Competing uses of biomass for energy and chemicals: implications for long‐term global CO2 mitigation potential

Biomass is considered a low carbon source for various energy or chemical options. This paper assesses it's different possible uses, the competition between these uses, and the implications for long‐term global energy demand and energy system emissions. A scenario analysis is performed using the TIMER energy system model. Under baseline conditions, 170 EJ yr^?1 of secondary bioenergy is consumed in 2100 (approximately 18% of total secondary energy demand), used primarily in the transport, buildings and nonenergy (chemical production) sectors. This leads to a reduction of 9% of CO₂ emissions compared to a counterfactual scenario where no bioenergy is used. Bioenergy can contribute up to 40% reduction in emissions at carbon taxes greater than 500/tC. As higher CO₂ taxes are applied, bioenergy is increasingly diverted towards electricity generation. Results are more sensitive to assumptions about resource availability than technological parameters. To estimate the effectiveness of bioenergy in specific sectors, experiments are performed in which bioenergy is only allowed in one sector at a time. The results show that cross‐sectoral leakage and emissions from biomass conversion limit the total emission reduction possible in each sector. In terms of reducing emissions per unit of bioenergy use, we show that the use of bioelectricity is the most effective, especially when used with carbon capture and storage. However, this technology only penetrates at a high carbon price (>100/tC) and competition with transport fuels may limit its adoption.     

Phenyl Disulfide Additive for Solution‐Mediated Carbon Dioxide Utilization in Li–CO2 Batteries

Phenyl disulfide (PDS) is employed as an electrolyte additive in lithium–carbon dioxide (Li–CO₂) batteries to allow for a solution‐mediated carbon dioxide reduction pathway. Thiophenolate anions, generated via electrochemical reduction of PDS, act as CO₂ capture agents by forming the adduct S‐phenyl carbonothioate (SPC^?) in solution. A mechanism of SPC^?‐mediated CO₂ capture and utilization is proposed and supported via carbon‐13 nuclear magnetic resonance spectroscopy and Fourier‐transform infrared spectroscopy. Reversible formation and decomposition of lithium carbonate and amorphous carbon during cycling, facilitated by the solution‐mediated pathway, are demonstrated with an array of characterization techniques. Li–CO₂ batteries employing the PDS additive show vastly improved capacity, energy efficiency, and cycle life. The enhanced Li–CO₂ battery performance offered by the proposed solution‐mediated reaction pathway offers a compelling step forward in the pursuit of reversible CO₂ utilization.     

CO2‐fixing liquid droplets: Towards a dissection of the microalgal pyrenoid

CO₂ enters the biosphere via the slow, oxygen‐sensitive carboxylase, Rubisco. To compensate, most microalgae saturate Rubisco with its substrate gas through a carbon dioxide concentrating mechanism. This strategy frequently involves compartmentalization of the enzyme in the pyrenoid, a non‐membrane enclosed compartment of the chloroplast stroma. Recently, tremendous advances have been achieved concerning the structure, physical properties, composition and in vitro reconstitution of the pyrenoid matrix from the green alga Chlamydomonas reinhardtii. The discovery of the intrinsically disordered multivalent Rubisco linker protein EPYC1 provided a biochemical framework to explain the subsequent finding that the pyrenoid resembles a liquid droplet in vivo. Reconstitution of the corresponding liquid‐liquid phase separation using pure Rubisco and EPYC1 allowed a detailed characterization of this process. Finally, a large high‐quality dataset of pyrenoidal protein‐protein interactions inclusive of spatial information provides ample substrate for rapid further functional dissection of the pyrenoid. Integrating and extending recent advances will inform synthetic biology efforts towards enhancing plant photosynthesis as well as contribute a versatile model towards experimentally dissecting the biochemistry of enzyme‐containing membraneless organelles.     

Overpotentials and Faraday Efficiencies in CO2 Electrocatalysis–the Impact of 1‐Ethyl‐3‐Methylimidazolium Trifluoromethanesulfonate

The mixtures of room temperature ionic liquid 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate ([EMIM]TFO) and water as electrolytes for reduction of CO₂ to CO are reported. Linear sweep voltammetry shows overpotentials for CO₂ reduction and the competing hydrogen evolution reaction (HER), both of which vary as a function of [EMIM]TFO concentration in the range from 4 × 10^?3m (0.006 mol%) to 4869 × 10^?3m (50 mol%). A steady lowering of overpotentials up to an optimum for 334 × 10?³m is identified. At 20 mol% and more of [EMIM]TFO, a significant CO₂ reduction plateau and inhibition of HER, which is limited by H₂O diffusion, is noted. Such a plateau in CO₂ reduction correlates to high CO Faraday efficiencies. In case of 50 mol% [EMIM]TFO, a broad plateau spanning over a potential range of 0.58 V evolves. At the same time, the overpotential for HER is increased by 1.20 V when compared to 334 × 10^?3m and, in turn, HER is largely inhibited. The Faraday efficiencies for CO and H₂ formation feature 95.6% ± 6.8% and 0.5% ± 0.3%, respectively, over a period of 3 h in a separator divided cell. Cathodic as well as anodic electrochemical stability of the electrolyte throughout this time period is corroborated in ¹H NMR spectroscopic measurements.     

Mechanistic Study Revealing the Role of the Br3−/Br2 Redox Couple in CO2‐Assisted Li–O2 Batteries

Replacing oxygen (O₂) with air is a critical step in the development of lithium (Li)–air batteries. A trace amount of carbon dioxide (CO₂) in the air is, however, influentially involved in the O₂ chemistry, which indicates that a fundamental understanding of the effect of CO₂ is required for the design of practical cells. When up to 30% CO₂ is added to Li–O₂ cells, CO₂ acts as an O₂^? scavenger. Their chemical reactions form soluble products, CO₄^2? and C₂O₆^2?, in the tetraglyme electrolyte solution, and enhance full capacity and cell cyclability. A critical challenge is, however, the sluggish decomposition of the coproduct Li₂CO₃ during recharge. To lower the charging overpotential, a Br₃^?/Br₂ redox couple is incorporated and its redox behavior is investigated using spectroscopic methods. The redox shuttle of Br₃^?/Br₂ decomposes amorphous Li₂CO₃ more efficiently than its crystalline counterpart. It is revealed that Br₂ combines with Br₃^? to form a Br₂···Br₃^? complex, which acts as a mobile catalyst in the electrolyte solution without swift precipitation of the nonpolar Br₂. This comprehensive study, revealing the molecular structure and redox process of mobile catalysts, provides an insight into improving the design of redox couples toward superior cycling performance.     

Bioenergy futures in Sweden – system effects of CO2 reduction and fossil fuel phase‐out policies

Bioenergy could contribute both to the reduction of greenhouse gases and to increased energy security, but the extent of this contribution strongly depends on the cost and potential of biomass resources. For Sweden, this study investigates how the implementation of policies for CO₂ reduction and for phase out of fossil fuels in road transport affect the future utilization of biomass, in the stationary energy system and in the transport sector, and its price. The analysis is based on the bottom‐up, optimization MARKAL_Sweden model, which includes a comprehensive representation of the national energy system. For the analysis, the biomass supply representation of MARKAL_Sweden is updated and improved by the use of, e.g., forestry forecasting modeling and through construction of detailed biomass supply curves. A time horizon up to 2050 is applied. The results indicate a potential for significantly higher use of bioenergy. In the main analysis scenario, in which CO₂ reduction of 80% by 2050 is imposed on the Swedish energy system, the total bioenergy utilization increases by 63% by 2050 compared to 2010. The largest increase occurs in the transport sector, which by 2050 accounts for 43% of the total primary bioenergy use. The high demand and strong competition significantly increase biomass prices and lead to the utilization of higher cost biomass sources such as stumps and cultivated energy forest, as well as use of pulpwood resources for energy purposes.     

Climatic role of terrestrial ecosystem under elevated CO2: a bottom‐up greenhouse gases budget

The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO₂) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO₂. We show that elevated CO₂ significantly stimulates plant C pool (NPP) by 20%, soil CO₂ fluxes by 24%, and methane (CH₄) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH₄ uptake of upland soils by 3.8%. Elevated CO₂ causes insignificant increases in soil nitrous oxide (N₂O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO₂‐induced increase in GHG emissions may decline with CO₂ enrichment levels. An elevated CO₂‐induced rise in soil CH₄ and N₂O emissions (2.76 Pg CO₂‐equivalent year^?1) could negate soil C enrichment (2.42 Pg CO₂ year^?1) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO₂ year^?1) under elevated CO₂. Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO₂ might have been largely offset by its induced increases in soil GHGs source strength.     

Towards a coupled paradigm of NH3‐CO2 biosphere–atmosphere exchange modelling

Stomatal conductance, one of the major plant physiological controls within NH₃ biosphere–atmosphere exchange models, is commonly estimated from semi‐empirical multiplicative schemes or simple light‐ and temperature‐response functions. However, due to their inherent parameterization on meteorological proxy variables, instead of a direct measure of stomatal opening, they are unfit for the use in climate change scenarios and of limited value for interpreting field‐scale measurements. Alternatives based on H₂O flux measurements suffer from uncertainties in the partitioning of evapotranspiration at humid sites, as well as a potential decoupling of transpiration from stomatal opening in the presence of hygroscopic particles on leaf surfaces. We argue that these problems may be avoided by directly deriving stomatal conductance from CO₂ fluxes instead. We reanalysed a data set of NH₃ flux measurements based on CO₂‐derived stomatal conductance, confirming the hypothesis that the increasing relevance of stomatal exchange with the onset of vegetation activity caused a rapid decrease of observed NH₃ deposition velocities. Finally, we argue that developing more mechanistic representations of NH₃ biosphere–atmosphere exchange can be of great benefit in many applications. These range from model‐based flux partitioning, over deposition monitoring using low‐cost samplers and inferential modelling, to a direct response of NH₃ exchange to climate change.     

The CO2‐concentrating mechanism in the bloom‐forming cyanobacterium Microcystis aeruginosa (Cyanophyceae) and effects of UVB radiation on its operation1

The bloom‐forming cyanobacterium Microcystis aeruginosa (Kütz.) Kütz. 854 was cultured with 1.05 W · m^?2 ultraviolet‐B radiation (UVBR) for 3 h every day, and the CO₂‐concentrating mechanism (CCM) within this species as well as effects of UVBR on its operation were investigated. Microcystis aeruginosa 854 possessed at least three inorganic carbon transport systems and could utilize external HCO₃^? and CO₂ for its photosynthesis. The maximum photosynthetic rate was approximately the same, but the apparent affinity for dissolved inorganic carbon was significantly decreased from 74.7 μmol · L^?1 in the control to 34.7 μmol · L^?1 in UVBR‐treated cells. At 150 μmol · L^?1 KHCO₃ and pH 8.0, Na⁺‐dependent HCO₃^? transport contributed 43.4%–40.2% to the photosynthesis in the control and 34.5%–31.9% in UVBR‐treated cells. However, the contribution of Na⁺‐independent HCO₃^? transport increased from 8.7% in the control to 18.3% in UVBR‐treated cells. The contribution of CO₂‐uptake systems showed little difference: 47.9%–51.0% in the control and 49.8%–47.2% in UVBR‐treated cells. Thus, the rate of total inorganic carbon uptake was only marginally affected, although UVBR had a differential effect on various inorganic carbon transporters. However, the number of carboxysomes in UVBR‐treated cells was significantly decreased compared to that in the control.     

Greenhouse gas budget of a poplar bioenergy plantation in Belgium: CO2 uptake outweighs CH4 and N2O emissions

Biomass from short‐rotation coppice (SRC) of woody perennials is being increasingly used as a bioenergy source to replace fossil fuels, but accurate assessments of the long‐term greenhouse gas (GHG) balance of SRC are lacking. To evaluate its mitigation potential, we monitored the GHG balance of a poplar (Populus) SRC in Flanders, Belgium, over 7 years comprising three rotations (i.e., two 2 year rotations and one 3 year rotation). In the beginning—that is, during the establishment year and during each year immediately following coppicing—the SRC plantation was a net source of GHGs. Later on—that is, during each second or third year after coppicing—the site shifted to a net sink. From the sixth year onward, there was a net cumulative GHG uptake reaching ?35.8 Mg CO₂ eq/ha during the seventh year. Over the three rotations, the total CO₂ uptake was ?51.2 Mg CO₂/ha, while the emissions of CH₄ and N₂O amounted to 8.9 and 6.5 Mg CO₂ eq/ha, respectively. As the site was non‐fertilized, non‐irrigated, and only occasionally flooded, CO₂ fluxes dominated the GHG budget. Soil disturbance after land conversion and after coppicing were the main drivers for CO₂ losses. One single N₂O pulse shortly after SRC establishment contributed significantly to the N₂O release. The results prove the potential of SRC biomass plantations to reduce GHG emissions and demonstrate that, for the poplar plantation under study, the high CO₂ uptake outweighs the emissions of non‐CO₂ greenhouse gases.     

How does elevated carbon dioxide (CO2) affect plant–herbivore interactions? A field experiment and meta‐analysis of CO2‐mediated changes on plant chemistry and herbivore performance

First, we report the results of the longest‐known field study (9 years) to examine the effects of elevated carbon dioxide (CO₂) on leaf miner densities in a scrub‐oak community at Kennedy Space Center, Florida. Here, the densities of all leaf miner species (6) on all host species (3) were lower in every year in elevated CO₂ than they were in ambient CO₂. Second, meta‐analyses were used to review the effects of elevated CO₂ on both plants (n=59 studies) and herbivores (n=75 studies). The log of the response ratio was chosen as the metric to calculate effect sizes. Results showed that elevated CO₂ significantly decreased herbivore abundance (−21.6%), increased relative consumption rates (+16.5%), development time (+3.87%) and total consumption (+9.2%), and significantly decreased relative growth rate (−8.3%), conversion efficiency (−19.9%) and pupal weight (−5.03%). No significant differences were observed among herbivore guilds. Host plants growing under enriched CO₂ environments exhibited significantly larger biomass (+38.4%), increased C/N ratio (+26.57%), and decreased nitrogen concentration (−16.4%), as well as increased concentrations of tannins (+29.9%) and other phenolics. Effects of changes on plant primary and secondary chemistry due to elevated CO₂ and consequences for herbivore growth and development are discussed.     

Studies of visible oscillating chemiluminescence in luminol–H2O2–KSCN–CuSO4 system using (2‐hydroxyethyl) trimethylammonium hydroxide

Visible oscillating chemiluminescence (CL) of luminol–H₂O₂–KSCN–CuSO₄ was studied using the organic base (2‐hydroxyethyl)trimethylammonium hydroxide. The effect of concentrations of luminol, H₂O₂, KSCN, CuSO₄ and the base were investigated in a batch reactor. This report shows how the concentration of components involved in the oscillating CL system influenced the oscillation period, light amplitude and total time of light emission. The oscillating CL with different bases was also investigated. Results indicated that using 2‐HETMAOH causes regular oscillating CL with nearly the same oscillating period. Copyright © 2008 John Wiley & Sons, Ltd.     

Microbial physiology and soil CO2 efflux after 9 years of soil warming in a temperate forest – no indications for thermal adaptations

Thermal adaptations of soil microorganisms could mitigate or facilitate global warming effects on soil organic matter (SOM) decomposition and soil CO₂ efflux. We incubated soil from warmed and control subplots of a forest soil warming experiment to assess whether 9 years of soil warming affected the rates and the temperature sensitivity of the soil CO₂ efflux, extracellular enzyme activities, microbial efficiency, and gross N mineralization. Mineral soil (0–10 cm depth) was incubated at temperatures ranging from 3 to 23 °C. No adaptations to long‐term warming were observed regarding the heterotrophic soil CO₂ efflux (R₁₀ warmed: 2.31 ± 0.15 μmol m^?2 s^?1, control: 2.34 ± 0.29 μmol m^?2 s^?1; Q₁₀ warmed: 2.45 ± 0.06, control: 2.45 ± 0.04). Potential enzyme activities increased with incubation temperature, but the temperature sensitivity of the enzymes did not differ between the warmed and the control soils. The ratio of C : N acquiring enzyme activities was significantly higher in the warmed soil. Microbial biomass‐specific respiration rates increased with incubation temperature, but the rates and the temperature sensitivity (Q₁₀ warmed: 2.54 ± 0.23, control 2.75 ± 0.17) did not differ between warmed and control soils. Microbial substrate use efficiency (SUE) declined with increasing incubation temperature in both, warmed and control, soils. SUE and its temperature sensitivity (Q₁₀ warmed: 0.84 ± 0.03, control: 0.88 ± 0.01) did not differ between warmed and control soils either. Gross N mineralization was invariant to incubation temperature and was not affected by long‐term soil warming. Our results indicate that thermal adaptations of the microbial decomposer community are unlikely to occur in C‐rich calcareous temperate forest soils.     

Water availability affects seasonal CO2‐induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO₂ (eCO₂) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (g_s) and increased soil water content (V_SWC) and second, through increased leaf internal CO₂ (C_i) and decreased stomatal limitations (S_lim). It is unclear if such findings from temperate grassland studies similarly pertain to warmer ecosystems with periodic water deficits. We tested these mechanisms in three important C₃ herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO₂‐induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%–50% with a 150 μmol mol^?1 increase in atmospheric CO₂ across seasons. This eCO₂‐induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily V_SWC. The highest photosynthetic enhancement by eCO₂ (>30%) was observed during the most water‐limited period, for example, with V_SWC <0.07 in this sandy surface soil. Under eCO₂ there was neither a significant decrease in g_s in the three herbaceous species, nor increases in V_SWC, indicating no “water‐savings effect” of eCO₂. Periods of low V_SWC showed lower g_s (less than ≈ 0.12 mol m^?2 s^?1), higher relative S_lim (>30%) and decreased C_i under the ambient CO₂ concentration (aCO₂), with leaf photosynthesis strongly carboxylation‐limited. The alleviation of S_lim by eCO₂ was facilitated by increasing C_i, thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO₂, controls g_s and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO₂ has the potential to alter vegetation functioning in a periodically dry woodland understory through changes in stomatal limitation to photosynthesis, not by the “water‐savings effect” usually invoked in grasslands.     

A role for β‐sitosterol to stigmasterol conversion in plant–pathogen interactions

Upon inoculation with pathogenic microbes, plants induce an array of metabolic changes that potentially contribute to induced resistance or even enhance susceptibility. When analysing leaf lipid composition during the Arabidopsis thaliana–Pseudomonas syringae interaction, we found that accumulation of the phytosterol stigmasterol is a significant plant metabolic process that occurs upon bacterial leaf infection. Stigmasterol is synthesized from β‐sitosterol by the cytochrome P450 CYP710A1 via C22 desaturation. Arabidopsis cyp710A1 mutant lines impaired in pathogen‐inducible expression of the C22 desaturase and concomitant stigmasterol accumulation are more resistant to both avirulent and virulent P. syringae strains than wild‐type plants, and exogenous application of stigmasterol attenuates this resistance phenotype. These data indicate that induced sterol desaturation in wild‐type plants favours pathogen multiplication and plant susceptibility. Stigmasterol formation is triggered through perception of pathogen‐associated molecular patterns such as flagellin and lipopolysaccharides, and through production of reactive oxygen species, but does not depend on the salicylic acid, jasmonic acid or ethylene defence pathways. Isolated microsomal and plasma membrane preparations exhibited a similar increase in the stigmasterol/β‐sitosterol ratio as whole‐leaf extracts after leaf inoculation with P. syringae, indicating that the stigmasterol produced is incorporated into plant membranes. The increased contents of stigmasterol in leaves after pathogen attack do not influence salicylic acid‐mediated defence signalling but attenuate pathogen‐induced expression of the defence regulator flavin‐dependent monooxygenase 1. P. syringae thus promotes plant disease susceptibility through stimulation of sterol C22 desaturation in leaves, which increases the stigmasterol to β‐sitosterol ratio in plant membranes.     

Assessment of grain quality in terms of functional group response to elevated [CO2], water,and nitrogen using a meta‐analysis: Grain protein,zinc, and iron under future climate

The increasing [CO₂] in the atmosphere increases crop productivity. However, grain quality of cereals and pulses are substantially decreased and consequently compromise human health. Meta‐analysis techniques were employed to investigate the effect of elevated [CO₂] (e[CO₂]) on protein, zinc (Zn), and iron (Fe) concentrations of major food crops (542 experimental observations from 135 studies) including wheat, rice, soybean, field peas, and corn considering different levels of water and nitrogen (N). Each crop, except soybean, had decreased protein, Zn, and Fe concentrations when grown at e[CO₂] concentration (≥550 μmol/mol) compared to ambient [CO₂] (a[CO₂]) concentration (≤380 μmol/mol). Grain protein, Zn, and Fe concentrations were reduced under e[CO₂]; however, the responses of protein, Zn, and Fe concentrations to e[CO₂] were modified by water stress and N. There was an increase in Fe concentration in soybean under medium N and wet conditions but nonsignificant. The reductions in protein concentrations for wheat and rice were ~5%–10%, and the reductions in Zn and Fe concentrations were ~3%–12%. For soybean, there was a small and nonsignificant increase of 0.37% in its protein concentration under medium N and dry water, while Zn and Fe concentrations were reduced by ~2%–5%. The protein concentration of field peas decreased by 1.7%, and the reductions in Zn and Fe concentrations were ~4%–10%. The reductions in protein, Zn, and Fe concentrations of corn were ~5%–10%. Bias in the dataset was assessed using a regression test and rank correlation. The analysis indicated that there are medium levels of bias within published meta‐analysis studies of crops responses to free‐air [CO₂] enrichment (FACE). However, the integration of the influence of reporting bias did not affect the significance or the direction of the [CO₂] effects.     

State of the science in reconciling top‐down and bottom‐up approaches for terrestrial CO2 budget

Robust estimates of CO₂ budget, CO₂ exchanged between the atmosphere and terrestrial biosphere, are necessary to better understand the role of the terrestrial biosphere in mitigating anthropogenic CO₂ emissions. Over the past decade, this field of research has advanced through understanding of the differences and similarities of two fundamentally different approaches: “top‐down” atmospheric inversions and “bottom‐up” biosphere models. Since the first studies were undertaken, these approaches have shown an increasing level of agreement, but disagreements in some regions still persist, in part because they do not estimate the same quantity of atmosphere–biosphere CO₂ exchange. Here, we conducted a thorough comparison of CO₂ budgets at multiple scales and from multiple methods to assess the current state of the science in estimating CO₂ budgets. Our set of atmospheric inversions and biosphere models, which were adjusted for a consistent flux definition, showed a high level of agreement for global and hemispheric CO₂ budgets in the 2000s. Regionally, improved agreement in CO₂ budgets was notable for North America and Southeast Asia. However, large gaps between the two methods remained in East Asia and South America. In other regions, Europe, boreal Asia, Africa, South Asia, and Oceania, it was difficult to determine whether those regions act as a net sink or source because of the large spread in estimates from atmospheric inversions. These results highlight two research directions to improve the robustness of CO₂ budgets: (a) to increase representation of processes in biosphere models that could contribute to fill the budget gaps, such as forest regrowth and forest degradation; and (b) to reduce sink–source compensation between regions (dipoles) in atmospheric inversion so that their estimates become more comparable. Advancements on both research areas will increase the level of agreement between the top‐down and bottom‐up approaches and yield more robust knowledge of regional CO₂ budgets.