The Potential Applications of Nanoporous Materials for the Adsorption,Separation, and Catalytic Conversion of Carbon Dioxide

Carbon capture and storage (CCS) technologies aiming at tackling CO₂ emission have attracted much attention from scientists of various backgrounds. Most CCS systems require an efficient adsorbent to remove CO₂ from sources such as fossil fuels (pre‐combustion) or flue gas from power generation (post‐combustion). Research on developing efficient adsorbents with a substantial capacity, good stability and recyclability has grown rapidly in the past decade. Because of their high surface area, highly porous structure, and high stability, various nanoporous materials have been viewed as good candidates for this challenging task. Here, recent developments in several classes of nanoporous materials, such as zeolites, metal organic frameworks (MOFs), mesoporous silicas, carbon nanotubes, and organic cage frameworks, for CCS are examined and potential future directions for CCS technology are discussed. The main criteria for a sustainable CO₂ adsorbent for industrial use are also rationalized. Moreover, catalytic transformations of CO₂ to other chemical species using nanoporous catalysts and their potential for large scale carbon capture and utilization (CCU) processes are also discussed. Application of CCU technologies avoids any potential hazard associated with CO₂ reservoirs and allows possible recovery of some running cost for CO₂ capture by manufacturing valuable chemicals.     

Methane and soil CO2 production from current‐season photosynthates in a rice paddy exposed to elevated CO2 concentration and soil temperature

Quantification of rhizodeposition (root exudates and root turnover) represents a major challenge for understanding the links between above‐ground assimilation and below‐ground anoxic decomposition of organic carbon in rice paddy ecosystems. Free‐air CO₂ enrichment (FACE) fumigating depleted ¹³CO₂ in rice paddy resulted in a smaller ¹³C/¹²C ratio in plant‐assimilated carbon, providing a unique measure by which we partitioned the sources of decomposed gases (CO₂ and CH₄) into current‐season photosynthates (new C) and soil organic matter (old C). In addition, we imposed a soil‐warming treatment nested within the CO₂ treatments to assess whether the carbon source was sensitive to warming. Compared with the ambient CO₂ treatment, the FACE treatment decreased the ¹³C/¹²C ratio not only in the rice‐plant carbon but also in the soil CO₂ and CH₄. The estimated new C contribution to dissolved CO₂ was minor (ca. 20%) at the tillering stage, increased with rice growth and was about 50% from the panicle‐formation stage onwards. For CH₄, the contribution of new C was greater than for heterotrophic CO₂ production; ca. 40–60% of season‐total CH₄ production originated from new C with a tendency toward even larger new C contribution with soil warming, presumably because enhanced root decay provided substrates for greater CH₄ production. The results suggest a fast and close coupling between photosynthesis and anoxic decomposition in soil, and further indicate a positive feedback of global warming by enhanced CH₄ emission through greater rhizodeposition.     

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

Deep peat warming increases surface methane and carbon dioxide emissions in a black spruce‐dominated ombrotrophic bog

Boreal peatlands contain approximately 500 Pg carbon (C) in the soil, emit globally significant quantities of methane (CH₄), and are highly sensitive to climate change. Warming associated with global climate change is likely to increase the rate of the temperature‐sensitive processes that decompose stored organic carbon and release carbon dioxide (CO₂) and CH₄. Variation in the temperature sensitivity of CO₂ and CH₄ production and increased peat aerobicity due to enhanced growing‐season evapotranspiration may alter the nature of peatland trace gas emission. As CH₄ is a powerful greenhouse gas with 34 times the warming potential of CO₂, it is critical to understand how factors associated with global change will influence surface CO₂ and CH₄ fluxes. Here, we leverage the Spruce and Peatland Responses Under Changing Environments (SPRUCE) climate change manipulation experiment to understand the impact of a 0–9°C gradient in deep belowground warming (“Deep Peat Heat”, DPH) on peat surface CO₂ and CH₄ fluxes. We find that DPH treatments increased both CO₂ and CH₄ emission. Methane production was more sensitive to warming than CO₂ production, decreasing the C‐CO₂:C‐CH₄ of the respired carbon. Methane production is dominated by hydrogenotrophic methanogenesis but deep peat warming increased the δ¹³C of CH₄ suggesting an increasing contribution of acetoclastic methanogenesis to total CH₄ production with warming. Although the total quantity of C emitted from the SPRUCE Bog as CH₄ is <2%, CH₄ represents >50% of seasonal C emissions in the highest‐warming treatments when adjusted for CO₂ equivalents on a 100‐year timescale. These results suggest that warming in boreal regions may increase CH₄ emissions from peatlands and result in a positive feedback to ongoing warming.     

The CO2‐equivalent balance of freshwater ecosystems is non‐linearly related to productivity

Eutrophication of fresh waters results in increased CO₂ uptake by primary production, but at the same time increased emissions of CH₄ to the atmosphere. Given the contrasting effects of CO₂ uptake and CH₄ release, the net effect of eutrophication on the CO₂‐equivalent balance of fresh waters is not clear. We measured carbon fluxes (CO₂ and CH₄ diffusion, CH₄ ebullition) and CH₄ oxidation in 20 freshwater mesocosms with 10 different nutrient concentrations (total phosphorus range: mesotrophic 39 µg/L until hypereutrophic 939 µg/L) and planktivorous fish in half of them. We found that the CO₂‐equivalent balance had a U‐shaped relationship with productivity, up to a threshold in hypereutrophic systems. CO₂‐equivalent sinks were confined to a narrow range of net ecosystem production (NEP) between 5 and 19 mmol O₂ m^?3 day^?1. Our findings indicate that eutrophication can shift fresh waters from sources to sinks of CO₂‐equivalents due to enhanced CO₂ uptake, but continued eutrophication enhances CH₄ emission and transforms freshwater ecosystems to net sources of CO₂‐equivalents to the atmosphere. Nutrient enrichment but also planktivorous fish presence increased productivity, thereby regulating the resulting CO₂‐equivalent balance. Increasing planktivorous fish abundance, often concomitant with eutrophication, will consequently likely affect the CO₂‐equivalent balance of fresh waters.     

Increased night temperature reduces the stimulatory effect of elevated carbon dioxide concentration on methane emission from rice paddy soil

To determine how elevated night temperature interacts with carbon dioxide concentration ([CO₂]) to affect methane (CH₄) emission from rice paddy soil, we conducted a pot experiment using four controlled‐environment chambers and imposed a combination of two [CO₂] levels (ambient: 380 ppm; elevated: 680 ppm) and two night temperatures (22 and 32 °C). The day temperature was maintained at 32 °C. Rice (cv. IR72) plants were grown outside until the early‐reproductive growth stage and then transferred to the chambers. After onset of the treatment, day and night CH₄ fluxes were measured every week. The CH₄ fluxes changed significantly with the growth stage, with the largest fluxes occurring around the heading stage in all treatments. The total CH₄ emission during the treatment period was significantly increased by both elevated [CO₂] (P=0.03) and elevated night temperature (P<0.01). Elevated [CO₂] increased CH₄ emission by 3.5% and 32.2% under high and low night temperature conditions, respectively. Elevated [CO₂] increased the net dry weight of rice plants by 12.7% and 38.4% under high and low night temperature conditions, respectively. These results imply that increasing night temperature reduces the stimulatory effect of elevated [CO₂] on both CH₄ emission and rice growth. The CH₄ emission during the day was larger than at night even under the high‐night‐temperature treatment (i.e. a constant temperature all day). This difference became larger after the heading stage. We observed significant correlations between the night respiration and daily CH₄ flux (P<0.01). These results suggest that net plant photosynthesis contributes greatly to CH₄ emission and that increasing night temperature reduces the stimulatory effect of elevated [CO₂] on CH₄ emission from rice paddy soil.     

Agricultural peatland restoration: effects of land‐use change on greenhouse gas (CO2 and CH4) fluxes in the Sacramento‐San Joaquin Delta

Agricultural drainage of organic soils has resulted in vast soil subsidence and contributed to increased atmospheric carbon dioxide (CO₂) concentrations. The Sacramento‐San Joaquin Delta in California was drained over a century ago for agriculture and human settlement and has since experienced subsidence rates that are among the highest in the world. It is recognized that drained agriculture in the Delta is unsustainable in the long‐term, and to help reverse subsidence and capture carbon (C) there is an interest in restoring drained agricultural land‐use types to flooded conditions. However, flooding may increase methane (CH₄) emissions. We conducted a full year of simultaneous eddy covariance measurements at two conventional drained agricultural peatlands (a pasture and a corn field) and three flooded land‐use types (a rice paddy and two restored wetlands) to assess the impact of drained to flooded land‐use change on CO₂ and CH₄ fluxes in the Delta. We found that the drained sites were net C and greenhouse gas (GHG) sources, releasing up to 341 g C m^?2 yr^?1 as CO₂ and 11.4 g C m^?2 yr^?1 as CH₄. Conversely, the restored wetlands were net sinks of atmospheric CO₂, sequestering up to 397 g C m^?2 yr^?1. However, they were large sources of CH₄, with emissions ranging from 39 to 53 g C m^?2 yr^?1. In terms of the full GHG budget, the restored wetlands could be either GHG sources or sinks. Although the rice paddy was a small atmospheric CO₂ sink, when considering harvest and CH₄ emissions, it acted as both a C and GHG source. Annual photosynthesis was similar between sites, but flooding at the restored sites inhibited ecosystem respiration, making them net CO₂ sinks. This study suggests that converting drained agricultural peat soils to flooded land‐use types can help reduce or reverse soil subsidence and reduce GHG emissions.     

Effects of simulated drought on the carbon balance of Everglades short‐hydroperiod marsh

Hydrology drives the carbon balance of wetlands by controlling the uptake and release of CO₂ and CH₄. Longer dry periods in between heavier precipitation events predicted for the Everglades region, may alter the stability of large carbon pools in this wetland's ecosystems. To determine the effects of drought on CO₂ fluxes and CH₄ emissions, we simulated changes in hydroperiod with three scenarios that differed in the onset rate of drought (gradual, intermediate, and rapid transition into drought) on 18 freshwater wetland monoliths collected from an Everglades short‐hydroperiod marsh. Simulated drought, regardless of the onset rate, resulted in higher net CO₂ losses net ecosystem exchange (NEE) over the 22‐week manipulation. Drought caused extensive vegetation dieback, increased ecosystem respiration (R_eco), and reduced carbon uptake gross ecosystem exchange (GEE). Photosynthetic potential measured by reflective indices (photochemical reflectance index, water index, normalized phaeophytinization index, and the normalized difference vegetation index) indicated that water stress limited GEE and inhibited R_eco. As a result of drought‐induced dieback, NEE did not offset methane production during periods of inundation. The average ratio of net CH₄ to NEE over the study period was 0.06, surpassing the 100‐year greenhouse warming compensation point for CH₄ (0.04). Drought‐induced diebacks of sawgrass (C₃) led to the establishment of the invasive species torpedograss (C₄) when water was resupplied. These changes in the structure and function indicate that freshwater marsh ecosystems can become a net source of CO₂ and CH₄ to the atmosphere, even following an extended drought. Future changes in precipitation patterns and drought occurrence/duration can change the carbon storage capacity of freshwater marshes from sinks to sources of carbon to the atmosphere. Therefore, climate change will impact the carbon storage capacity of freshwater marshes by influencing water availability and the potential for positive feedbacks on radiative forcing.     

Carbon fluxes from a tropical peat swamp forest floor

A tropical ombrotrophic peatland ecosystem is one of the largest terrestrial carbon stores. Flux rates of carbon dioxide (CO₂) and methane (CH₄) were studied at various peat water table depths in a mixed‐type peat swamp forest floor in Central Kalimantan, Indonesia. Temporary gas fluxes on microtopographically differing hummock and hollow peat surfaces were combined with peat water table data to produce annual cumulative flux estimates. Hummocks formed mainly from living and dead tree roots and decaying debris maintained a relatively steady CO₂ emission rate regardless of the water table position in peat. In nearly vegetation‐free hollows, CO₂ emission rates were progressively smaller as the water table rose towards the peat surface. Methane emissions from the peat surface remained small and were detected only in water‐saturated peat. By applying long‐term peat water table data, annual gas emissions from the peat swamp forest floor were estimated to be 3493±316 g CO₂ m^?2 and less than 1.36±0.57 g CH₄ m^?2. On the basis of the carbon emitted, CO₂ is clearly a more important greenhouse gas than CH₄. CO₂ emissions from peat are the highest during the dry season, when the oxic peat layer is at its thickest because of water table lowering.     

High carbon emissions from thermokarst lakes and their determinants in the Tibet Plateau

Thermokarst lakes are potentially important sources of methane (CH₄) and carbon dioxide (CO₂). However, considerable uncertainty exists regarding carbon emissions from thermokarst lakes owing to a limited understanding of their patterns and motivators. In this study, we measured CH₄ and CO₂ diffusive fluxes in 163 thermokarst lakes in the Qinghai–Tibet Plateau (QTP) over 3 years from May to October. The median carbon emissions from the QTP thermokarst lakes were 1440 mg CO₂ m⁻² day⁻¹ and 60 mg CH₄ m⁻² day⁻¹, respectively. The diffusive rates of CO₂ and CH₄ are related to the catchment land cover type. Sediment microbial abundance and hydrochemistry explain 51.9% and 38.3% of the total variance in CH₄ diffusive emissions, respectively, while CO₂ emissions show no significant relationship with environmental factors. When upscaling carbon emissions from the QTP thermokarst lakes, the annual average CH₄ release per lake area is equal to that of the pan-Arctic region. Our findings highlight the importance of incorporating in situ observation data with different emission pathways for different land cover types in predicting carbon emissions from thermokarst lakes in the future.     

Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

Wetlands are important sources of methane (CH₄) and sinks of carbon dioxide (CO₂). However, little is known about CH₄ and CO₂ fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH₄ and CO₂ in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO₂ but strong sources of CH₄, particularly during inundation when reducing conditions in soils increase CH₄ production and limit CO₂ release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH₄‐C m^?2 d^?1 and absorbed 1.6 ± 0.2 g CO₂‐C m^?2 d^?1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH₄ emissions decreased significantly (0.002 ± 0.001 g CH₄‐C m^?2 d^?1) but remained a net source, while the net CO₂ flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH₄ fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO₂ and CH₄ were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m^?2 (as CH₄‐C + CO₂‐C) in anaerobic phases and emitting 76 g C m^?2in aerobic phases), high CH₄ effluxes during the anaerobic flooded phase and modest CH₄ effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.     

Soil-atmospheric exchange of CO2, CH4, and N2O in three subtropical forest ecosystems in southern China

The magnitude, temporal, and spatial patterns of soil‐atmospheric greenhouse gas (hereafter referred to as GHG) exchanges in forests near the Tropic of Cancer are still highly uncertain. To contribute towards an improvement of actual estimates, soil‐atmospheric CO₂, CH₄, and N₂O fluxes were measured in three successional subtropical forests at the Dinghushan Nature Reserve (hereafter referred to as DNR) in southern China. Soils in DNR forests behaved as N₂O sources and CH₄ sinks. Annual mean CO₂, N₂O, and CH₄ fluxes (mean±SD) were 7.7±4.6 Mg CO₂‐C ha^?1 yr^?1, 3.2±1.2 kg N₂O‐N ha^?1 yr^?1, and 3.4±0.9 kg CH₄‐C ha^?1 yr^?1, respectively. The climate was warm and wet from April through September 2003 (the hot‐humid season) and became cool and dry from October 2003 through March 2004 (the cool‐dry season). The seasonality of soil CO₂ emission coincided with the seasonal climate pattern, with high CO₂ emission rates in the hot‐humid season and low rates in the cool‐dry season. In contrast, seasonal patterns of CH₄ and N₂O fluxes were not clear, although higher CH₄ uptake rates were often observed in the cool‐dry season and higher N₂O emission rates were often observed in the hot‐humid season. GHG fluxes measured at these three sites showed a clear increasing trend with the progressive succession. If this trend is representative at the regional scale, CO₂ and N₂O emissions and CH₄ uptake in southern China may increase in the future in light of the projected change in forest age structure. Removal of surface litter reduced soil CO₂ effluxes by 17–44% in the three forests but had no significant effect on CH₄ absorption and N₂O emission rates. This suggests that microbial CH₄ uptake and N₂O production was mainly related to the mineral soil rather than in the surface litter layer.     

Microbial processes influencing methane emission from rice fields

Irrigated rice fields are an important source of atmospheric methane. In order to improve our understanding of the controlling processes, we measured in situ CH₄ emission and CH₄ oxidation in an Italian rice field in 1998 and 1999, and studied CH₄ production in soil and root samples. The CH₄ emission rates were correlated with diurnal temperature variations and showed pronounced seasonal and interannual variations. The contribution of CH₄ oxidation to total CH₄ flux, determined by specific inhibition with difluoromethane, decreased from 40% at the beginning to zero at the end of the season. The stable carbon isotopic composition of the emitted CH₄ also decreased. The CH₄‐oxidizing bacteria probably became limited by nitrogen as indicated by the seasonal decrease of NH₄⁺. Thus, CH₄ oxidation had little effect on CH₄ emission. Methane production on rice roots was relatively constant over the season. Methane production in soil slowly increased after flooding and was highest in the middle of the season. Pore water concentrations of CH₄ showed a similar seasonal pattern. In 1999, CH₄ production increased later in the season and reached lower rates than in 1998. An additional drainage in 1999 resulted in higher ferric iron concentrations, higher soil redox potentials and lower acetate concentrations. As a result, acetate‐utilizing methanogens were probably out‐competed by iron‐reducers so that a larger percentage of [2–¹⁴C]acetate was converted to ¹⁴CO₂ instead of ¹⁴CH₄. The residual CH₄ production was relatively low and was mainly due to H₂/CO₂‐dependent methanogenesis. Experiments with radioactive bicarbonate and with methyl fluoride as specific inhibitor showed that the theoretical ratio of 7:3 of methanogenesis from acetate vs. H₂/CO₂ was only reached later in the season when total CH₄ production was at the maximum. In conclusion, our results give a mechanistic explanation for the intraseasonal and interannual differences in CH₄ emission.     

Environmental controls of temporal and spatial variability in CO2 and CH4 fluxes in a neotropical peatland

Tropical peatlands play an important role in the global storage and cycling of carbon (C) but information on carbon dioxide (CO₂) and methane (CH₄) fluxes from these systems is sparse, particularly in the Neotropics. We quantified short and long‐term temporal and small scale spatial variation in CO₂ and CH₄ fluxes from three contrasting vegetation communities in a domed ombrotrophic peatland in Panama. There was significant variation in CO₂ fluxes among vegetation communities in the order Campnosperma panamensis > Raphia taedigera > Cyperus. There was no consistent variation among sites and no discernible seasonal pattern of CH₄ flux despite the considerable range of values recorded (e.g. ?1.0 to 12.6 mg m^?2 h^?1 in 2007). CO₂ fluxes varied seasonally in 2007, being greatest in drier periods (300–400 mg m^?2 h^?1) and lowest during the wet period (60–132 mg m^?2 h^?1) while very high emissions were found during the 2009 wet period, suggesting that peak CO₂ fluxes may occur following both low and high rainfall. In contrast, only weak relationships between CH₄ flux and rainfall (positive at the C. panamensis site) and solar radiation (negative at the C. panamensis and Cyperus sites) was found. CO₂ fluxes showed a diurnal pattern across sites and at the Cyperus sp. site CO₂ and CH₄ fluxes were positively correlated. The amount of dissolved carbon and nutrients were strong predictors of small scale within‐site variability in gas release but the effect was site‐specific. We conclude that (i) temporal variability in CO₂ was greater than variation among vegetation communities; (ii) rainfall may be a good predictor of CO₂ emissions from tropical peatlands but temporal variation in CH₄ does not follow seasonal rainfall patterns; and (iii) diurnal variation in CO₂ fluxes across different vegetation communities can be described by a Fourier model.     

Biotic controls on CO2 and CH4 exchange in wetlands – a closed environment study

Wetlands are significant sources of the important greenhouse gas CH₄. Here we explore the use of an experimental system developed for the determination of continuous fluxes of CO₂ and CH₄ in closed ecosystem monoliths including the capture of ¹⁴CO₂ and ¹⁴CH₄ following pulse labelling with ¹⁴CO₂. We show that, in the ecosystem studied, ebullition (bubble emission) may account for 18 to 50% of the total CH₄ emission, representing fluxes that have been difficult to estimate accurately in the past. Furthermore, using plant removal and ¹⁴C labelling techniques, we use the system to detail the direct influence of vascular plants on CH₄ emission. This influence is observed to be dependent on the amount of vascular plants present. The results that may be produced using the presented experimental set-up have implications for an improved understanding of wetland ecosystem/atmosphere interactions, including possible feedback effects on climate change. In recent years much attention has been devoted to ascertaining and subsequently using the relationship between net ecosystem productivity and CH₄ emission as a basis for extrapolation of fluxes across large areas. The experimental system presented may be used to study the complex relationship between vascular plants and CH₄ emission and here we show examples of how this may vary considerably in nature between and even within ecosystems.     

Elevated UV-B radiation alters fluxes of methane and carbon dioxide in peatland microcosms

Increases in solar ultraviolet‐B radiation (UV‐B; 280–320 nm) reaching the earth have been estimated to continue until 2050s in the boreal and subarctic regions with an abundant peatland cover. Peatlands are significant sinks for carbon dioxide (CO₂) and sources for methane (CH₄). To assess whether the future increases in UV‐B could affect the fluxes of CO₂ and CH₄ in peatlands via an impact on vegetation, we exposed peatland microcosms to modulated 30% supplementation of erythemally weighted UV‐B at an outdoor facility for one growing season. The experimental design included appropriate controls for UV‐A and ambient radiation. The UV‐B caused a significant reduction in gross photosynthesis, net ecosystem CO₂ exchange, and CH₄ emission of the peatland microcosms. These changes in the carbon gas cycling can be partly explained by UV‐B‐induced morphological changes in Eriophorum vaginatum which acts as a conduit for CH₄. Leaf cross section and the percentage of CH₄‐conducting aerenchymatous tissue in E. vaginatum were significantly reduced by UV‐B. Methanol‐extractable UV‐B absorbing compounds decreased under both UV‐B and UV‐A in Sphagnum angustifolium, and tended to accumulate under UV‐B in S. papillosum. Membrane permeability to magnesium (Mg) and calcium (Ca) ions was higher in UV‐B exposed S. angustifolium. Amount of chlorophyll and carotenoid pigments was increased by UV‐A in S. magellanicum. The observed changes in Sphagnum mosses did not coincide with those in carbon gas fluxes but occurred at the time of the highest UV intensity in the mid summer. Our findings indicate that increasing UV‐B may have more substantial effects on gas exchange in peatlands than previously thought.     

Multiyear greenhouse gas balances at a rewetted temperate peatland

Drained peat soils are a significant source of greenhouse gas (GHG) emissions to the atmosphere. Rewetting these soils is considered an important climate change mitigation tool to reduce emissions and create suitable conditions for carbon sequestration. Long‐term monitoring is essential to capture interannual variations in GHG emissions and associated environmental variables and to reduce the uncertainty linked with GHG emission factor calculations. In this study, we present GHG balances: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) calculated for a 5‐year period at a rewetted industrial cutaway peatland in Ireland (rewetted 7 years prior to the start of the study); and compare the results with an adjacent drained area (2‐year data set), and with ten long‐term data sets from intact (i.e. undrained) peatlands in temperate and boreal regions. In the rewetted site, CO₂ exchange (or net ecosystem exchange (NEE)) was strongly influenced by ecosystem respiration (R_eco) rather than gross primary production (GPP). CH₄ emissions were related to soil temperature and either water table level or plant biomass. N₂O emissions were not detected in either drained or rewetted sites. Rewetting reduced CO₂ emissions in unvegetated areas by approximately 50%. When upscaled to the ecosystem level, the emission factors (calculated as 5‐year mean of annual balances) for the rewetted site were (±SD) ?104 ± 80 g CO₂‐C m^?2 yr^?1 (i.e. CO₂ sink) and 9 ± 2 g CH₄‐C m^?2 yr^?1 (i.e. CH₄ source). Nearly a decade after rewetting, the GHG balance (100‐year global warming potential) had reduced noticeably (i.e. less warming) in comparison with the drained site but was still higher than comparative intact sites. Our results indicate that rewetted sites may be more sensitive to interannual changes in weather conditions than their more resilient intact counterparts and may switch from an annual CO₂ sink to a source if triggered by slightly drier conditions.     

Tunable Polyaniline‐Based Porous Carbon with Ultrahigh Surface Area for CO2 Capture at Elevated Pressure

Natural gas is the cleanest fossil fuel source. However, natural gas wells typically contain considerable amounts of CO₂, with on‐site CO₂ capture necessary. Solid sorbents are advantageous over traditional amine scrubbing due to their relatively low regeneration energies and non‐corrosive nature. However, it remains a challenge to improve the sorbent's CO₂ capacity at elevated pressures relevant to natural gas purification. Here, the synthesis of porous carbons derived from a 3D hierarchical nanostructured polymer hydrogel, with simple and effective tunability over the pore size distribution is reported. The optimized surface area reaches 4196 m² g^?1, which is among the highest of carbon‐based materials, with abundant micro‐ and narrow mesopores (2.03 cm³ g^?1 with d < 4 nm). This carbon exhibits a record‐high CO₂ capacity among reported carbons at elevated pressure (i.e., 28.3 mmol g^?1 total adsorption at 25 °C and 30 bar). This carbon also shows good CO₂/CH₄ selectivity and excellent cyclability. Molecular simulations suggest increased CO₂ density in micro‐ and narrow mesopores at high pressures. This is consistent with the observation that these pores are mainly responsible for the material's high‐pressure CO₂ capacity. This work provides insights into material design and further development for CO₂ capture from natural gas.     

Partitioning of CH4 and CO2 Production Originating from Rice Straw,Soil and Root Organic Carbon in Rice Microcosms

Flooded rice fields are an important source of the greenhouse gas CH₄. Possible carbon sources for CH₄ and CO₂ production in rice fields are soil organic matter (SOM), root organic carbon (ROC) and rice straw (RS), but partitioning of the flux between the different carbon sources is difficult. We conducted greenhouse experiments using soil microcosms planted with rice. The soil was amended with and without ¹³C-labeled RS, using two ¹³C-labeled RS treatments with equal RS (5 g kg⁻¹ soil) but different δ¹³C of RS. This procedure allowed to determine the carbon flux from each of the three sources (SOM, ROC, RS) by determining the δ¹³C of CH₄ and CO₂ in the different incubations and from the δ¹³C of RS. Partitioning of carbon flux indicated that the contribution of ROC to CH₄ production was 41% at tillering stage, increased with rice growth and was about 60% from the booting stage onwards. The contribution of ROC to CO₂ was 43% at tillering stage, increased to around 70% at booting stage and stayed relatively constant afterwards. The contribution of RS was determined to be in a range of 12–24% for CH₄ production and 11–31% for CO₂ production; while the contribution of SOM was calculated to be 23–35% for CH₄ production and 13–26% for CO₂ production. The results indicate that ROC was the major source of CH₄ though RS application greatly enhanced production and emission of CH₄ in rice field soil. Our results also suggest that data of CH₄ dissolved in rice field could be used as a proxy for the produced CH₄ after tillering stage.     

High emissions of greenhouse gases from grasslands on peat and other organic soils

Drainage has turned peatlands from a carbon sink into one of the world's largest greenhouse gas (GHG) sources from cultivated soils. We analyzed a unique data set (12 peatlands, 48 sites and 122 annual budgets) of mainly unpublished GHG emissions from grasslands on bog and fen peat as well as other soils rich in soil organic carbon (SOC) in Germany. Emissions and environmental variables were measured with identical methods. Site‐averaged GHG budgets were surprisingly variable (29.2 ± 17.4 t CO₂‐eq. ha^?1 yr^?1) and partially higher than all published data and the IPCC default emission factors for GHG inventories. Generally, CO₂ (27.7 ± 17.3 t CO₂ ha^?1 yr^?1) dominated the GHG budget. Nitrous oxide (2.3 ± 2.4 kg N₂O‐N ha^?1 yr^?1) and methane emissions (30.8 ± 69.8 kg CH₄‐C ha^?1 yr^?1) were lower than expected except for CH₄ emissions from nutrient‐poor acidic sites. At single peatlands, CO₂ emissions clearly increased with deeper mean water table depth (WTD), but there was no general dependency of CO₂ on WTD for the complete data set. Thus, regionalization of CO₂ emissions by WTD only will remain uncertain. WTD dynamics explained some of the differences between peatlands as sites which became very dry during summer showed lower emissions. We introduced the aerated nitrogen stock (N_air) as a variable combining soil nitrogen stocks with WTD. CO₂ increased with N_air across peatlands. Soils with comparatively low SOC concentrations showed as high CO₂ emissions as true peat soils because N_air was similar. N₂O emissions were controlled by the WTD dynamics and the nitrogen content of the topsoil. CH₄ emissions can be well described by WTD and ponding duration during summer. Our results can help both to improve GHG emission reporting and to prioritize and plan emission reduction measures for peat and similar soils at different scales.