Free-air CO2 Enrichment Research Articles

Elevated Atmospheric Co2 Levels Impact Soil Protist Functional Core Community Compositions

Protists, known as microeukaryotes, are a significant portion of soil microbial communities. They are crucial predators of bacteria and depend on bacterial community dynamics for the growth and evolution of protistan communities. In parallel, increased levels of atmospheric CO2 significantly impact bacterial metabolic activity in rhizosphere soils. In this study, we investigated the effect of elevated atmospheric CO2 levels on the metabolically active protist community composition and function and their co-occurrences with bacteria from bulk and rhizosphere soils from the Giessen Free-Air CO2 enrichment grassland experiment. Metabarcoding sequencing data analyses of partial 18S rRNA from total soil RNA showed that elevated CO2 concentrations stimulated only a few ASVs of phagotrophic predators of bacteria and other microeukaryotes, affecting protist community composition (P = 0.006, PERMANOVA). In parallel, phagotrophic and parasitic lineages appeared slightly favoured under ambient CO2 conditions, results that were corroborated by microbial signature analyses. Cross-comparisons of protist-bacteria co-occurrences showed mostly negative relations between prokaryotes and microeukaryotes, indicating that the ongoing increase in atmospheric CO2 will lead to changes in microbial soil communities and their interactions, potentially cascading to higher trophic levels in soil systems.

Elevated atmospheric CO2 alters the multi-element stoichiometry of pollen-bearing oak flowers, with possible negative effects on bees

Increasing atmospheric CO2 levels change the elemental composition in plants, altering their nutritional quality and affecting consumers and ecosystems. Ecological stoichiometry provides a framework for investigating how CO2-driven nutrient dilution in pollen affects bees by linking changes in pollen chemical element proportions to the nutritional needs of bees. We investigated the consequences of five years of Free Air CO2 Enrichment (FACE) in a mature oak-dominated temperate forest on the elemental composition of English oak (Quercus robur) pollen. We measured the concentrations and proportions of 12 elements (C, N, P, S, K, Na, Ca, Mg, Cu, Zn, Fe, and Mn) in Q. robur pollen-bearing flowers collected from the Birmingham Institute for Forest Research (BIFoR) FACE facility. An elevated CO2 (eCO2) level of 150 ppm above ambient significantly reduced the S, K, and Fe levels and altered the multi-element ratio, with different elements behaving differently. This shift in pollen multi-element composition may have subsequent cascading effects on higher trophic levels. To assess the impact on bees, we calculated the stoichiometric mismatch (a measure of the discrepancy between consumer needs and food quality) for two bee species, Osmia bicornis (red mason bee) and Apis mellifera (honey bee), that consume oak pollen in nature. We observed stoichiometric mismatches for P and S, in pollen under eCO2, which could negatively affect bees. We highlight the need for a comprehensive understanding of the changes in pollen multi-element stoichiometry under eCO2, which leads to nutrient limitations under climate change with consequences for bees.

Getting allometry right at the Oak Ridge free‐air CO2 enrichment experiment: Old problems and new opportunities for global change experiments

Societal Impact StatementFree‐air CO2 enrichment (FACE) experiments provide essential data on forest responses to increasing atmospheric CO2 for evaluations of climate change impacts on humanity. Understanding and reducing the uncertainty in the experimental results is critical to ensure scientific and public confidence in the models and policy initiatives that derive therefrom. One source of uncertainty is the estimation of tree biomass using mathematical relationships between biomass and easily obtained and non‐destructive measurements (allometry). We evaluated the robustness of the allometric relationships established at the beginning of a FACE experiment and discuss the challenges and opportunities for the new generation of FACE experiments.Summary Long‐term field experiments to elucidate forest responses to rising atmospheric CO2 concentration require allometric equations to estimate tree biomass from non‐destructive measurements of tree size. We analyzed whether the allometric equations established at the beginning of a free‐air CO2 enrichment (FACE) experiment in a Liquidambar styraciflua plantation were still valid at the end of the 12 year experiment. Aboveground woody biomass was initially predicted by an equation that included bole diameter, taper, and height, assuming that including taper and height as predictors would accommodate changes in tree structure that might occur over time and in response to elevated CO2. At the conclusion of the FACE experiment, we harvested 23 trees, measured dimensions and dry mass of boles and branches, and extracted and measured the woody root mass of 10 trees. Although 10 of the harvested trees were larger than the trees used to establish the allometric relationship, measured aboveground woody biomass was well predicted by the original allometry. The initial linear equation between bole basal area and woody root biomass underestimated final root biomass by 28%, but root biomass was just 21% of total wood mass, and errors in aboveground and belowground estimates were offsetting. The allometry established at the beginning of the experiment provided valid predictions of tree biomass throughout the experiment. New allometric approaches using terrestrial laser scanning should reduce an important source of uncertainty in decade‐long forest experiments and in assessments of centuries‐long forest biomass accretion used in evaluating carbon offsets and climate mitigation.

Enhanced woody biomass production in a mature temperate forest under elevated CO2

Enhanced CO2 assimilation by forests as atmospheric CO2 concentration rises could slow the rate of CO2 increase if the assimilated carbon is allocated to long-lived biomass. Experiments in young tree plantations support a CO2 fertilization effect as atmospheric CO2 continues to increase. Uncertainty exists, however, as to whether older, more mature forests retain the capacity to respond to elevated CO2. Here, aided by tree-ring analysis and canopy laser scanning, we show that a 180-year-old Quercus robur L. woodland in central England increased the production of woody biomass when exposed to free-air CO2 enrichment (FACE) for 7 years. Further, elevated CO2 increased exudation of carbon from fine roots into the soil with likely effects on nutrient cycles. The increase in tree growth and allocation to long-lived woody biomass demonstrated here substantiates the major role for mature temperate forests in climate change mitigation.

Soil organic carbon under decadal elevated CO2: Pool size unchanged but stability reduced

Soil organic carbon (SOC) dynamics under elevated atmospheric CO2 concentration has been widely reported, however, in which the behaviors of active and passive fractions remain inadequately explored. Here we studied this issue using three pairs of active and passive fractions of SOC under a 10-year free-air CO2 enrichment experiment (550 ​± ​17 ​ppm) in a cropland in the North China Plain. We found that decadal elevated CO2 increased the root biomass, root exudation rate and microbial biomass, but had little effects on SOC pool size. Elevated CO2 increased the readily oxidizable organic carbon (ROOC) and particulate organic carbon (POC) due to the increments of root C input, but decreased their paired passive fractions possibly because of the carbon input-induced positive priming effect. Our results indicate the reduced stability of SOC pool under elevated CO2. This is significant for better predicting SOC feedback to future climate change.

Carbon-phosphorus cycle models overestimate CO2 enrichment response in a mature Eucalyptus forest.

The importance of phosphorus (P) in regulating ecosystem responses to climate change has fostered P-cycle implementation in land surface models, but their CO2 effects predictions have not been evaluated against measurements. Here, we perform a data-driven model evaluation where simulations of eight widely used P-enabled models were confronted with observations from a long-term free-air CO2 enrichment experiment in a mature, P-limited Eucalyptus forest. We show that most models predicted the correct sign and magnitude of the CO2 effect on ecosystem carbon (C) sequestration, but they generally overestimated the effects on plant C uptake and growth. We identify leaf-to-canopy scaling of photosynthesis, plant tissue stoichiometry, plant belowground C allocation, and the subsequent consequences for plant-microbial interaction as key areas in which models of ecosystem C-P interaction can be improved. Together, this data-model intercomparison reveals data-driven insights into the performance and functionality of P-enabled models and adds to the existing evidence that the global CO2-driven carbon sink is overestimated by models.

Individual grain mass of inbred rice cultivars does not benefit from elevated [CO2]

Climate warming has led to a reduction of global crop yields. Elevated atmospheric [CO2] is believed to promote crop production by increasing photosynthesis, and thus partly offset the yield losses due to climate warming. However, photosynthetic acclimation may occur when the plant is exposed to long-term high [CO2] conditions, suggesting that elevated [CO2] (e[CO2]) might not bring benefits for cereal crops as the growing season proceeds. To assess the effect of long-term e[CO2] on rice yield, particularly on individual grain mass determined post-heading, we synthesized existing data from FACE (free-air CO2 enrichment) experiments across four locations in Japan and China. We also conducted a five-year field experiment with different [CO2] treatments using OTC (open-top chamber) facility. A novel experiment of pot replacement at heading was conducted in 2018 to evaluate the effect of post-heading e[CO2] on individual grain mass of rice. Meanwhile, we measured net photosynthetic rates under ambient [CO2] (a[CO2]) and e[CO2] (ambient + 200 μmol mol−1) at different developmental stages to identify the occurrence of photosynthetic acclimation. We show that FACE condition did not increase individual grain mass across thirty-six inbred rice cultivars at various rates of nitrogen application, and that the replacement of potted plants either from a[CO2] to e[CO2] or from e[CO2] to a[CO2] did not impact the individual grain mass. The yield benefit from e[CO2] was primarily attributed to an increase in the spikelet density determined pre-heading. We conclude that the individual grain mass of inbred rice does not benefit from e[CO2], which is most likely attributed to a loss of the advantage in CO2 gain induced by photosynthetic acclimation. Our findings suggest the necessity for selective breeding strategies to make better use of post-heading e[CO2] and for crop modelers to incorporate updated knowledge into models in the context of elevated [CO2].

Five decades of ecological and meteorological data enhance the mechanistic understanding of global change impacts on the treeline ecotone in the European Alps

Understanding the dynamics of treeline ecotones under global change requires long-term ecological and environmental data. The Stillberg ecological treeline research site in the Swiss Alps was established in 1975 by planting 92,000 seedlings of Larix decidua, Pinus cembra and Pinus mugo ssp. uncinata, and has been continuously monitored since then. Here, we present curated long-term data acquired over almost 50 years at the Stillberg site, and we synthesise the major research findings. The long-term datasets comprise 6.5 million ecological and environmental records from the 40-year afforestation experiment, as well as from a 9-year free-air CO2 enrichment experiment crossed with a 6-year soil warming experiment, a 12-year nutrient addition experiment and an 8-year multifactorial tree seedling recruitment experiment. Our datasets further include 38 million records of 25 meteorological parameters measured at an hourly resolution from 1975 to 1996, and at a 10 min resolution since 1997. We provide all datasets and the corresponding metadata as open research data. Almost five decades of research in this treeline ecotone showed high mortality after tree establishment that was closely related to microclimatic variability. The two Pinus species survived at a much lower rate than L. decidua, due to indirect pathogen interactions. Furthermore, CO2 enrichment only increased growth of L. decidua, while warming increased growth of P. mugo ssp. uncinata and two Vaccinium shrub species. Enhanced nutrient availability stimulated growth in tree and understorey shrub species. In addition, soil warming and CO2 enrichment stimulated microbial activity and decreased soil carbon stocks. These findings improve our understanding of ecological processes in the treeline ecotone under global change and confirm the importance of tree growth and establishment limitations. The enhanced availability and quality of these long-term data are expected to foster whole-system approaches and transdisciplinary research syntheses, supporting the development of effective global change adaptation strategies.

Variability in the responses of rice ecotypes to elevated CO2 based on data from FACE studies in China and Japan: Implications for climate change adaptation

Elevated CO2 increases rice yields, and the response level varies across locations and genotypes. Previous analyses of genotypic variations from diverse Free-Air CO2 Enrichment (FACE) studies lacked specificity, limiting their applicability in simulating the responses of crop growth to elevated CO2. Using meta-analysis approach and the ORYZA (v3) model with historical and projected climatic data, this study evaluated the differences in the responses of rice ecotypes to elevated CO2 and identified adaptive measures. Meta-analytical findings indicated that Chinese inbred indica (indicai) and hybrid indica (indicah) rice exhibited comparable yield response rates (28.4% and 31.1%, respectively) to elevated CO2, surpassing those of Chinese japonica rice and Japanese indicai and japonica rice. Achieving higher adaptation to elevated CO2, exemplified by Chinese indicah rice, necessitates the consideration of balanced yield components, with individual contributions to yield responses ranging from 9.8% to 36.2%. This study highlighted the susceptibility of japonica rice to adverse effects of maximum temperatures on yield component responses to elevated CO2 compared to indicai or indicah rice. Strategic adjustments in sowing dates can enhance rice production under climate change, with high-response genotypes benefiting more from optimal sowing periods. Furthermore, for genotypes with less responsiveness to elevated CO2, augmenting nitrogen application in conjunction with sowing date adjustments was beneficial for yield optimization. This research not only advances our understanding of the diverse adaptation strategies of rice genotypes under varying climatic conditions but also enhances the precision of crop growth simulations by accounting for the varied responses to CO2 enrichment. These insights are pivotal for developing targeted breeding and management practices aimed at enhancing climate resilience in rice production.

Effect of Elevated CO2 and Temperature on Chlorophyll Content and Growth Attributes of Rice-wheat Cropping System in Central India

Rice-wheat is a major cropping system in India and it is predicted that the productivity of both crops will decline due to climate change factors including elevated CO2 and temperature. To define the mechanisms, a field experiment was carried out to evaluate the effect of elevated CO2 and temperature on growth attributes of rice and wheat crops using a Free Air CO2 Enrichment (FACE) system. The treatments were ambient CO2+ambient temperature, elevatedCO2of 600ppm+ambient temperature, ambient CO2 + elevated temperature (+2°C), ambient CO2+elevated temperature (+3°C), elevated CO2 600 ppm +elevated temperature (+2°C) and elevated CO2 600ppm +elevated temperature (+3°C). Elevated CO2 and elevated temperature (+2°C or +3°C) strongly affected the crop growth. Elevated CO2 stimulated leaf chlorophyll content, root-shoot length and biomass yield. However, elevated temperature inhibited chlorophyll content in both the crops. Elevated CO2enhanced chlorophyll content by 12.9–19% in rice and 8.8–16.5%in wheat. Elevated temperature reduced chlorophyll content by a range of 20.5-27.3% in rice and 6.3-11.5% in wheat. Combined effect of elevated CO2and elevated temperature decreased the leaf total chlorophyll and plant biomass in both crops. Study highlights that elevated CO2 concentration and rising temperature may affect the photosynthesis and productivity of rice and wheat crop in central India.

The effects of elevated CO2 and temperature on soil organic carbon and total nitrogen contents and mineralization in the 0 to 50 cm paddy soil layer were masked by different land use history

Global warming can accelerate soil organic matter (SOM) decomposition resulting in faster carbon (C) loss and positive climate-C feedback. Previous studies on response of SOM decomposition to climate change mainly focus on plow soil layer. However, the effects of elevated CO2 and soil warming on soil organic carbon (SOC) and total nitrogen (TN) contents and mineralization are rarely studied in subsoil layer. In this study, soil samples were collected from the 0–50-cm paddy soil layer of the Tsukuba free-air CO2 enrichment experimental site with elevated CO2 (+200 ppm) and soil warming (+2 °C), Japan, after 5-year rice growth season. The amounts of SOC, TN, δ13C, and δ15N were analyzed. A 4-week anaerobic incubation experiment was conducted to measure C decomposition and N mineralization potentials. Due to the intrinsic variation in SOC and TN contents in soil layers and fields, the effects of elevated CO2 and soil warming on C decomposition and N mineralization potentials could not be determined here. However, the effect of elevated CO2 on δ13C was only found in 0‒10-cm soil layer. In the 0–50-cm soil profiles, significant correlations were observed among SOC and TN, δ13C and δ15N, decomposed C and mineralized N, and δ13C of decomposed C. The variables associated with soil C and N pools, and dynamics showed large spatial heterogeneity within paddy field, due to variation in the original land use history. Therefore, great caution should be exercised when evaluating the effects of elevated CO2 and temperature on SOM decomposition and sequestration in paddy soil profiles.

Forests high on CO2: A glimpse into how scientists study the biochemical machinery of forest ecosystems

Are forests across the planet storing more carbon as a result of rising atmospheric carbon dioxide (CO2) concentration? In this text, I propose to reflect on the work of researchers who are asking that very question. The focus is on a particular approach called free-air CO2 enrichment (FACE) experiments, in which forests in their local environment are subjected to a CO2 concentration higher than in the ambient air, to test the CO2 fertilization hypothesis. This hypothesis predicts that forests store more carbon in those conditions. FACE studies held the promise of establishing cause–effect relations and transcending the here and now. They emerged in the United States in the late 1980s, at a time when ecology scaled up to make itself relevant to the then nascent Earth System science. FACE experiments in forests are now conducted in Australia, the United Kingdom, and Brazil, and their results are closely scrutinized by a small research community. The text, first, presents the engineering feat that underpins FACE experiments in forests and discuss two problems highlighted by scientists because they impede hasty generalization from the data: the timescale needed to witness forests change and the spatial heterogeneity of forest soils. It then shows how scientists contend with their incomplete understanding of forest ecosystems as they bring together computer models and empirical data to build consensus around the sources of epistemic uncertainty. To conclude, I propose to rework the idea of “the macroscope” to conceptualize what scientists do when they seek to study forests as complex, layered, multi-scale systems. I wish to capture how researchers, who willingly endorse a planetary gaze, problematize their own predicament. This, as I suggest, has to do with the immanence of forests in the world.

Shining a Light on How Soil Organic Carbon Behaves at Fine Scales under Long-Term Elevated CO2: An 8 Year Free-Air Carbon Dioxide Enrichment Study.

Building and protecting soil organic carbon (SOC) are critical to agricultural productivity, soil health, and climate change mitigation. We aim to understand how mechanisms at the organo-mineral interfaces influence SOC persistence in three contrasting soils (Luvisol, Vertisol, and Calcisol) under long-term free air CO2 enrichment conditions. A continuous wheat-field pea-canola rotation was maintained. For the first time, we provided evidence to a novel notion that persistent SOC is molecularly simple even under elevated CO2 conditions. We found that the elevated CO2 condition did not change the total SOC content or C forms compared with the soils under ambient CO2 as identified by synchrotron-based soft X-ray analyses. Furthermore, synchrotron-based infrared microspectroscopy confirmed a two-dimensional microscale distribution of similar and less diverse C forms in intact microaggregates under long-term elevated CO2 conditions. Strong correlations between the distribution of C forms and O-H groups of clays can explain the steady state of the total SOC content. However, the correlations between C forms and clay minerals were weakened in the coarse-textured Calcisol under long-term elevated CO2. Our findings suggested that we should emphasize identifying management practices that increase the physical protection of SOC instead of increasing complexity of C. Such information is valuable in developing more accurate C prediction models under elevated CO2 conditions and shift our thinking in developing management practices for maintaining and building SOC for better soil fertility and future environmental sustainability.

Polycyclic aromatic hydrocarbons degradation by free-air CO2 enriched (FACE) bacteria into low molecular easy to degrade organic compounds

Polycyclic aromatic hydrocarbons degradation by free-air CO2 enriched (FACE) bacteria into low molecular easy to degrade organic compounds

Global change impacts on mineral nutritional quality of cereal grains: Coordinated datasets and analyses to advance a systems-based understanding

ContextGlobal nutritional health outcomes are directly reliant on agroecosystem nutrient outputs. Appropriately, there is concern surrounding the impacts of a changing climate not only on crop yields, but also on crop nutritional quality (e.g., mineral nutrient concentrations). Quantifying the impacts of elevated CO2 concentrations, elevated temperature, drought stress, edaphic factors, and agronomic management on crop yields and mineral nutrition is critical, yet a systems-level understanding of these interactive factors is poorly developed, limiting our ability to effectively target solutions. Empirical data for climate impacts on crop nutritional quality remain scarce, with much of the research emerging from valuable, but geographically limited, Free-air CO2 Enrichment (FACE) experiments, several of which suggest that human nutrition will be adversely impacted by e[CO2]. Specific concerns center on observed declines in grain protein, iron, and zinc concentrations due to already wide-spread human nutritional deficiencies in these nutrients. ObjectivesAs global change experiments expand to pursue questions regarding interactive climate impacts on crop yields and nutritional quality, it is imperative to interrogate the measurements, data standardization, and metadata needed for unifying synthesis. The data reported for shifts in crop nutritional quality are often incomplete, precluding the generalizability and comparability of results. MethodsWe frame this review around six inter-reliant methods, tools, and practices to support maximally useful experimental datasets to inform questions of global change impacts on crop nutrition and aid in detecting genotypic differences in mineral nutrient density. The bulk of the data and discussion centers on wheat (Triticum aestivum L.) due to the central role this crop plays in human nutrition and sustained biofortification efforts. ResultsTo permit experimental comparability and synthesis, datasets should (1) clearly delineate analytical methods and standards and (2) link mean nutrient concentrations with the covariate of yield. (3) Multi-year, multi-location data is required to identify genotypes with significant deviations in nutrient concentrations, with (4) data normalized for yield within appropriate analytical frameworks. (5) Inclusion of data on soil properties, weather, and abiotic and biotic stresses as well as (6) agronomic practices and nutrient management is essential for understanding global change impacts on nutritional outcomes. ConclusionsCoordinated, multi-dimensional data will permit the syntheses and meta-analyses needed to identify and quantify climate impacts on nutrition. ImplicationsThis work is essential to effectively target nutritional solutions, to develop modeling tools to support nutritional planning, and to identify areas where agronomic management and breeding can minimize climate impacts on nutritional outcomes.

Modelled forest ecosystem carbon–nitrogen dynamics with integrated mycorrhizal processes under elevated CO2

Abstract. Almost 95 % of all terrestrial plant species form symbioses with mycorrhizal fungi that mediate plant–soil interactions: mycorrhizae facilitate plant nitrogen (N) acquisition and are, therefore, vital for plant growth, but they also build a pathway for plant-assimilated carbon (C) into the rhizosphere. Therefore, mycorrhizae likely play an important role in shaping the response of ecosystems to environmental changes such as rising atmospheric carbon dioxide (CO2) concentrations, which can increase plant N demand and the transfer of plant C assimilation to the soil. While the importance of mycorrhizal fungi is widely recognised, they are rarely represented in current terrestrial biosphere models (TBMs) explicitly. Here, we present a novel, dynamic plant–mycorrhiza–soil model as part of the QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system) TBM. This new model is based on mycorrhizal functional types that either actively mine soil organic matter (SOM) for N or enhance soil microbial activity through increased transfer of labile C into the rhizosphere, thereby (passively) priming SOM decomposition. Using the Duke Free-Air CO2 Enrichment (FACE) experiment, we show that mycorrhizal fungi can have important effects on projected SOM turnover and plant nutrition under ambient as well as elevated-CO2 treatments. Specifically, we find that including enhanced active mining of SOM for N in the model allows one to more closely match the observations with respect to observed decadal responses of plant growth, plant N acquisition and soil C dynamics to elevated CO2, whereas a simple enhancement of SOM turnover by increased below-ground C transfer of mycorrhizae is unable to replicate the observed responses. We provide an extensive parameter uncertainty study to investigate the robustness of our findings with respect to model parameters that cannot readily be constrained by observations. Our study points to the importance of implementing mycorrhizal functionalities in TBMs as well as to further observational needs to better constrain mycorrhizal models and to close the existing major knowledge gaps in actual mycorrhizal functioning.

Using Free Air CO2 Enrichment data to constrain land surface model projections of the terrestrial carbon cycle

Abstract. Predicting the responses of terrestrial ecosystem carbon to future global change strongly relies on our ability to model accurately the underlying processes at a global scale. However, terrestrial biosphere models representing the carbon and nitrogen cycles and their interactions remain subject to large uncertainties, partly because of unknown or poorly constrained parameters. Parameter estimation is a powerful tool that can be used to optimise these parameters by confronting the model with observations. In this paper, we identify sensitive model parameters from a recent version of the ORgainzing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model that includes the nitrogen cycle. These sensitive parameters include ones involved in parameterisations controlling the impact of the nitrogen cycle on the carbon cycle and, in particular, the limitation of photosynthesis due to leaf nitrogen availability. We optimise these ORCHIDEE parameters against carbon flux data collected on sites from the FLUXNET network. However, optimising against present-day observations does not automatically give us confidence in future projections of the model, given that environmental conditions are likely to shift compared to the present day. Manipulation experiments give us a unique look into how the ecosystem may respond to future environmental changes. One such type of manipulation experiment, the Free Air CO2 Enrichment (FACE) experiment, provides a unique opportunity to assess vegetation response to increasing CO2 by providing data under ambient and elevated CO2 conditions. Therefore, to better capture the ecosystem response to increased CO2, we add the data from two FACE sites to our optimisations, in addition to the FLUXNET data. We use data from both CO2 conditions of FACE, which allows us to gain extra confidence in the model simulations using this set of parameters. We find that we are able to improve the magnitude of modelled productivity. Although we are unable to correct the interannual variability fully, we start to simulate possible progressive nitrogen limitation at one of the sites. Using an idealised simulation experiment based on increasing atmospheric CO2 by 1 % yr−1 over 100 years, we find that optimising against only FLUXNET data tends to imply a large fertilisation effect, whereas optimising against FLUXNET and FACE data (with information about nutrient limitation and acclimation of plants) decreases it significantly.

Differential effects of elevated CO2 on awn and glume metabolism in durum wheat (Triticum durum).

While the effect of CO2 enrichment on wheat (Triticum spp.) photosynthesis, nitrogen content or yield has been well-studied, the impact of elevated CO2 on metabolic pathways in organs other than leaves is poorly documented. In particular, glumes and awns, which may refix CO2 respired by developing grains and be naturally exposed to higher-than-ambient CO2 mole fraction, could show specific responses to elevated CO2 . Here, we took advantage of a free-air CO2 enrichment experiment and performed multilevel analyses, including metabolomics, ionomics, proteomics, major hormones and isotopes in Triticum durum . While in leaves, elevated CO2 tended to accelerate amino acid metabolism with many significantly affected metabolites, the effect on glumes and awns metabolites was modest. There was a lower content in compounds of the polyamine pathway (along with uracile and allantoin) under elevated CO2 , suggesting a change in secondary N metabolism. Also, cytokinin metabolism appeared to be significantly affected under elevated CO2 . Despite this, elevated CO2 did not affect the final composition of awn and glume organic matter, with the same content in carbon, nitrogen and other elements. We conclude that elevated CO2 mostly impacts on leaf metabolism but has little effect in awns and glumes, including their composition at maturity.

Increased leaf area index and efficiency drive enhanced production under elevated atmospheric [CO2 ] in a pine-dominated stand showing no progressive nitrogen limitation.

Enhancement of net primary production (NPP) in forests as atmospheric [CO2 ] increases is likely limited by the availability of other growth resources. The Duke Free Air CO2 Enrichment (FACE) experiment was located on a moderate-fertility site in the southeastern US, in a loblolly pine (Pinus taeda L.) plantation with broadleaved species growing mostly in mid-canopy and understory. Duke FACE ran from 1994 to 2010 and combined elevated [CO2 ] (eCO2 ) with nitrogen (N) additions. We assessed the spatial and temporal variation of NPP response using a dataset that includes previously unpublished data from 6 years of the replicated CO2 × N experiment and extends to 2 years beyond the termination of enrichment. Averaged over time (1997-2010), NPP of pine and broadleaved species were 38% and 52% higher under eCO2 compared to ambient conditions. Furthermore, there was no evidence of a decline in enhancement over time in any plot regardless of its native site quality. The relation between spatial variation in the response and native site quality was suggested but inconclusive. Nitrogen amendments under eCO2 , in turn, resulted in an additional 11% increase in pine NPP. For pine, the eCO2 -induced increase in NPP was similar above- and belowground and was driven by both increased leaf area index (L) and production efficiency (PE = NPP/L). For broadleaved species, coarse-root biomass production was more than200% higher under eCO2 and accounted for the entire production response, driven by increased PE. Notably, the fraction of annual NPP retained in total living biomass was higher under eCO2 , reflecting a slight shift in allocation fraction to woody mass and a lower mortality rate. Our findings also imply that tree growth may not have been only N-limited, but perhaps constrained by the availability of other nutrients. The observed sustained NPP enhancement, even without N-additions, demonstrates no progressive N limitation.

Field phenotyping of ten wheat cultivars under elevated CO2 shows seasonal differences in chlorophyll fluorescence, plant height and vegetation indices.

In the context of climate change and global sustainable development goals, future wheat cultivation has to master various challenges at a time, including the rising atmospheric carbon dioxide concentration ([CO2]). To investigate growth and photosynthesis dynamics under the effects of ambient (~434 ppm) and elevated [CO2] (~622 ppm), a Free-Air CO2 Enrichment (FACE) facility was combined with an automated phenotyping platform and an array of sensors. Ten modern winter wheat cultivars (Triticum aestivum L.) were monitored over a vegetation period using a Light-induced Fluorescence Transient (LIFT) sensor, ground-based RGB cameras and a UAV equipped with an RGB and multispectral camera. The LIFT sensor enabled a fast quantification of the photosynthetic performance by measuring the operating efficiency of Photosystem II (Fq'/Fm') and the kinetics of electron transport, i.e. the reoxidation rates Fr1' and Fr2'. Our results suggest that elevated [CO2] significantly increased Fq'/Fm' and plant height during the vegetative growth phase. As the plants transitioned to the senescence phase, a pronounced decline in Fq'/Fm' was observed under elevated [CO2]. This was also reflected in the reoxidation rates Fr1' and Fr2'. A large majority of the cultivars showed a decrease in the harvest index, suggesting a different resource allocation and indicating a potential plateau in yield progression under e[CO2]. Our results indicate that the rise in atmospheric [CO2] has significant effects on the cultivation of winter wheat with strong manifestation during early and late growth.