Increased atmospheric CO2 and the transit time of carbon in terrestrial ecosystems.

The response of terrestrial ecosystems to increased atmospheric CO2 concentrations is controversial and not yet fully understood, with previous large-scale forest manipulation experiments exhibiting contrasting responses. Although there is consensus that increased CO2 has a relevant effect on instantaneous processes such as photosynthesis and transpiration, there are large uncertainties regarding the fate of extra assimilated carbon in ecosystems. Filling this research gap is challenging because tracing the movement of new carbon across ecosystem compartments involves the study of multiple processes occurring over a wide range of timescales, from hours to millennia. We posit that a comprehensive quantification of the effect of increased CO2 must answer two interconnected questions: How much and for how long is newly assimilated carbon stored in ecosystems? Therefore, we propose that the transit time distribution of carbon is the key concept needed to effectively address these questions. Here, we show how the transit time distribution of carbon can be used to assess the fate of newly assimilated carbon and the timescales at which it is cycled in ecosystems. We use as an example a transit time distribution obtained from a tropical forest and show that most of the 60% of fixed carbon is respired in less than 1 year; therefore, we infer that under increased CO2 , most of the new carbon would follow a similar fate unless increased CO2 would cause changes in the rates at which carbon is cycled and transferred among ecosystem compartments. We call for a more frequent adoption of the transit time concept in studies seeking to quantify the ecosystem response to increased CO2 .

Links across ecological scales: Plant biomass responses to elevated CO2.

The degree to which elevated CO2 concentrations (e[CO2 ]) increase the amount of carbon (C) assimilated by vegetation plays a key role in climate change. However, due to the short-term nature of CO2 enrichment experiments and the lack of reconciliation between different ecological scales, the effect of e[CO2 ] on plant biomass stocks remains a major uncertainty in future climate projections. Here, we review the effect of e[CO2 ] on plant biomass across multiple levels of ecological organization, scaling from physiological responses to changes in population-, community-, ecosystem-, and global-scale dynamics. We find that evidence for a sustained biomass response to e[CO2 ] varies across ecological scales, leading to diverging conclusions about the responses of individuals, populations, communities, and ecosystems. While the distinct focus of every scale reveals new mechanisms driving biomass accumulation under e[CO2 ], none of them provides a full picture of all relevant processes. For example, while physiological evidence suggests a possible long-term basis for increased biomass accumulation under e[CO2 ] through sustained photosynthetic stimulation, population-scale evidence indicates that a possible e[CO2 ]-induced increase in mortality rates might potentially outweigh the effect of increases in plant growth rates on biomass levels. Evidence at the global scale may indicate that e[CO2 ] has contributed to increased biomass cover over recent decades, but due to the difficulty to disentangle the effect of e[CO2 ] from a variety of climatic and land-use-related drivers of plant biomass stocks, it remains unclear whether nutrient limitations or other ecological mechanisms operating at finer scales will dampen the e[CO2 ] effect over time. By exploring these discrepancies, we identify key research gaps in our understanding of the effect of e[CO2 ] on plant biomass and highlight the need to integrate knowledge across scales of ecological organization so that large-scale modeling can represent the finer-scale mechanisms needed to constrain our understanding of future terrestrial C storage.

Integrating the evidence for a terrestrial carbon sink caused by increasing atmospheric CO2.

Atmospheric carbon dioxide concentration ([CO2 ]) is increasing, which increases leaf-scale photosynthesis and intrinsic water-use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2 ] increase and thus climate change. However, ecosystem CO2 responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO2 ]-driven terrestrial carbon sink can appear contradictory. Here we synthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO2 ] (iCO2 ) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre-industrial times. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2 responses are high in comparison to experiments and predictions from theory. Plant mortality and soil carbon iCO2 responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2 , albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.

Ecosystem responses to elevated CO2 governed by plant-soil interactions and the cost of nitrogen acquisition.

Contents Summary 507 I. Introduction 507 II. The return on investment approach 508 III. CO2 response spectrum 510 IV. Discussion 516 Acknowledgements 518 References 518 SUMMARY: Land ecosystems sequester on average about a quarter of anthropogenic CO2 emissions. It has been proposed that nitrogen (N) availability will exert an increasingly limiting effect on plants' ability to store additional carbon (C) under rising CO2 , but these mechanisms are not well understood. Here, we review findings from elevated CO2 experiments using a plant economics framework, highlighting how ecosystem responses to elevated CO2 may depend on the costs and benefits of plant interactions with mycorrhizal fungi and symbiotic N-fixing microbes. We found that N-acquisition efficiency is positively correlated with leaf-level photosynthetic capacity and plant growth, and negatively with soil C storage. Plants that associate with ectomycorrhizal fungi and N-fixers may acquire N at a lower cost than plants associated with arbuscular mycorrhizal fungi. However, the additional growth in ectomycorrhizal plants is partly offset by decreases in soil C pools via priming. Collectively, our results indicate that predictive models aimed at quantifying C cycle feedbacks to global change may be improved by treating N as a resource that can be acquired by plants in exchange for energy, with different costs depending on plant interactions with microbial symbionts.

Experimental soil warming shifts the fungal community composition at the alpine treeline.

Increased CO2 emissions and global warming may alter the composition of fungal communities through the removal of temperature limitation in the plant-soil system, faster nitrogen (N) cycling and changes in the carbon (C) allocation of host plants to the rhizosphere. At a Swiss treeline featuring Larix decidua and Pinus uncinata, the effects of multiple years of CO2 enrichment and experimental soil warming on the fungal community composition in the organic horizons were analysed using 454-pyrosequencing of ITS2 amplicons. Sporocarp production and colonization of ectomycorrhizal root tips were investigated in parallel. Fungal community composition was significantly altered by soil warming, whereas CO2 enrichment had little effect. Tree species influenced fungal community composition and the magnitude of the warming responses. The abundance of ectomycorrhizal fungal taxa was positively correlated with N availability, and ectomycorrhizal taxa specialized for conditions of high N availability proliferated with warming, corresponding to considerable increases in inorganic N in warmed soils. Traits related to N utilization are important in determining the responses of ectomycorrhizal fungi to warming in N-poor cold ecosystems. Shifts in the overall fungal community composition in response to higher temperatures may alter fungal-driven processes with potential feedbacks on ecosystem N cycling and C storage at the alpine treeline.

Model-data synthesis for the next generation of forest free-air CO2 enrichment (FACE) experiments.

The first generation of forest free-air CO2 enrichment (FACE) experiments has successfully provided deeper understanding about how forests respond to an increasing CO2 concentration in the atmosphere. Located in aggrading stands in the temperate zone, they have provided a strong foundation for testing critical assumptions in terrestrial biosphere models that are being used to project future interactions between forest productivity and the atmosphere, despite the limited inference space of these experiments with regards to the range of global ecosystems. Now, a new generation of FACE experiments in mature forests in different biomes and over a wide range of climate space and biodiversity will significantly expand the inference space. These new experiments are: EucFACE in a mature Eucalyptus stand on highly weathered soil in subtropical Australia; AmazonFACE in a highly diverse, primary rainforest in Brazil; BIFoR-FACE in a 150-yr-old deciduous woodland stand in central England; and SwedFACE proposed in a hemiboreal, Pinus sylvestris stand in Sweden. We now have a unique opportunity to initiate a model-data interaction as an integral part of experimental design and to address a set of cross-site science questions on topics including responses of mature forests; interactions with temperature, water stress, and phosphorus limitation; and the influence of biodiversity.

Production and turnover of ectomycorrhizal extramatrical mycelial biomass and necromass under elevated CO2 and nitrogen fertilization.

Extramatrical mycelia (EMM) of ectomycorrhizal fungi are important in carbon (C) and nitrogen (N) cycling in forests, but poor knowledge about EMM biomass and necromass turnovers makes the quantification of their role problematic. We studied the impacts of elevated CO2 and N fertilization on EMM production and turnover in a Pinus taeda forest. EMM C was determined by the analysis of ergosterol (biomass), chitin (total bio- and necromass) and total organic C (TOC) of sand-filled mycelium in-growth bags. The production and turnover of EMM bio- and necromass and total C were estimated by modelling. N fertilization reduced the standing EMM biomass C to 57% and its production to 51% of the control (from 238 to 122 kg C ha(-1)  yr(-1) ), whereas elevated CO2 had no detectable effects. Biomass turnover was high (˜13 yr(-1) ) and unchanged by the treatments. Necromass turnover was slow and was reduced from 1.5 yr(-1) in the control to 0.65 yr(-1) in the N-fertilized treatment. However, TOC data did not support an N effect on necromass turnover. An estimated EMM production ranging from 2.5 to 6% of net primary production stresses the importance of its inclusion in C models. A slow EMM necromass turnover indicates an importance in building up forest humus.

Increases in atmospheric CO2 have little influence on transpiration of a temperate forest canopy.

Models of forest energy, water and carbon cycles assume decreased stomatal conductance with elevated atmospheric CO2 concentration ([CO2]) based on leaf-scale measurements, a response not directly translatable to canopies. Where canopy-atmosphere are well-coupled, [CO2 ]-induced structural changes, such as increasing leaf-area index (LD), may cause, or compensate for, reduced mean canopy stomatal conductance (GS), keeping transpiration (EC) and, hence, runoff unaltered. We investigated GS responses to increasing [CO2] of conifer and broadleaved trees in a temperate forest subjected to 17-yr free-air CO2 enrichment (FACE; + 200 µmol mol(-1)). During the final phase of the experiment, we employed step changes of [CO2] in four elevated-[CO2 ] plots, separating direct response to changing [CO2] in the leaf-internal air-space from indirect effects of slow changes via leaf hydraulic adjustments and canopy development. Short-term manipulations caused no direct response up to 1.8 × ambient [CO2], suggesting that the observed long-term 21% reduction of GS was an indirect effect of decreased leaf hydraulic conductance and increased leaf shading. Thus, EC was unaffected by [CO2] because 19% higher canopy LD nullified the effect of leaf hydraulic acclimation on GS . We advocate long-term experiments of duration sufficient for slow responses to manifest, and modifying models predicting forest water, energy and carbon cycles accordingly.

Elevated CO2 facilitates C and N accumulation in a rice paddy ecosystem.

Elevated CO2 can stimulate wetland carbon (C) and nitrogen (N) exports through gaseous and dissolved pathways, however, the consequent influences on the C and N pools are still not fully known. Therefore, we set up a free-air CO2 enrichment experiment in a paddy field in Eastern China. After five year fumigation, we studied C and N in the plant-water-soil system. The results showed: (1) elevated CO2 stimulated rice aboveground biomass and N accumulations by 19.1% and 12.5%, respectively. (2) Elevated CO2 significantly increased paddy soil TOC and TN contents by 12.5% and 15.5%, respectively in the 0-15 cm layer, and 22.7% and 26.0% in the 15-30 cm soil layer. (3) Averaged across the rice growing period, elevated CO2 greatly increased TOC and TN contents in the surface water by 7.6% and 11.4%, respectively. (4) The TOC/TN ratio and natural δ15N value in the surface soil showed a decreasing trend under elevated CO2. The above results indicate that elevated CO2 can benefit C and N accumulation in paddy fields. Given the similarity between the paddies and natural wetlands, our results also suggest a great potential for long-term C and N accumulation in natural wetlands under future climate patterns.

CO2 enrichment alters diurnal stem radius fluctuations of 36-yr-old Larix decidua growing at the alpine tree line.

To understand how trees at high elevations might use water differently in the future, we investigated the effects of CO2 enrichment and soil warming (separately and combined) on the water relations of Larix decidua growing at the tree line in the Swiss Alps. We assessed diurnal stem radius fluctuations using point dendrometers and applied a hydraulic plant model using microclimate and soil water potential data as inputs. Trees exposed to CO2 enrichment for 9 yr showed smaller diurnal stem radius contractions (by 46 ± 16%) and expansions (42 ± 16%) compared with trees exposed to ambient CO2 . Additionally, there was a delay in the timing of daily maximum (40 ± 12 min) and minimum (63 ± 14 min) radius values for trees growing under elevated CO2 . Parameters optimized with the hydraulic model suggested that CO2 -enriched trees had an increased flow resistance between the xylem and bark, representing a more buffered water supply system. Soil warming did not alter diurnal fluctuation dynamics or the CO2 response. Elevated CO2 altered the hydraulic water flow and storage system within L. decidua trees, which might have contributed to enhanced growth during 9 yr of CO2 enrichment and could ultimately influence the future competitive ability of this key tree-line species.

Where does the carbon go? A model-data intercomparison of vegetation carbon allocation and turnover processes at two temperate forest free-air CO2 enrichment sites.

Elevated atmospheric CO2 concentration (eCO2) has the potential to increase vegetation carbon storage if increased net primary production causes increased long-lived biomass. Model predictions of eCO2 effects on vegetation carbon storage depend on how allocation and turnover processes are represented. We used data from two temperate forest free-air CO2 enrichment (FACE) experiments to evaluate representations of allocation and turnover in 11 ecosystem models. Observed eCO2 effects on allocation were dynamic. Allocation schemes based on functional relationships among biomass fractions that vary with resource availability were best able to capture the general features of the observations. Allocation schemes based on constant fractions or resource limitations performed less well, with some models having unintended outcomes. Few models represent turnover processes mechanistically and there was wide variation in predictions of tissue lifespan. Consequently, models did not perform well at predicting eCO2 effects on vegetation carbon storage. Our recommendations to reduce uncertainty include: use of allocation schemes constrained by biomass fractions; careful testing of allocation schemes; and synthesis of allocation and turnover data in terms of model parameters. Data from intensively studied ecosystem manipulation experiments are invaluable for constraining models and we recommend that such experiments should attempt to fully quantify carbon, water and nutrient budgets.

Evaluation of 11 terrestrial carbon-nitrogen cycle models against observations from two temperate Free-Air CO2 Enrichment studies.

We analysed the responses of 11 ecosystem models to elevated atmospheric [CO2 ] (eCO2 ) at two temperate forest ecosystems (Duke and Oak Ridge National Laboratory (ORNL) Free-Air CO2 Enrichment (FACE) experiments) to test alternative representations of carbon (C)-nitrogen (N) cycle processes. We decomposed the model responses into component processes affecting the response to eCO2 and confronted these with observations from the FACE experiments. Most of the models reproduced the observed initial enhancement of net primary production (NPP) at both sites, but none was able to simulate both the sustained 10-yr enhancement at Duke and the declining response at ORNL: models generally showed signs of progressive N limitation as a result of lower than observed plant N uptake. Nonetheless, many models showed qualitative agreement with observed component processes. The results suggest that improved representation of above-ground-below-ground interactions and better constraints on plant stoichiometry are important for a predictive understanding of eCO2 effects. Improved accuracy of soil organic matter inventories is pivotal to reduce uncertainty in the observed C-N budgets. The two FACE experiments are insufficient to fully constrain terrestrial responses to eCO2 , given the complexity of factors leading to the observed diverging trends, and the consequential inability of the models to explain these trends. Nevertheless, the ecosystem models were able to capture important features of the experiments, lending some support to their projections.

Effects of elevated CO2 on the extractable amino acids of leaf litter and fine roots.

Elevated atmospheric CO2 concentrations can change chemistry and input rate of plant tissue to soil, potentially influencing above- and below-ground biogeochemical cycles. Given the important role played by leaf and root litter chemistry in controlling ecosystem function and vulnerability to environmental stresses, we investigated the hydrolyzable amino acid distribution and concentration in leaf and fine root litter among control and elevated CO2 treatments at the Rhinelander free air CO2 enrichment (FACE) experiment (WI, USA). We extracted hydrolyzable amino acids from leaf litter and fine (< 2 mm) roots at three depths for both control and elevated CO2 plots. We found that elevated CO2 decreased the proportion of total leaf amino acid carbon (C), but had no effect on total leaf amino acid nitrogen (N). There was no treatment effect for total root amino acid N or amino acid C for any depth. The decrease in leaf amino acids is probably a result of the shift of protein compounds to more structural compounds. Despite the decrease in leaf amino acid C concentrations, the overall increase in annual plant production under elevated CO2 would result in an increase in plant amino acids to the soil.

Fungal functioning in a pine forest: evidence from a ¹5N-labeled global change experiment.

• We used natural and tracer nitrogen (N) isotopes in a Pinus taeda free air CO2 enrichment (FACE) experiment to investigate functioning of ectomycorrhizal and saprotrophic fungi in N cycling. • Fungal sporocarps were sampled in 2004 (natural abundance and (15) N tracer) and 2010 (tracer) and δ(15)N patterns were compared against litter and soil pools. • Ectomycorrhizal fungi with hydrophobic ectomycorrhizas (e.g. Cortinarius and Tricholoma) acquired N from the Oea horizon or deeper. Taxa with hydrophilic ectomycorrhizas acquired N from the Oi horizon (Russula and Lactarius) or deeper (Laccaria, Inocybe, and Amanita). (15)N enrichment patterns for Cortinarius and Amanita in 2010 did not correspond to any measured bulk pool, suggesting that a persistent pool of active organic N supplied these two taxa. Saprotrophic fungi could be separated into those colonizing pine cones (Baeospora), wood, litter (Oi), and soil (Ramariopsis), with δ(15)N of taxa reflecting substrate differences. (15)N enrichment between sources and sporocarps varied across taxa and contributed to δ(15)N patterns. • Natural abundance and (15)N tracers proved useful for tracking N from different depths into fungal taxa, generally corresponded to literature estimates of fungal activity within soil profiles, and provided new insights into interpreting natural abundance δ(15)N patterns.

Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests.

Three young northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO2 ) and tropospheric ozone (O3 ) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO2 enhanced ecosystem C content by 11%, whereas elevated O3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO2 and O3 . Treatment effects on ecosystem C content resulted primarily from changes in the near-surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r(2) = 0.96). Elevated CO2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m(-2) ) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (∆NPP/∆N) decreased through time with further canopy development, the O3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O3 and less soil C from 0.1 to 0.2 m in depth under elevated CO2 . Overall, these results suggest that elevated CO2 may create a sustained increase in NPP, whereas the long-term effect of elevated O3 on NPP will be smaller than expected. However, changes in soil C are not well-understood and limit our ability to predict changes in ecosystem C content.

Soil warming alters microbial substrate use in alpine soils.

Will warming lead to an increased use of older soil organic carbon (SOC) by microbial communities, thereby inducing C losses from C-rich alpine soils? We studied soil microbial community composition, activity, and substrate use after 3 and 4 years of soil warming (+4 °C, 2007-2010) at the alpine treeline in Switzerland. The warming experiment was nested in a free air CO2 enrichment experiment using depleted (13)CO2 (δ(13)C = -30‰, 2001-2009). We traced this depleted (13)C label in phospholipid fatty acids (PLFA) of the organic layer (0-5 cm soil depth) and in C mineralized from root-free soils to distinguish substrate ages used by soil microorganisms: fixed before 2001 ('old'), from 2001 to 2009 ('new') or in 2010 ('recent'). Warming induced a sustained stimulation of soil respiration (+38%) without decline in mineralizable SOC. PLFA concentrations did not reveal changes in microbial community composition due to soil warming, but soil microbial metabolic activity was stimulated (+66%). Warming decreased the amount of new and recent C in the fungal biomarker 18:2ω6,9 and the amount of new C mineralized from root-free soils, implying a shift in microbial substrate use toward a greater use of old SOC. This shift in substrate use could indicate an imbalance between C inputs and outputs, which could eventually decrease SOC storage in this alpine ecosystem.

Elevated atmospheric CO2 stimulates soil fungal diversity through increased fine root production in a semiarid shrubland ecosystem.

Soil fungal communities are likely to be central in mediating microbial feedbacks to climate change through their effects on soil carbon (C) storage, nutrient cycling, and plant health. Plants often produce increased fine root biomass in response to elevated atmospheric carbon dioxide (CO2 ), but the responses of soil microbial communities are variable and uncertain, particularly in terms of species diversity. In this study, we describe the responses of the soil fungal community to free air CO2 enrichment (FACE) in a semiarid chaparral shrubland in Southern California (dominated by Adenomstoma fasciculatum) using large subunit rRNA gene sequencing. Community composition varied greatly over the landscape and responses to FACE were subtle, involving a few specific groups. Increased frequency of Sordariomycetes and Leotiomycetes, the latter including the Helotiales, a group that includes many dark septate endophytes known to associate positively with roots, was observed in the FACE plots. Fungal diversity, both in terms of richness and evenness, increased consistently in the FACE treatment, and was relatively high compared to other studies that used similar methods. Increases in diversity were observed across multiple phylogenetic levels, from genus to class, and were distributed broadly across fungal lineages. Diversity was also higher in samples collected close to (5 cm) plants compared to samples in canopy gaps (30 cm away from plants). Fungal biomass correlated well with soil organic matter (SOM) content, but patterns of diversity were correlated with fine root production rather than SOM. We conclude that the fungal community in this ecosystem is tightly linked to plant fine root production, and that future changes in the fungal community in response to elevated CO2 and other climatic changes will be primarily driven by changes in plant belowground allocation. Potential feedbacks mediated by soil fungi, such as soil C sequestration, nutrient cycling, and pathogenesis, are discussed.

NosZ I carrying microorganisms determine N2O emissions from the subtropical paddy field under elevated CO2 and strongly CO2-responsive cultivar.

Elevated CO2 (eCO2) decreases N2O emissions from subtropical paddy fields, but the underlying mechanisms remain to be investigated. Herein, the response of key microbial nitrogen cycling genes to eCO2 (ambient air +200 µmol CO2 mol-1) in four rice cultivars, including two weakly CO2-responsive (W27, H5) and two strongly CO2-responsive cultivars (Y1540, L1988), was investigated. Except for nosZ I, eCO2 did not significantly alter the abundance of the other genes. NosZ I was a crucial factor governing N2O emissions, especially under eCO2 and a strongly responsive cultivar. eCO2 affected the nosZ I gene abundance (p < 0.05), for instance, the nosZ I gene abundance of cultivar W27 increased from 1.53 × 107 to 2.86 × 107 copies g-1 dw soil (p < 0.05). In the nosZ I microbial community, the known taxa were mainly Pseudomonadota (phylum) (19.74-31.72 %) and Alphaproteobacteria (class) (0.56-13.12 %). In the nosZ I community assembly process, eCO2 enhanced the role of stochasticity, increasing from 35 % to 85 % (p < 0.05), thereby inducing diffusion limitations of weakly responsive cultivars to dominate (67 %). Taken together, the increase in nosZ I gene abundance is a potential reason for the alleviation of N2O emissions from subtropical paddy fields under eCO2.

Differential Effect of Free-Air CO2 Enrichment (FACE) in Different Organs and Growth Stages of Two Cultivars of Durum Wheat.

Wheat is a target crop within the food security context. The responses of wheat plants under elevated concentrations of CO2 (e[CO2]) have been previously studied; however, few of these studies have evaluated several organs at different phenological stages simultaneously under free-air CO2 enrichment (FACE) conditions. The main objective of this study was to evaluate the effect of e[CO2] in two cultivars of wheat (Triumph and Norin), analyzed at three phenological stages (elongation, anthesis, and maturation) and in different organs at each stage, under FACE conditions. Agronomic, biomass, physiological, and carbon (C) and nitrogen (N) dynamics were examined in both ambient CO2 (a[CO2]) fixed at 415 µmol mol-1 CO2 and e[CO2] at 550 µmol mol-1 CO2. We found minimal effect of e[CO2] compared to a[CO2] on agronomic and biomass parameters. Also, while exposure to 550 µmol mol-1 CO2 increased the photosynthetic rate of CO2 assimilation (An), the current study showed a diminishment in the maximum carboxylation (Vc,max) and maximum electron transport (Jmax) under e[CO2] conditions compared to a[CO2] at physiological level in both cultivars. However, even if no significant differences were detected between cultivars on photosynthetic machinery, differential responses between cultivars were detected in C and N dynamics at e[CO2]. Triumph showed starch accumulation in most organs during anthesis and maturation, but a decline in N content was observed. Contrastingly, in Norin, a decrease in starch content during the three stages and an increase in N content was observed. The amino acid content decreased in grain and shells at maturation in both cultivars, which might indicate a minimal translocation from source to sink organs. These results suggest a greater acclimation to e[CO2] enrichment in Triumph than Norin, because both the elongation stage and e[CO2] modified the source-sink relationship. According to the differences between cultivars, future studies should be performed to test genetic variation under FACE technology and explore the potential of cultivars to cope with projected climate scenarios.

Changes in plant nutrient status following combined elevated [CO2] and canopy warming in winter wheat.

Projected global climate change is a potential threat to nutrient utilization in agroecosystems. However, the combined effects of elevated [CO2] and canopy warming on plant nutrient concentrations and translocations are not well understood. Here we conducted an open-air field experiment to investigate the impact of factorial elevated [CO2] (up to 500 µmol mol-1) and canopy air warming (+2°C) on nutrient (N, P, and K) status during the wheat growing season in a winter wheat field. Compared to ambient conditions, soil nutrient status was generally unchanged under elevated [CO2] and canopy warming. In contrast, elevated [CO2] decreased K concentrations by 11.0% and 11.5% in plant shoot and root, respectively, but had no impact on N or P concentration. Canopy warming increased shoot N, P and K concentrations by 8.9%, 7.5% and 15.0%, but decreased root N, P, and K concentrations by 12.3%, 9.0% and 31.6%, respectively. Accordingly, canopy warming rather than elevated [CO2] increased respectively N, P and K transfer coefficients (defined as the ratio of nutrient concentrations in the shoot to root) by 22.2%, 27.9% and 84.3%, which illustrated that canopy warming played a more important role in nutrient translocation from belowground to aboveground than elevated [CO2]. These results suggested that the response of nutrient dynamics was more sensitive in plants than in soil under climate change.