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Plant responses to the projected future levels of CO2 were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO2 were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO2 Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO2 on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO2 under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO2 stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO2 improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO2 stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO2 does not directly stimulate C4 photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO2 in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Summary •  Expression of 4600 poplar expressed sequence tags (ESTs) was studied over the 2001–2002 growing seasons using trees of the moderately ozone (O3)‐tolerant trembling aspen (Populus tremuloides) clone 216 exposed to elevated CO2 and/or O3 for their entire 5‐yr life history. •  Based on replication of the experiment in years 2001 and 2002, 238 genes showed qualitatively similar expression in at least one treatment and were retained for analysis. Of these 238 genes, 185 were significantly regulated (1.5‐fold) from one year to the other in at least one treatment studied. Less than 1% of the genes were regulated 2‐fold or more. •  In the elevated CO2 treatment, relatively small numbers of genes were up‐regulated, whereas in the O3 treatment, higher expression of many signaling and defense‐related genes and lower expression of several photosynthesis and energy‐related genes were observed. Senescence‐associated genes (SAGs) and genes involved in the flavanoid pathway were also up‐regulated under O3, with or without CO2 treatment. Interestingly, the combined treatment of CO2 plus O3 resulted in the differential expression of genes that were not up‐regulated with individual gas treatments. •  This study represents the first investigation into gene expression following long‐term exposure of trees to the interacting effects of elevated CO2 and O3 under field conditions. Patterns of gene‐specific regulation described in this study correlated with previously published physiological responses of aspen clone 216.

BackgroundProcess-based ecophysiological crop models are pivotal in assessing responses of crop productivity and designing strategies of adaptation to climate change. Most existing crop models generally over-estimate the effect of elevated atmospheric [CO2], despite decades of experimental research on crop growth response to [CO2].AnalysisA review of the literature indicates that the quantitative relationships for a number of traits, once expressed as a function of internal plant nitrogen status, are altered little by the elevated [CO2]. A model incorporating these nitrogen-based functional relationships and mechanisms simulated photosynthetic acclimation to elevated [CO2], thereby reducing the chance of over-estimating crop response to [CO2]. Robust crop models to have small parameterization requirements and yet generate phenotypic plasticity under changing environmental conditions need to capture the carbon–nitrogen interactions during crop growth.ConclusionsThe performance of the improved models depends little on the type of the experimental facilities used to obtain data for parameterization, and allows accurate projections of the impact of elevated [CO2] and other climatic variables on crop productivity.

Subterranean termites are ecosystem engineers that modulate the flow of carbon from dead wood to the atmosphere and soil, yet their contributions to the latter pool are largely unaccounted for in carbon cycling models. The fate of C from wood utilized by Reticulitermes flavipes (Kollar) was determined using a reductionist design in a closed system with δ13C labeled wood as a stable isotope tracer. The percentage of wood‐based carbon in termite respiratory gases, tissues, and organic deposits (frass and construction materials) was measured for five colonies to budget wood‐C mass distributed into metabolic and behavioral pathways during a 160‐h incubation period. We found that termites emitted 42% of the C from wood as gas (largely as carbon dioxide), returned 40% to the environment as organic deposits (frass and construction materials), and retained 18% in their tissues (whole alimentary tracts and de‐gutted bodies). Our findings affirm that termites are a source of greenhouse gases but are also ecosystem engineers that return approximately half the C from dead wood as organic deposits into their surrounding environment.

PurposeElevated CO2 concentration (eCO2) may stimulate plant growth and influence the soil microbial community, but questions remain for whether microbial responses to elevated CO2 would vary by different CO2-responsive plants. We thus attempted to elaborate the changes of soil microbiome to different rice cultivars under the eCO2 condition.Materials and methodsTwo rice cultivars, i.e., the CO2-tolerant cultivar, Wuyunjing23 (WYJ23), and the CO2-sensitive one, Yandao 6 (YD6), were grown under eCO2 and ambient CO2 (aCO2) conditions. The contents of dissolved organic carbon (DOC) and nitrogen (DON) in soil were measured. Real-time qualitative PCR (qPCR) and high-throughput sequencing techniques were employed to characterize the bacterial community. Furthermore, co-occurrence network analysis was applied to reveal the ecological interactions among bacterial taxa.Results and discussionNo significant differences were found among all treatments in terms of bacterial population, alpha-diversity indices, and bacterial community structure. However, the topological parameters of ecological networks highlighted the distinct co-occurrence patterns among treatments. YD6 under eCO2 led to more links, lower modularity, and greater centralization degree compared to that under aCO2. Opposite trends of those parameters were observed for WYJ23 under eCO2 compared to that under aCO2. Besides, more Proteobacteria and Acidobacteria served as keystone taxa in the CO2-sensitive cultivar treatments, compared to those in WYJ23, implying the different influences of rice cultivars on the microbial ecological network.ConclusionsDifferent rice cultivars under eCO2 did not influence the alpha- and beta-diversity of the soil bacterial community, but changed the co-occurrence network of the community. More attention should be paid to the assembly mechanisms of the soil microbial microbiome when evaluating the impacts of productive crops on the soil-plant ecosystem under the eCO2 condition.

Purpose Although micronutrients are essential to higher plants, it remains unclear whether the projected future climate change would affect their availability to plants. The objective of this study was to investigate the effect of carbon dioxide (CO 2 ) enrichment and warming on soil micronutrient availability and plant uptake. Materials and methods This study was conducted in an open field experiment with CO 2 enrichment and plant canopy warming. Four treatments were included: (1) free-air CO 2 enrichment up to 500 ppm (CE); (2) canopy warming by plus 2 °C (WA); (3) CO 2 enrichment combined with canopy warming (CW), and (4) ambient condition as control. Plant and soil samples were collected, respectively, at the jointing, heading, and ripening stage over the whole wheat growing season in 2014. The micronutrient concentrations both in soil and plant were both analyzed, and the accumulated uptake by wheat harvest was assessed. Results and discussion Both CO 2 enrichment and warming increased the availability of most soil micronutrients. The availability of Fe, Mn, Cu, and Zn under CO 2 enrichment increased by 47.7, 22.5, 59.8, and 114.1 %, respectively. Warming increased the availability of Fe, Cu, and Zn by 60.4, 23.8, and 15.3 %, respectively. The plant growth induced changes in soil pH and in soil microbial biomass carbon (MBC) accounted to the changes in soil micronutrient availability. The enrichment of CO 2 and warming had significant effects on micronutrient uptake by wheat. The enrichment of CO 2 decreased the concentration of Fe by 9.3 %, while it increased the concentrations of Mn and Zn by 18.9 and 8.1 % in plant shoot, respectively. Warming increased the concentration of Fe and Cu by 24.3 and 7.6 % in plant shoot, respectively. The increase in soil micronutrient availability did not always lead to the increase in micronutrient uptake. The element types and crop growth stage affected the uptake of micronutrients by wheat under CO 2 enrichment and warming. Additionally, CO 2 enrichment decreased the translocation of Fe and Zn by 25.3 and 10.0 %, respectively, while warming increased the translocation of Fe, Mn, Cu, and Zn across stages. Conclusions Our results demonstrated that CO 2 enrichment and warming would improve availability of some micronutrients and their uptake by wheat. However, it is still unclear whether a net removal of micronutrient through crop straw harvest would occur under CO 2 enrichment and warming.

Prediction of the impact of climate change requires the response of carbon (C) flow in plant-soil systems to increased CO₂ to be understood. A mechanism by which grassland C sequestration might be altered was investigated by pulse-labelling Lolium perenne swards, which had been subject to CO₂ enrichment and two levels of nitrogen (N) fertilization for 10 yr, with ¹⁴CO₂. Over a 6-d period 40-80% of the ¹⁴C pulse was exported from mature leaves, 1-2% remained in roots, 2-7% was lost as below-ground respiration, 0.1% was recovered in soil solution, and 0.2-1.5% in soil. Swards under elevated CO₂ with the lower N supply fixed more ¹⁴C than swards grown in ambient CO₂, exported more fixed ¹⁴C below ground and respired less than their high-N counterparts. Sward cutting reduced root ¹⁴C, but plants in elevated CO₂ still retained 80% more ¹⁴C below ground than those in ambient CO₂. The potential for below-ground C sequestration in grasslands is enhanced under elevated CO₂, but any increase is likely to be small and dependent upon grassland management.

Emissions of CO₂ from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO₂ to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO₂ Enrichment (FACE) experiment, we examined how forest growth at elevated (+ 200 ppmv) atmospheric CO₂ concentration affects soil CO₂ dynamics over 7 years of continuous enrichment. Soil respiration, soil CO₂ concentrations, and the isotopic signature of soil CO₂ were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO₂ efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO₂ enrichment effect on soil CO₂ efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO₂ concentrations and depletion in the δ¹³C of soil CO₂ at all but the shallowest soil depths.

The effect of elevated carbon dioxide (CO 2 ) concentration on symbiotic nitrogen fixation in soybean under open-air conditions has not been reported. Two soybean cultivars ( Glycine max (L.) Merr. cv. Zhonghuang 13 and cv. Zhonghuang 35) were grown to maturity under ambient (415 ± 16 μmol mol −1 ) and elevated (550 ± 17 μmol mol −1 ) [CO 2 ] at the free-air carbon dioxide enrichment experimental facility in northern China. Elevated [CO 2 ] increased above- and below-ground biomass by 16–18% and 11–20%, respectively, but had no significant effect on the tissue C/N ratio at maturity. Elevated [CO 2 ] increased the percentage of N derived from the atmosphere (%Ndfa, estimated by natural abundance) from 59% to 79% for Zhonghuang 13, and the amount of N fixed from 166 to 275 kg N ha −1 , but had no significant effect on either parameter for Zhonghuang 35. These results suggest that variation in N 2 fixation ability in response to elevated [CO 2 ] should be used as key trait for selecting cultivars for future climate with respect to meeting the higher N demand driven by a carbon-rich atmosphere.

Increased atmospheric [CO₂] is likely to affect photosynthesis, plant growth, and yield potential of plants. Mustard (Brassica juncea L.) is an important oil seed crop that is widely grown in India. Therefore, the impact of elevated [CO₂] (585 μmol mol⁻¹) on pigment and protein content, chlorophyll a fluorescence, photosynthetic electron transport reactions, CO₂ assimilation, biomass production, and seed yield potential was measured in B. juncea cv Pusa Bold, grown inside free air carbon dioxide enrichment (FACE) rings installed on the campus of Jawaharlal Nehru University, New Delhi, India. Plants were grown for three consecutive winter seasons (2010—2013), in ambient (385 μmol mol⁻¹) or elevated [CO₂], in field conditions. Elevated [CO₂] had no significant effect on the minimal chlorophyll fluorescence (F ₀), while the quantum efficiency of Photosystem II, measured as variable fluorescence (F ᵥ = F ₘ–F ₀) to maximum fluoresence (F ₘ), increased by 3 %. Electron transport rate, photosystem I, photosystem II, and whole chain electron transport rates increased by 8 % in elevated [CO₂]. However, the net photosynthesis rate increased by ≈50 % in three growing seasons under elevated [CO₂] condition. The stomatal conductance and transpiration rate decreased resulting in higher photosynthetic water use efficiency. The photosynthesizing surface, i.e., leaf area index substantially increased leading to higher biomass and seed yield under elevated [CO₂] condition. Acclimatory downregulation of photosynthesis and plant productivity was not observed in three consecutive growing years suggesting that in the absence of nutrient limitation, B. juncea is highly responsive to elevated CO₂ whose yield potential shall increase in changing climatic conditions.