Mineral Carbonation as the Core of an Industrial Symbiosis for Energy‐Intensive Minerals Conversion

The longer term sustainability of the minerals sector may hinge, in large part, on finding innovative solutions to the challenges of energy intensity and carbon dioxide (CO₂) management. This article outlines the need for large‐scale “carbon solutions” that might be shared by several colocated energy‐intensive and carbon‐intensive industries. In particular, it explores the potential for situating a mineral carbonation plant as a carbon sink at the heart of a minerals and energy complex to form an industrial symbiosis. Several resource‐intensive industries can be integrated synergistically in this way, to enable a complex that produces energy and mineral products with low net CO₂ emissions. An illustrative hypothetical case study of such a system within New South Wales, Australia, has been constructed, on the basis of material and energy flows derived from Aspen modeling of a serpentine carbonation process. The synergies and added value created have the potential to significantly offset the energy and emission penalties and direct costs of CO₂ capture and storage. This suggests that greenfield minerals beneficiation and metals refining plants should consider closer integration with the power production and energy provision plants on which they depend, together with a carbon solution, such as mineral carbonation, as a critical element of such integration. Other sustainability considerations are highlighted.     

The role of bioenergy for global deep decarbonization: CO2 removal or low‐carbon energy?

Bioenergy is expected to have a prominent role in limiting global greenhouse emissions to meet the climate change target of the Paris Agreement. Many studies identify negative emissions from bioenergy generation with carbon capture and storage (BECCS) as its key contribution, but assume that no other CO₂ removal technologies are available. We use a global integrated assessment model, TIAM‐UCL, to investigate the role of bioenergy within the global energy system when direct air capture and afforestation are available as cost‐competitive alternatives to BECCS. We find that the presence of other CO₂ removal technologies does not reduce the pressure on biomass resources but changes the use of bioenergy for climate mitigation. While we confirm that when available BECCS offers cheaper decarbonization pathways, we also find that its use delays the phase‐out of unabated fossil fuels in industry and transport. Furthermore, it displaces renewable electricity generation, potentially increasing the likelihood of missing the Paris Agreement target. We found that the most cost‐effective solution is to invest in a basket of CO₂ removal technologies. However, if these technologies rely on CCS, then urgent action is required to ramp up the necessary infrastructure. We conclude that a sustainable biomass supply is critical for decarbonizing the global energy system. Since only a few world regions carry the burden of producing the biomass resource and store CO₂ in geological storage, adequate international collaboration, policies and standards will be needed to realize this resource while avoiding undesired land‐use change.     

Multi‐year carbon budget of a mature commercial short rotation coppice willow plantation

Energy derived from second generation perennial energy crops is projected to play an increasingly important role in the decarbonization of the energy sector. Such energy crops are expected to deliver net greenhouse gas emissions reductions through fossil fuel displacement and have potential for increasing soil carbon (C) storage. Despite this, few empirical studies have quantified the ecosystem‐level C balance of energy crops and the evidence base to inform energy policy remains limited. Here, the temporal dynamics and magnitude of net ecosystem carbon dioxide (CO₂) exchange (NEE) were quantified at a mature short rotation coppice (SRC) willow plantation in Lincolnshire, United Kingdom, under commercial growing conditions. Eddy covariance flux observations of NEE were performed over a four‐year production cycle and combined with biomass yield data to estimate the net ecosystem carbon balance (NECB) of the SRC. The magnitude of annual NEE ranged from ?147 ± 70 to ?502 ± 84 g CO₂‐C m^?2 year^?1 with the magnitude of annual CO₂ capture increasing over the production cycle. Defoliation during an unexpected outbreak of willow leaf beetle impacted gross ecosystem production, ecosystem respiration, and net ecosystem exchange during the second growth season. The NECB was ?87 ± 303 g CO₂‐C m^?2 for the complete production cycle after accounting for C export at harvest (1,183 g C m^?2), and was approximately CO₂‐C neutral (?21 g CO₂‐C m^?2 year^?1) when annualized. The results of this study are consistent with studies of soil organic C which have shown limited changes following conversion to SRC willow. In the context of global decarbonization, the study indicates that the primary benefit of SRC willow production at the site is through displacement of fossil fuel emissions.     

Full carbon and greenhouse gas balances of fertilized and nonfertilized reed canary grass cultivations on an abandoned peat extraction area in a dry year

Bioenergy crop cultivation on former peat extraction areas is a potential after‐use option that provides a source of renewable energy while mitigating climate change through enhanced carbon (C) sequestration. This study investigated the full C and greenhouse gas (GHG) balances of fertilized (RCG‐F) and nonfertilized (RCG‐C) reed canary grass (RCG; Phalaris arundinacea) cultivation compared to bare peat (BP) soil within an abandoned peat extraction area in western Estonia during a dry year. Vegetation sampling, static chamber and lysimeter measurements were carried out to estimate above‐ and belowground biomass production and allocation, fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in cultivated strips and drainage ditches as well as the dissolved organic carbon (DOC) export, respectively. Heterotrophic respiration was determined from vegetation‐free trenched plots. Fertilization increased the above‐ to belowground biomass production ratio and the autotrophic to heterotrophic respiration ratio. The full C balance (incl. CO₂, CH₄ and DOC fluxes from strips and ditches) was 96, 215 and 180 g C m^?2 yr^?1 in RCG‐F, RCG‐C and BP, respectively, suggesting that all treatments acted as C sources during the dry year. The C balance was driven by variations in the net CO₂ exchange, whereas the combined contribution of CH₄ and DOC fluxes was <5%. The GHG balances were 3.6, 7.9 and 6.6 t CO₂ eq ha^?1 yr^?1 in RCG‐F, RCG‐C and BP, respectively. The CO₂ exchange was also the dominant component of the GHG balance, while the contributions of CH₄ and N₂O were <1% and 1–6%, respectively. Overall, this study suggests that maximizing plant growth and the associated CO₂ uptake through adequate water and nutrient supply is a key prerequisite for ensuring sustainable high yields and climate benefits in RCG cultivations established on organic soils following drainage and peat extraction.     

The role of bioenergy and biochemicals in CO2 mitigation through the energy system – a scenario analysis for the Netherlands

Bioenergy as well as bioenergy with carbon capture and storage are key options to embark on cost‐efficient trajectories that realize climate targets. Most studies have not yet assessed the influence on these trajectories of emerging bioeconomy sectors such as biochemicals and renewable jet fuels (RJFs). To support a systems transition, there is also need to demonstrate the impact on the energy system of technology development, biomass and fossil fuel prices. We aim to close this gap by assessing least‐cost pathways to 2030 for a number of scenarios applied to the energy system of the Netherlands, using a cost‐minimization model. The type and magnitude of biomass deployment are highly influenced by technology development, fossil fuel prices and ambitions to mitigate climate change. Across all scenarios, biomass consumption ranges between 180 and 760 PJ and national emissions between 82 and 178 Mt CO₂. High technology development leads to additional 100–270 PJ of biomass consumption and 8–20 Mt CO₂ emission reduction compared to low technology development counterparts. In high technology development scenarios, additional emission reduction is primarily achieved by bioenergy and carbon capture and storage. Traditional sectors, namely industrial biomass heat and biofuels, supply 61–87% of bioenergy, while wind turbines are the main supplier of renewable electricity. Low technology pathways show lower biochemical output by 50–75%, do not supply RJFs and do not utilize additional biomass compared to high technology development. In most scenarios the emission reduction targets for the Netherlands are not met, as additional reduction of 10–45 Mt CO₂ is needed. Stronger climate policy is required, especially in view of fluctuating fossil fuel prices, which are shown to be a key determinant of bioeconomy development. Nonetheless, high technology development is a no‐regrets option to realize deep emission reduction as it also ensures stable growth for the bioeconomy even under unfavourable conditions.     

Belowground impacts of perennial grass cultivation for sustainable biofuel feedstock production in the tropics

Perennial grasses can sequester soil organic carbon (SOC) in sustainably managed biofuel systems, directly mitigating atmospheric CO₂ concentrations while simultaneously generating biomass for renewable energy. The objective of this study was to quantify SOC accumulation and identify the primary drivers of belowground C dynamics in a zero‐tillage production system of tropical perennial C4 grasses grown for biofuel feedstock in Hawaii. Specifically, the quantity, quality, and fate of soil C inputs were determined for eight grass accessions – four varieties each of napier grass and guinea grass. Carbon fluxes (soil CO₂ efflux, aboveground net primary productivity, litterfall, total belowground carbon flux, root decay constant), C pools (SOC pool and root biomass), and C quality (root chemistry, C and nitrogen concentrations, and ratios) were measured through three harvest cycles following conversion of a fallow field to cultivated perennial grasses. A wide range of SOC accumulation occurred, with both significant species and accession effects. Aboveground biomass yield was greater, and root lignin concentration was lower for napier grass than guinea grass. Structural equation modeling revealed that root lignin concentration was the most important driver of SOC pool: varieties with low root lignin concentration, which was significantly related to rapid root decomposition, accumulated the greatest amount of SOC. Roots with low lignin concentration decomposed rapidly, but the residue and associated microbial biomass/by‐products accumulated as SOC. In general, napier grass was better suited for promoting soil C sequestration in this system. Further, high‐yielding varieties with low root lignin concentration provided the greatest climate change mitigation potential in a ratoon system. Understanding the factors affecting SOC accumulation and the net greenhouse gas trade‐offs within a biofuel production system will aid in crop selection to meet multiple goals toward environmental and economic sustainability.     

Greenhouse gas mitigation potential in crop production with biochar soil amendment—a carbon footprint assessment for cross‐site field experiments from China

Biochar soil amendment (BSA) had been advocated as a promising approach to mitigate greenhouse gas (GHG) emissions in agriculture. However, the net GHG mitigation potential of BSA remained unquantified with regard to the manufacturing process and field application. Carbon footprint (CF) was employed to assess the mitigating potential of BSA by estimating all the direct and indirect GHG emissions in the full life cycles of crop production including production and field application of biochar. Data were obtained from 7 sites (4 sites for paddy rice production and 3 sites for maize production) under a single BSA at 20 t/ha^?1 across mainland China. Considering soil organic carbon (SOC) sequestration and GHG emission reduction from syngas recycling, BSA reduced the CFs by 20.37–41.29 t carbon dioxide equivalent ha^?1 (CO₂‐eq ha^?1) and 28.58–39.49 t CO₂‐eq ha^?1 for paddy rice and maize production, respectively, compared to no biochar application. Without considering SOC sequestration and syngas recycling, the net CF change by BSA was in a range of ?25.06 to 9.82 t CO₂‐eq ha^?1 and ?20.07 to 5.95 t CO₂‐eq ha^?1 for paddy rice and maize production, respectively, over no biochar application. As the largest contributors among the others, syngas recycling in the process of biochar manufacture contributed by 47% to total CF reductions under BSA for rice cultivation while SOC sequestration contributed by 57% for maize cultivation. There was a large variability of the CF reductions across the studied sites whether in paddy rice or maize production, due likely to the difference in GHG emission reductions and SOC increments under BSA across the sites. This study emphasized that SOC sequestration should be taken into account the CF calculation of BSA. Improved biochar manufacturing technique could achieve a remarkable carbon sink by recycling the biogas for traditional fossil‐fuel replacement.     

Land management and climate change determine second‐generation bioenergy potential of the US Northern Great Plains

Bioenergy with carbon capture and storage (BECCS) has been proposed as a potential climate mitigation strategy raising concerns over trade‐offs with existing ecosystem services. We evaluate the feasibility of BECCS in the Upper Missouri River Basin (UMRB), a landscape with diverse land use, ownership, and bioenergy potential. We develop land‐use change scenarios and a switchgrass (Panicum virgatum L.) crop functional type to use in a land‐surface model to simulate second‐generation bioenergy production. By the end of this century, average annual switchgrass production over the UMRB ranges from 60 to 210 Tg dry mass/year and is dependent on the Representative Concentration Pathway for greenhouse gas emissions and on land‐use change assumptions. Under our simple phase‐in assumptions this results in a cumulative total production of 2,000–6,000 Tg C over the study period with the upper estimates only possible in the absence of climate change. Switchgrass yields decreased as average CO₂ concentrations and temperatures increased, suggesting the effect of elevated atmospheric CO₂ was small because of its C4 photosynthetic pathway. By the end of the 21st century, the potential energy stored annually in harvested switchgrass averaged between 1 and 4 EJ/year assuming perfect conversion efficiency, or an annual electrical generation capacity of 7,000–28,000 MW assuming current bioenergy efficiency rates. Trade‐offs between bioenergy and ecosystem services were identified, including cumulative direct losses of 1,000–2,600 Tg C stored in natural ecosystems from land‐use change by 2090. Total cumulative losses of ecosystem carbon stocks were higher than the potential ~300 Tg C in fossil fuel emissions from the single largest power plant in the region over the same time period, and equivalent to potential carbon removal from the atmosphere from using biofuels grown in the same region. Numerous trade‐offs from BECCS expansion in the UMRB must be balanced against the potential benefits of a carbon‐negative energy system.     

Life Cycle Assessment of Carbon Dioxide–Based Production of Methane and Methanol and Derived Polymers

Previous studies showed that using carbon dioxide (CO₂) as a raw material for chemical syntheses may provide an opportunity for achieving greenhouse gas (GHG) savings and a low‐carbon economy. Nevertheless, it is not clear whether carbon capture and utilization benefits the environment in terms of resource efficiency. We analyzed the production of methane, methanol, and synthesis gas as basic chemicals and derived polyoxymethylene, polyethylene, and polypropylene as polymers by calculating the output‐oriented indicator global warming impact (GWI) and the resource‐based indicators raw material input (RMI) and total material requirement (TMR) on a cradle‐to‐gate basis. As carbon source, we analyzed the capturing of CO₂ from air, raw biogas, cement plants, lignite‐fired power, and municipal waste incineration plants. Wind power serves as an energy source for hydrogen production. Our data were derived from both industrial processes and process simulations. The results demonstrate that the analyzed CO₂‐based process chains reduce the amount of GHG emissions in comparison to the conventional ones. At the same time, the CO₂‐based process chains require an increased amount of (abiotic) resources. This trade‐off between decreased GHG emissions and increased resource use is assessed. The decision about whether or not to recycle CO₂ into hydrocarbons depends largely on the source and amount of energy used to produce hydrogen.     

Environmental Implications and Costs of Municipal Solid Waste‐Derived Ethylene

Carbon recycling, in which organic waste is recycled into chemical feedstock for material production, may provide benefits in resource efficiency and a more cyclical economy—but may also create “trade‐offs” in increased impacts elsewhere. We investigate the system‐wide environmental burdens and cost associated with carbon recycling routes capable of converting municipal solid waste (MSW) by gasification and Fischer‐Tropsch synthesis into ethylene. Results are compared to business‐as‐usual (BAU) cases in which ethylene is derived from fossil resources and waste is either landfilled with methane and energy recovery (BAU#1) or incinerated (BAU#2) with energy recovery. Monte Carlo and sensitivity analysis is used to assess uncertainties of the results. Results indicate that carbon recycling may lead to a reduction in cumulative energy demand (CED), total material requirement (TMR), and acidification, when compared to BAU#1. Global warming potential is found to be similar or slightly lower than BAU#1 and BAU#2. In comparison to BAU#2, carbon recycling results in higher CED, TMR, acidification, and smog potential, mainly as a result of larger (fossil‐based) energy offsets from energy recovery. However, if a renewable power mix (envisioned for the future) is assumed to be offset, BAU#2 impacts may be similar or higher than carbon recycling routes. Production cost per kilogram (kg) MSW‐derived ethylene range between US$1.85 and US$2.06 (Jan 2011 US$). This compares to US$1.17 per kg for fossil‐based ethylene. Waste‐derived ethylene breaks even with its fossil‐based counterpart at a tipping fee of roughly US$42 per metric ton of waste feedstock.     

Effect of Change of Forest Carbon Storage on Net Carbon Dioxide Balance of Wood Use for Energy in Japan

This study analyzed the net carbon dioxide (CO₂) emission reductions between 2005 and 2050 by using wood for energy under various scenarios of forest management and energy conversion technology in Japan, considering both CO₂ emission reductions from replacement of fossil fuels and changes in carbon storage in forests. According to our model, wood production for energy results in a significant reduction of carbon storage levels in forests (by 46% to 77% in 2050 from the 2005 level). Thus, the net CO₂ emission reduction when wood is used for energy becomes drastically smaller. Conventional tree production for energy increases net CO₂ emissions relative to preserving forests, but fast‐growing tree production may reduce net CO₂ emissions more than preserving forests does. When wood from fast‐growing trees is used to generate electricity with gas turbines, displacing natural gas, the net CO₂ emission reduction from the combination of fast‐growing trees and electricity generation with gas turbines is about 58% of the CO₂ emission reduction from electricity generation from gas turbines alone in 2050, and an energy conversion efficiency of around 20% or more is required to obtain net reductions over the entire period until 2050. When wood is used to produce bioethanol, displacing gasoline, net reductions are realized after 2030, provided that heat energy is recovered from residues from ethanol production. These results show the importance of considering the change in carbon storage when estimating the net CO₂ emission reduction effect of the wood use for energy.     

Toward a Carbon Dioxide Neutral Industrial Park

The industrial park of Herdersbrug (Brugge, Flanders, Belgium) comprises 92 small and medium‐sized enterprises, a waste‐to‐energy incinerator, and a power plant (not included in the study) on its site. To study the carbon dioxide (CO₂) neutrality of the park, we made a park‐wide inventory for 2007 of the CO₂ emissions due to energy consumption (electricity and fossil fuel) and waste incineration, as well as an inventory of the existing renewable electricity and heat generation. The definition of CO₂ neutrality in Flanders only considers CO₂ released as a consequence of consumption or generation of electricity, not the CO₂ emitted when fossil fuel is consumed for heat generation. To further decrease or avoid CO₂ emissions, we project and evaluate measures to increase renewable energy generation. The 21 kilotons (kt) of CO₂ emitted due to electricity consumption are more than compensated by the 25 kt of CO₂ avoided by generation of renewable electricity. Herdersbrug Industrial Park is thus CO₂ neutral, according to the definition of the Flemish government. Only a small fraction (6.6%) of the CO₂ emitted as a consequence of fossil fuel consumption (heat generation) and waste incineration is compensated by existing and projected measures for renewable heat generation. Of the total CO₂ emission (149 kt) due to energy consumption (electricity + heat generation) and waste incineration on the Herdersbrug Industrial Park in 2007, 70.5% is compensated by existing and projected renewable energy generated in the park. Forty‐seven percent of the yearly avoided CO₂ corresponds to renewable energy generated from waste incineration and biomass fermentation.     

Challenges and opportunities for hydrogen production from microalgae

The global population is predicted to increase from ~7.3 billion to over 9 billion people by 2050. Together with rising economic growth, this is forecast to result in a 50% increase in fuel demand, which will have to be met while reducing carbon dioxide (CO₂) emissions by 50–80% to maintain social, political, energy and climate security. This tension between rising fuel demand and the requirement for rapid global decarbonization highlights the need to fast‐track the coordinated development and deployment of efficient cost‐effective renewable technologies for the production of CO₂ neutral energy. Currently, only 20% of global energy is provided as electricity, while 80% is provided as fuel. Hydrogen (H₂) is the most advanced CO₂‐free fuel and provides a ‘common’ energy currency as it can be produced via a range of renewable technologies, including photovoltaic (PV), wind, wave and biological systems such as microalgae, to power the next generation of H₂ fuel cells. Microalgae production systems for carbon‐based fuel (oil and ethanol) are now at the demonstration scale. This review focuses on evaluating the potential of microalgal technologies for the commercial production of solar‐driven H₂ from water. It summarizes key global technology drivers, the potential and theoretical limits of microalgal H₂ production systems, emerging strategies to engineer next‐generation systems and how these fit into an evolving H₂ economy.     

A Heterogeneous Carbon Nitride–Nickel Photocatalyst for Efficient Low‐Temperature CO2 Methanation

The Sabatier reaction, i.e., the hydrogenation of CO₂ to methane (CH₄) using hydrogen (H₂), constitutes a potentially scalable method to store energy in a product with a high energy density. However, up to today, this reaction has been mainly thermally driven and conducted at high temperatures (typically 400–600 °C). Using light as a renewable energy source will allow for a more sustainable process by lowering the reaction temperature. Here, it is demonstrated that Ni nanoparticles support on graphitic carbon nitride (g‐CN) are a highly efficient and stable photocatalyst for the gas‐phase CO₂ methanation at low temperature (150 °C). Detailed mechanistic studies reveal a very low activation energy for the reaction and high activity under visible light, leading to a remarkable and continuous CH₄ production of 28 µmol g^?1 h^?1 of CH₄ for 24 h.     

Atmospheric impact of bioenergy based on perennial crop (reed canary grass,Phalaris arundinaceae,L.) cultivation on a drained boreal organic soil

Marginal organic soils, abundant in the boreal region, are being increasingly used for bioenergy crop cultivation. Using long‐term field experimental data on greenhouse gas (GHG) balance from a perennial bioenergy crop [reed canary grass (RCG), Phalaris arundinaceae L.] cultivated on a drained organic soil as an example, we show here for the first time that, with a proper cultivation and land‐use practice, environmentally sound bioenergy production is possible on these problematic soil types. We performed a life cycle assessment (LCA) for RCG on this organic soil. We found that, on an average, this system produces 40% less CO₂‐equivalents per MWh of energy in comparison with a conventional energy source such as coal. Climatic conditions regulating the RCG carbon exchange processes have a high impact on the benefits from this bioenergy production system. Under appropriate hydrological conditions, this system can even be carbon‐negative. An LCA sensitivity analysis revealed that net ecosystem CO₂ exchange and crop yield are the major LCA components, while non‐CO₂ GHG emissions and costs associated with crop production are the minor ones. Net bioenergy GHG emissions resulting from restricted net CO₂ uptake and low crop yields, due to climatic and moisture stress during dry years, were comparable with coal emissions. However, net bioenergy emissions during wet years with high net uptake and crop yield were only a third of the coal emissions. As long‐term experimental data on GHG balance of bioenergy production are scarce, scientific data stemming from field experiments are needed in shaping renewable energy source policies.     

Metal‐Organic Frameworks for Carbon Dioxide Capture and Methane Storage

In the global transition to a sustainable low‐carbon economy, CO₂ capture and storage technology still plays a critical role for deep emission reduction, particularly for the stationary sources in power generation and industry. However, for small and mobile emission sources in transportation, CO₂ capture is not suitable and it is more practical to use relatively clean energy, such as natural gas. In these two low‐carbon energy technologies, designing highly selective sorbents is one of the key and most challenging steps. Toward this end, metal‐organic frameworks (MOFs) have received continuously intensive attention in the past decades for their highly porous and diversified structures. In this review, the recent progress in developing MOFs for selective CO₂ capture from post‐combustion flue gas and CH₄ storage for vehicle applications are summarized. For CO₂ capture, several promising strategies being used to improve CO₂ adsorption uptake at low pressures are highlighted and compared. In addition, the conventional and novel regeneration techniques for MOFs are also discussed. In the case of CH₄ storage, the flexible and rigid MOFs, whose CH₄ storage capacity is close to the target set by U.S. Department of Energy are particularly emphasized. Finally, the challenge of using MOFs for CH₄ storage is discussed.     

Periplasmic expression of carbonic anhydrase in Escherichia coli: A new biocatalyst for CO2 hydration

Carbonic anhydrase is a valuable and efficient catalyst for CO₂ hydration. Most often the free enzyme is employed which complicates catalyst recycling, and can increase cost due to the need for protein purification. Immobilization of the enzyme may address these shortcomings. Here we report the development of whole‐cell biocatalysts for CO₂ hydration via periplasmic expression of two forms of carbonic anhydrase in Escherichia coli using two different targeting sequences. The enzymatic turnover numbers (k_cat) and catalytic efficiencies (k_cat/K_M) were decreased by an order of magnitude as compared to the free soluble enzyme, indicating the introduction of transport limitations. However, the thermal stabilities were improved for most configurations (>88% activity retention up to 95°C for three of four whole‐cell biocatalysts), operational stabilities were more than satisfactory (100% retention after 24 h of use for all four whole‐cell biocatalysts), and CO₂ hydration was significantly enhanced relative to the uncatalyzed reaction (～50–70% increase in CaCO₃ precipitate formed). A significant advantage of the whole‐cell approach is that protein purification is no longer necessary, and the cells can be easily separated and recycled in future applications including biofuel production, biosensors, and carbon capture and storage. Biotechnol. Bioeng. 2013; 110: 1865–1873. © 2013 Wiley Periodicals, Inc.     

ELEVATED ATMOSPHERIC CARBON DIOXIDE INCREASES ORGANIC CARBON FIXATION BY EMILIANIA HUXLEYI (HAPTOPHYTA), UNDER NUTRIENT‐LIMITED HIGH‐LIGHT CONDITIONS1

Phytoplankton play a key role in determining the partitioning of CO₂ between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw‐down of atmospheric CO₂ and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon‐to‐nitrogen (C:N) and carbon‐to‐phosphorus (C:P) assimilation ratios modify the oceanic CO₂ storage capacity. Here we show that increased atmospheric CO₂ concentration enhances CO₂ fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO₂ led to further increases in the particulate C:N and C:P ratios. Enhanced CO₂ uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO₂, could increase carbon storage in the nitrogen‐limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO₂ levels.     

Carbon budgeting in plant–soil mesocosms under elevated CO2: locally missing carbon?

Studies have suggested that more carbon is fixed due to a large increase in photosynthesis in plant–soil systems exposed to elevated CO₂ than could subsequently be found in plant biomass and soils –‐ the locally missing carbon phenomenon. To further understand this phenomenon, an experiment was carried out using EcoCELLs which are open‐flow, mass‐balance systems at the mesocosm scale. Naturally occurring ¹³C tracers were also used to separately measure plant‐derived carbon and soil‐derived carbon. The experiment included two EcoCELLs, one under ambient atmospheric CO₂ and the other under elevated CO₂ (ambient plus 350 μL L^{? 1}). By matching carbon fluxes with carbon pools, the issue of locally missing carbon was investigated. Flux‐based net primary production (NPP_f) was similar to pool‐based primary production (NPP_p) under ambient CO₂, and the discrepancy between the two carbon budgets (12 g C m^{? 2}, or 4% of NPP_f) was less than measurement errors. Therefore, virtually all carbon entering the system under ambient CO₂ was accounted for at the end of the experiment. Under elevated CO₂, however, the amount of NPP_f was much higher than NPP_p, resulting in missing carbon of approximately 80 g C m^{? 2} or 19% of NPP_f which was much higher than measurement errors. This was additional to the 96% increase in rhizosphere respiration and the 50% increase in root growth, two important components of locally missing carbon. The mystery of locally missing carbon under elevated CO₂ remains to be further investigated. Volatile organic carbon, carbon loss due to root washing, and measurement errors are discussed as some of the potential contributing factors.