active sites of carbon dioxide (CO2) and methane (CH4) production and accumulation. These potent carbon greenhouse gases (C‐GHG) can be emitted to the atmosphere during water column mixing and, for hydropower reservoirs, through deep water discharge from the outflow. Deep CO2 and CH4 concentrations are regulated by sediment‐water flux, aerobic respiration, methanogenesis, CH4 oxidation, and vertical diffusion. These processes are interlinked but have mostly been studied individually, making their relative importance and interactions difficult to disentangle. In this study, we combined empirical observations with an integrative process‐based modeling to examine ecosystem‐scale dynamics of CO2 and CH4 in the deep layer of a permanently stratified reservoir located on the tropical Borneo Island (Malaysia). Oxygen (O2) was a key modulator of deep CO2 and CH4 metabolism, and shaped their spatial and temporal patterns within the reservoir. Our model reproduced the shape of the vertical profiles of gas concentration and isotopic signature, and suggested that deep C‐GHG stocks reached a steady‐state about 10 years after impoundment. Vertical gas diffusivity was the most determinant parameter controlling reservoir deep C‐GHG accumulation through its direct effect on gas transport and its interaction with water column metabolism through O2 regulation. Model results highlighted the interactions, relative importance, and uncertainties of processes controlling deep C‐GHG fluxes. This case study offers an in‐depth comprehension of C‐GHG storage and emissions in a tropical system, as well as a mechanistic framework transferable to other lakes and reservoirs.

Dynamic model of CO2 reduction policies: Another step towards a cleaner generation expansion

Cap-and-trade mechanisms and emission taxes are becoming increasingly widespread CO2 reduction policies in liberalized electricity markets. To set the design parameters of a CO2 reduction mechanism, analytical optimization frameworks are a commonly used tools by policy makers. However, such frameworks can suffer from issues related to emerging dynamic behaviors in electricity markets. This paper focuses on the relation between the parameter settings of CO2 reduction policies and dynamics of the electricity market. The authors present an analytical framework based on the system dynamics modeling that improves the understanding of the relation between the quantified impacts of CO2 reduction policies and parameter settings. The framework is based on the insight that the quantified impacts of CO2 reduction policies are attributed to the policy design, delays and mutual interactions inherent in the electricity market. The framework addresses both the quantified impacts and the rationale behind dynamic behaviors in the electricity market after implementing CO2 reduction policies. The framework is based on a causal loop diagram, illustrating the causal relation between different components within a system of electricity market. It offers policy makers a tool to assess the interactions between policy parameters and the structure of the electricity market. The framework is simulated under a power system using different scenarios of CO2 reduction policies and illustrates possible quantification of cleaner production, outperforming the base case scenario up to 63% CO2 reduction.

Short-Term Physiological and Transcriptomic Analysis of Saxitoxin-Producing and Nonproducing Raphidiopsis raciborskii (Cyanobacteria) under an Extremely High CO2 Environment

Findings about CO2 dynamics in the Earth’s ancestral atmosphere have suggested much higher concentrations in past eras. Along this line, cyanobacteria are an early evolved photosynthetic group that is suggested to have experienced both high and low CO2 availability since their Precambrian origin, and therefore, it is reasonable to assume that these microbes have the potential to cope with these scenarios by rapidly adjusting to various carbon dioxide levels. Thus, in this work, we performed a short-term (72-h) investigation of the physiological parameters (cell growth, photosynthesis and saxitoxin production) of toxic and nontoxic strains of the cyanobacterium Raphidiopsis raciborskii challenged by an extremely high pCO2 (40,000 ppm). Additionally, the transcriptomic profile (regarding the carbon concentrating mechanism and photosynthesis) of selected toxic and nontoxic strains is also presented. We found that short-term exposure to extremely elevated CO2 concentrations did not affect R. raciborskii physiology regardless of toxin production. However, transcripts related to bicarbonate transporters and the RuBisCO enzyme indicated the upregulation of CCM and downregulation of the Calvin cycle, respectively. According to our findings, at least at the initial growth phase, R. raciborskii was able to cope with a very high CO2 level, which shed light on the understanding that this species might have the potential to cope with carbon dioxide in water above the predicted levels.

Slow CO2 Diffusion Governed by Steric Hindrance of Rotatory Ligands in Small Pores of a Metal-Organic Framework.

Understanding the adsorption and diffusional dynamics of CO2 in metal-organic frameworks (MOFs) is essential in the application of these materials to CO2 capture and separation. We show that the dynamics of adsorbed CO2 is related to the rotational motion of ligands located in the narrow pore windows of a MOF using solid-state nuclear magnetic resonance (NMR) spectroscopy. NMR analyses of local dynamics reveal that CO2 adsorbed in the pore hinders the rotation of the ligands. The rate of diffusion of adsorbed CO2 monitored by 13C NMR is much less than that in the larger pores of MOFs and decreases cooperatively with ligand mobility, which indicates that the rate of diffusion is influenced by the steric hindrance of the rotatory ligands. Adsorbed CH4 also showed slow diffusion in the MOF, suggesting molecular size-selective effect of the mobile steric hindrance on the rate of adsorbate diffusion.

Drivers of spatial and seasonal variations of CO2 and CH4 fluxes at the sediment water interface in a shallow eutrophic lake

Shallow eutrophic lakes contribute disproportional to the emissions of CO2 and CH4 from inland waters. The processes that contribute to these fluxes, their environmental controls, and anthropogenic influences, however, are poorly constrained. Here, we studied the spatial variability and seasonal dynamics of CO2 and CH4 fluxes across the sediment-water interface, and their relationships to porewater nutrient concentrations in Lake Ulansuhai, a shallow eutrophic lake located in a semi-arid region in Northern China. The mean concentrations of CO2 and CH4 in porewater were 877.8 ± 31.0 µmol L−1 and 689.2 ± 45.0 µmol L−1, which were more than 50 and 20 times higher than those in the water column, respectively. The sediment was always a source of both gases for the water column. Porewater CO2 and CH4 concentrations and diffusive fluxes across the sediment-water interface showed significant temporal and spatial variations with mean diffusive fluxes of 887.3 ±124.7 µmol m−2 d−1 and 607.1 ± 68.0 µmol m−2 d−1 for CO2 and CH4, respectively. The temporal and spatial variations of CO2 and CH4 concentrations in porewater were associated with corresponding variations in dissolved organic carbon and dissolved nitrogen species. Temperature and dissolved organic carbon in surface porewater were the most important drivers of temporal variations in diffusive fluxes, whereas dissolved organic carbon and nitrogen were the main drivers of their spatial variations. Diffusive fluxes generally increased with increasing dissolved organic carbon and nitrogen in the porewater from the inflow to the outflow region of the lake. The estimated fluxes of both gases at the sediment-water interface were one order of magnitude lower than the emissions at the water surface, which were measured in a companion study. This indicates that diffusive fluxes across the sediment-water interface were not the main pathway for CO2 and CH4 emissions to the atmosphere. To improve the mechanistic understanding and predictability of greenhouse gas emissions from shallow lakes, future studies should aim to close the apparent gap in the CO2 and CH4 budget by combining improved flux measurement techniques with process-based modeling.

Exploring Molecular Dynamics of Adsorbed CO2 Species in Amine-Modified Porous Silica by Solid-State NMR Relaxation.

Previous studies on CO2 adsorbents have mainly addressed the identification and quantification of adsorbed CO2 species in amine-modified porous materials. Investigation of molecular motion of CO2 species in confinement has not been explored in depth yet. This work entails a comprehensive study of molecular dynamics of the different CO2 species chemi- and physisorbed at amine-modified silica materials through the determination of the rotating frame spin–lattice relaxation times (T1ρ) by solid-state NMR. Rotational correlation times (τC) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen–Purcell–Pound theories. As expected, the τC values for the two physisorbed CO2 species are considerably shorter (32 and 20 μs) than for the three identified chemisorbed CO2 species (162, 62, and 123 μs). The differences in molecular dynamics between the different chemisorbed species correlate well with the structures previously proposed. In the case of the physisorbed CO2 species, the τC values of the CO2 species displaying faster molecular dynamics falls in the range of viscous liquids, whereas the species presenting slower dynamics exhibit T1ρ and τC values compatible with a CO2 layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and 1H–13C heteronuclear dipolar couplings have also been estimated from T1ρ measurements, for each adsorbed CO2 species. The CSA tensor parameters obtained from fitting the relaxation data agree with the experimentally measured CSA values, thus showing that the theories are well suited to study CO2 dynamics in silica surfaces.

Greenhouse gases concentrations and emissions from a small subtropical cascaded river-reservoir system

The damming of rivers can significantly intercept carbon transportation and regulate dynamic biogeochemistry, which then greatly influence carbon gases (CO2 and CH4) metabolism and emissions. However, little is known about the spatial–temporal pattern and controls of carbon emissions from a cascaded river-reservoir system. In this study, we investigated the spatial and seasonal partial pressure of CO2 and CH4 (i.e. pCO2 and pCH4) dynamics along a subtropical cascaded dammed river (Wubu River) located in Southwest China. The pCO2 and pCH4 in the Wubu River basin showed significantly spatial–temporal and were generally supersaturated with gas fluxes of 89.5 ± 89.7 mmol CO2 m−2 d-1 and 1.19 ± 1.18 mmol CH4 m−2 d-1. The pCO2 and pCH4 were all increasing from upstream to downstream. And, most of river sections had significantly higher pCO2 and CO2 than their downstream reservoir sections, while the pCH4 and CH4 flux were the opposite. Meanwhile, released waters had invariably high pCO2 and pCH4 and were carbon gas emission hotspots in the river-reservoir system. The intercepts of carbon and nutrients by cascade damming, created a series of eutrophic hotspots and enhanced primary productivity, could inhibit riverine CO2 emissions but promote CH4 production. Thus, the spatial patterns of riverine pCO2, pCH4 and the “pCO2/pCH4” in cascade hydroelectric river were importantly altered. It was evaluated that cascade damming would reduce about 17.9 % of the total CO2 evasion amount but increase approximately 16.3 % of CH4. The global warming potential of rivers would be enhanced when organic carbon was captured and mineralized to CH4 rather than to CO2 in cascade damming river system. pH, dissolved oxygen and chlorophyll a (chl-a) were indicated as natural controls of pCO2 in such a cascade river-reservoir system, while dissolved organic carbon, dissolved organic carbon and chl-a were significantly correlated with pCH4 and acted as good predictors of CH4 emissions. The temperature and autotrophic production were responsible for the different seasonal patterns of carbon gas concentrations observed between river sections and reservoir sections. Our study highlighted that cascaded damming decreased CO2 emissions and created CH4 hotspots in river systems and may have significantly increased the complexity of the spatial–temporal pattern and construction of riverine carbon gases emissions. Given the increasing damming of rivers worldwide for energy demand, researches are needed to quantify the role of cascaded damming in the riverine C budget.