Long-term Elevated CO2 Research Articles

Enhanced Growth Performance and Steviol Glycosides Content in Stevia Rebaudiana Under Elevated Carbon Dioxide

In the current climate-changing scenario with a steadily rising CO2 concentration, there is a chance that crop performance will be affected in terms of growth, yield, and quality. Therefore, an experiment was conducted in a glasshouse using a randomized complete block design with four replications to investigate the effect of short and long-term elevated CO2 on growth performance and chemical markers of Stevia rebaudiana Bertoni. The CO2 in the glasshouse was gradually elevated from 400 ppm to 1800 ppm weekly. The plants were exposed to elevated CO2 for four months (T1), two months (T2), and one month (T3), while the control plants (T4) were grown under ambient CO2 (aCO2) levels to assess the effect of elevated atmospheric carbon dioxide (eCO2) on stevia crop growth performance and steviol glycosides content. The number of branches per plant, plant height, number of leaves per branch, and plant biomass were found to be significantly increased under eCO2 treatment over aCO2 treatment. The eCO2 increased photosynthetic rate by 46% for T1, 45% for T2, and 29% for T3 over control plants (T4) at 3rd month of planting. The enhancement in photosynthesis is attributed to an increase in stevioside; with a 33% increase for T1 28.83% for T2 and 11% for T3 over aCO2. Similarly, the rebaudiosides A were also significantly increased by 32.8% for T1, 25% for T2, and 15% for T3 compared to the control under aCO2. Based on our findings, we concluded that eCO2 levels positively influenced the growth, biomass, and glycoside content by enhancing the physiological performance of Stevia rebaudiana Bertoni.

Contributions of microbial necromass and plant lignin to soil organic carbon stock in a paddy field under simulated conditions of long-term elevated CO2 and warming

Global climate change has various fundamental impacts on plant productivity and soil microbial communities, altering the formation and sequestration of soil organic carbon (SOC). However, the effect of climate change on different SOC components such as plant-derived C and microbial-derived C, remains poorly understood. A 3-year field experiment was conducted from 2018 to 2020 to examine the impacts of elevating atmospheric CO2 levels (550 ppm) and warming (+2 °C) on microbial necromass, plant lignin and phospholipid fatty acid (PLFA) in a Chinese rice paddy. Results showed that elevated CO2 and warming conditions increased the SOC stock by 16.5% and 8.6%, respectively. Elevated CO2 increased the accumulation of microbial necromass (mainly fungal) C by 24.6% and total lignin phenols by 15.8%, while also increasing the biomass of fungal PLFAs by 33.4%. In comparison, warming increased the accumulation of microbial necromass C (mainly bacterial necromass) by 11.1% and bacterial biomass by 27.1%, while it decreased total lignin phenols and their contribution to SOC by 8.3% and 15.7%, respectively. The reduction in lignin phenols and their contribution to SOC under warming conditions was mainly attributed to the lower level of plant productivity and increased activities of the enzymes β-1,4-glucosidase, β-cellobiohydrolase and xylanase. This resulted in increased plant residue conversion to microbial necromass in warmed soils. Random forest and correlation analysis indicated that soil pH, fungal biomass, root biomass and C-acquiring enzyme activities were the major factors affecting microbial necromass, while lignin phenols were mainly regulated by the ratio of fungal/bacterial necromass and fungal biomass. Overall, the combined effects of CO2 enrichment and warming conditions increased the storage and sequestration of SOC by enhancing the accumulation of microbial necromass, which was affected by soil properties, plant root C inputs and microbial communities within soil.

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.

High CO2 exposure due to facemask wear is unlikely to impair cognition even in a warm environment after a long-term adaptation

There is a lack of consensus regarding the impact of elevated indoor CO2 exposure on cognition. COVID-19 provided an opportunity to study responses to long-term elevated CO2 exposure from facemask wear. Such an opportunity allows us to avoid exposing participants to elevated CO2 levels not typically experienced in real-life settings, potentially increasing the ecological validity of our experimental design. Further, the worsening warmness during summer necessitates studies and understanding of peoples’ cognition with combined stressors of both heat and CO2 accumulation. In this work, we recruited 60 college students to understand whether facemask wear elevates local CO2 levels and, if so, the extent to which it impacts cognition in warm conditions. Subjects remained in a controlled summer environmental room (temperature 31.5 °C, relative humidity 30 %) for 90 min with or without facemasks. Participants completed six cognitive tests in a random order and answered surveys using computer-based software. Ten experimental subjects had a second 30 min visit to measure CO2 concentration at the ala of the nose with and without surgical masks. The results show that wearing a surgical mask sharply increased CO2 concentration near the nose by 15,000 ppm. Our analysis showed that the experimental group with facemask wear did not exhibit significantly different cognition performance except for short-term memory which was higher instead of lower than the control group. Participants with facemask wear showed significantly lower risk-taking, possibly attributed to thermal discomfort. Nevertheless, no significance in cognition or decision-making was observed after controlling the familywise error rate using the Bonferroni correction. We hypothesize that the insignificant difference might be caused by adaptation to long-term wear and high CO2 exposure in daily life during COVID-19, which cannot be revealed in the studies prior to the pandemic.

Methanogenic and methanotrophic communities determine lower CH4 fluxes in a subtropical paddy field under long-term elevated CO2

Clarifying the effects of elevated CO2 concentration (e[CO2]) on CH4 emissions from paddy fields and its mechanisms is a crucial part of the research on agricultural systems in response to global climate change. However, the response of CH4 fluxes from rice fields to long-term e[CO2] (e[CO2] duration >10 years) and its microbial mechanism is still lacking. In this study, we used a long-term free-air CO2 enrichment experiment to examine the relationship between CH4 fluxes and the methanogenic and methanotrophic consortia under long- and short-term e[CO2]. We demonstrated that contrary to the effect of short-term e[CO2], long-term e[CO2] decreased CH4 fluxes. This may be associated with the reduction of methanogenic abundance and the increase of methanotrophic abundance under long-term e[CO2]. In addition, long-term e[CO2] also changed the community structure and composition of methanogens and methanotrophs compared with short-term e[CO2]. Partial least squares path modeling analysis showed that long-term e[CO2] also could affect the abundance and composition of methanogens and methanotrophs indirectly by influencing soil physical and chemical properties, thereby ultimately altering CH4 fluxes in paddy soils. These findings suggest that current estimates of short-term e[CO2]-induced CH4 fluxes from paddy fields may be overestimated. Therefore, a comprehensive assessment of climate‑carbon cycle feedbacks may need to consider the microbial regulation of CH4 production and oxidation processes in paddy ecosystems under long-term e[CO2].

Soil organic carbon stability and exogenous nitrogen fertilizer influence the priming effect of paddy soil under long-term exposure to elevated atmospheric CO2.

Soil organic carbon (SOC) stability and dynamics are greatly influenced by long-term elevated atmospheric CO2 [CO2]. The priming effect (PE) is vital in SOC stability and dynamics, but its role in paddy soil under long-term elevated [CO2] remains unclear. To examine how SOC stability changed in paddy soil after long-term elevated atmospheric CO2 enrichment, the PE was quantified through a 13C-glucose-induced experiment with different N levels for topsoil (0-20cm) from paddy free-air CO2 enrichment (FACE) platform. Compared with the ambient CO2 concentration ([CO2]), 10years of elevated [CO2] (500µmol·mol-1) significantly increased SOC and TN content by 18.4% and 19.0%, respectively, while the C/N ratio was not changed. The labile C fractions including dissolved organic carbon (DOC) and readily oxidizable organic carbon (ROC), but excluding microbial biomass C (MBC), accumulated faster than SOC in paddy soil, which implied the reduced SOC stability for long-term elevated [CO2] enrichment. With the decline of SOC stability, the exogenously induced cumulative specific PE (PE per gram of SOC) remarkably increased by 41.1-72.7% for elevated [CO2] fumigation. The cumulative PE, especially the cumulative specific PE, was found significantly linearly correlated with the ROC content or ROC/SOC ratio (labile SOC pool). Furthermore, the application of nitrogen fertilizer slowed down the PE under elevated [CO2] condition. Our results showed that long-term elevated [CO2] enrichment reduced SOC stability and, together with exogenous nitrogen fertilizer, regulated the PE in paddy soil.

Legacy Effect of Long-Term Elevated CO2 and Warming on Soil Properties Controls Soil Organic Matter Decomposition

Plant litter quality is one of the key factors that control soil organic matter (SOM) decomposition. Under climate change, although significant change in litter quality has been intensively reported, the effect of litter quality change on SOM decomposition is poorly understood. This limits our ability to model the dynamics of soil carbon under climate change. To determine the effect of litter quality and soil property change on SOM decomposition, we performed a controlled, reciprocal transplant and litter decomposition experiments. The soils and plant litters were collected from a long-term field experiment, where four treatments were designed, including: (1) the control without warming at ambient CO2; (2) elevated atmospheric CO2 up to 500 ppm (C); (3) warming plant canopy by 2 °C (T); (4) elevated CO2 plus warming (CT). We found that elevated CO2 and warming altered the litter quality significantly in terms of macronutrients’ content and their stoichiometry. Elevated CO2 decreased the concentration of N in rice and wheat straw, while warming decreased the concentration of N and K in wheat straw. However, the change in plant litter quality did not lead to a shift in SOM decomposition. On the contrary, the legacy effect of long-term elevated CO2 and warming on soil properties dominated the decomposition rate of SOM. Elevated atmospheric CO2 suppressed SOM decomposition mainly by increasing phosphorous availability and lowering the soil C/N, fungi/bacteria ratio, and N-acetyl-glucosaminidase activity, while warming or elevated CO2 plus warming had no effect on SOM decomposition. Our results demonstrated that the changes in soil property other than litter quality control the decomposition of SOM under climate change, and soil property change in respond to climate change should be considered in model developing to predict terrestrial soil carbon dynamics under elevated atmospheric CO2 and warming.

Distribution of Microorganisms and Fractionation of Sulphur in Anthropogenic Wetlands under Long-term Elevated CO2 Soil

The fate of extra carbon accessing soil under elevated CO2 levels, as well as the repercussions for plant nutrition, is primarily determined by soil microbe activity. However, most increased CO2 research has reported changes usually increases in soil organic carbon and reduction in the pH of the soil which is merely the first step in understanding how soil processes are altered. We analyzed these variables by assessing enzyme activities and identifying the individual components impacted by high CO2 and those that reflect changes in soil organic matter pools. The majority of the microbial variables studied showed a significant increase under eCO2 conditions, The rise in dehydrogenase activity suggests that the increased biomass of bacteria coincided with an increase in their activity. The rise in phosphatase activities implies that organic matter breakdown is being stimulated overall. The sulphur fractions had a significantly increased number of substrates consumed by soil microorganisms under increasing CO2. Moreover, direct examination of data from these perspective steep shifts in soil biological activity points to possible areas of investigation.

Multigenerational elevated atmospheric CO2 concentration induced changes of wheat grain quality via altering nitrogen reallocation and starch catabolism

Previous studies indicated the grain yield and compositions of wheat (Triticum aestivum L.) were affected by long-term elevated atmospheric CO2 concentration conditions. However, the roles of protein expression in wheat grain quality changes under multigenerational elevated atmospheric CO2 concentration are still rarely known. This study explored that the changes of grain quality in wheat offspring induced by multigenerational elevated atmospheric CO2 concentration exposure and analyzed the roles of the differently expressed proteins using 4D proteomics in regulating the wheat grain quality. The changes of grain protein accumulation, gluten index and dough development time indicated that the nutritional and end-use quality of wheat grains were directly affected by elevated atmospheric CO2 concentration. This was mainly due to the changed expressions of α-amylase inhibitors, glutamine synthetase, glutamate dehydrogenase, formamidase and β-glucosidase, which regulated the starch accumulation and nitrogen metabolism in grains. This study elucidates the mechanisms underlying the effects of long-term elevated atmospheric CO2 concentration on wheat grain quality.

Anatomical adjustments of the tree hydraulic pathway decrease canopy conductance under long-term elevated CO2.

The cause of reduced leaf-level transpiration under elevated CO2 remains largely elusive. Here, we assessed stomatal, hydraulic, and morphological adjustments in a long-term experiment on Aleppo pine (Pinus halepensis) seedlings germinated and grown for 22-40 months under elevated (eCO2; c. 860 ppm) or ambient (aCO2; c. 410 ppm) CO2. We assessed if eCO2-triggered reductions in canopy conductance (gc) alter the response to soil or atmospheric drought and are reversible or lasting due to anatomical adjustments by exposing eCO2 seedlings to decreasing [CO2]. To quantify underlying mechanisms, we analyzed leaf abscisic acid (ABA) level, stomatal and leaf morphology, xylem structure, hydraulic efficiency, and hydraulic safety. Effects of eCO2 manifested in a strong reduction in leaf-level gc (-55%) not caused by ABA and not reversible under low CO2 (c. 200 ppm). Stomatal development and size were unchanged, while stomatal density increased (+18%). An increased vein-to-epidermis distance (+65%) suggested a larger leaf resistance to water flow. This was supported by anatomical adjustments of branch xylem having smaller conduits (-8%) and lower conduit lumen fraction (-11%), which resulted in a lower specific conductivity (-19%) and leaf-specific conductivity (-34%). These adaptations to CO2 did not change stomatal sensitivity to soil or atmospheric drought, consistent with similar xylem safety thresholds. In summary, we found reductions of gc under elevated CO2 to be reflected in anatomical adjustments and decreases in hydraulic conductivity. As these water savings were largely annulled by increases in leaf biomass, we do not expect alleviation of drought stress in a high CO2 atmosphere.

Response of Yield, CH4, and N2O Emissions from Paddy Fields to Long-term Elevated CO2 Concentrations

Elevated atmospheric CO2 concentrations([CO2]e) are the main driving force of global climate change, which directly and indirectly affect carbon and nitrogen cycling in the paddy ecosystems. Therefore, understanding the response of rice yield and greenhouse gas emissions to long-term(more than 10 years)[CO2]e from paddy fields is of great significance for food security and future climate change assessment. In this study, strongly and weakly responsive cultivars were used as the experimental materials. Based on a free-air CO2 enrichment(FACE) platform continuously run for 14 years, two treatments of different[CO2] were set:a control(i.e., normal[CO2] and[CO2]a) and a 200 μmol·mol-1 higher than[CO2]a condition, ([CO2]e). CH4 and N2O emissions from the rice paddy fields were monitored in situ by static transparent chamber-gas chromatography, and grain yields were also obtained. The results showed that compared with the[CO2]a treatment, long-term[CO2]e increased grain yields of the strongly and weakly responsive cultivars by 29%-31%(P<0.05) and 12%-14%(P>0.05), and CH4 emissions of the strongly and weakly responsive cultivars were reduced by 21%-59% and 11%-54%, respectively. Furthermore, N2O emissions from the strongly and weakly responsive cultivars were significantly reduced by 70%(P<0.05) and 40%(P<0.05), respectively. The short- and long-term responses of grain yields and CH4 emissions from rice paddy fields to[CO2]e were significantly different. Specifically, with the increase in the duration of[CO2]e, the increases in rice yields and CH4 emissions significantly decreased, while the N2O emissions showed no significant changes. Therefore, under long-term[CO2]e conditions, the strongly responsive cultivar has a high potential to reduce greenhouse gas emission and increase grain yields.

Long-term warming and elevated CO2 increase ammonia-oxidizing microbial communities and accelerate nitrification in paddy soil

Soil nitrification is a crucial process that increases nitrogen (N) availability for plants and drives nitrous oxide (N2O) emissions and nitrate (NO3−) leaching. Ongoing climate warming and elevated CO2 in the atmosphere may affect nitrification in opposite directions. However, their interactions are not known, especially in the long-term and in flooded conditions common in paddy soils. We conducted a long-term (9 years) field experiment with simultaneous manipulation of temperature (+2 °C above ambient) and CO2 (+60–100 ppm above ambient) to evaluate the impacts of warming and CO2 enrichment on the community structure, diversity, and population size of ammonia-oxidizing archaea (AOA) and bacteria (AOB), soil nitrification and physico-chemical characteristics. Elevated CO2 increased soil organic carbon (SOC) content (18%), microbial biomass N (65%), and carbon (27%). These changes accompanied an increment in AOB functional-gene abundance (181%) and soil nitrification rate (96%). Elevated temperature also increased (58%) soil nitrification rate. The combination of warming and CO2 enrichment enhanced AOB gene abundance (232%) and nitrification rate (133%). Impacts of these climate change factors on nitrification strongly depended on AOB gene abundance and microbial biomass, but not on AOA abundance. Higher root C inputs by rhizodeposition and pH under these climate change factors explained the substantial increases in AOB gene abundance, as nutrient-rich and alkaline soils favor AOB growth. By contrast, the AOA community was less sensitive to these climate change factors. Community composition, diversity, and richness of nitrifiers were remarkably resilient in response to individual and interactive impacts of warming and elevated CO2. Consequently, climate change will increase AOB population size but without community restructuring and will increase nitrification in paddy soils, which enhances N availability with consequences for crop productivity, N2O emissions, and NO3− leaching.

Long-term elevated CO2 and warming enhance microbial necromass carbon accumulation in a paddy soil

Long-term elevated CO2 and warming enhance microbial necromass carbon accumulation in a paddy soil

Long-term effects of elevated CO2 on the population dynamics of the seagrass Cymodocea nodosa: Evidence from volcanic seeps

Population reconstruction techniques was used to assess for the first time the population dynamics of a seagrass, Cymodocea nodosa, exposed to long-term elevated CO2 near three volcanic seeps and compared them with reference sites away from the seeps. Under high CO2, the density of shoots and of individuals (apical shoots), and the vertical and horizontal elongation and production rates, were higher than at the reference sites. Nitrogen limitation effects on rhizome elongation and production rates and on biomass were more evident than CO2 as these were highest at the location where the limitation of nitrogen was highest. At the seep where the availability of CO2 was highest and nitrogen lowest, density of shoots and individuals were highest, probably due to CO2 effects on shoot differentiation and induced reproductive output, respectively. At the three seeps, there was higher short- and long-term shoot recruitment than at the reference sites, and growth rates was around zero, indicating that elevated CO2 increases the turnover of C. nodosa shoots.

Canopy position affects photosynthesis and anatomy in mature Eucalyptus trees in elevated CO2.

Leaves are exposed to different light conditions according to their canopy position, resulting in structural and anatomical differences with consequences for carbon uptake. While these structure-function relationships have been thoroughly explored in dense forest canopies, such gradients may be diminished in open canopies, and they are often ignored in ecosystem models. We tested within-canopy differences in photosynthetic properties and structural traits in leaves in a mature Eucalyptus tereticornis canopy exposed to long-term elevated CO2 for up to three years. We explored these traits in relation to anatomical variation and diffusive processes for CO2 (i.e., stomatal conductance, gs and mesophyll conductance, gm) in both upper and lower portions of the canopy receiving ambient and elevated CO2. While shade resulted in 13% lower leaf mass per area ratio (MA) in lower versus upper canopy leaves, there was no relationship between leaf Nmass and canopy gap fraction. Both maximum carboxylation capacity (Vcmax) and maximum electron transport (Jmax) were~18% lower in shaded leaves and were also reduced by ~ 22% with leaf aging. In mature leaves, we found no canopy differences for gm or gs, despite anatomical differences in MA, leaf thickness and mean mesophyll thickness between canopy positions. There was a positive relationship between net photosynthesis and gm or gs in mature leaves. Mesophyll conductance was negatively correlated with mean parenchyma length, suggesting that long palisade cells may contribute to a longer CO2 diffusional pathway and more resistance to CO2 transfer to chloroplasts. Few other relationships between gm and anatomical variables were found in mature leaves, which may be due to the open crown of Eucalyptus. Consideration of shade effects and leaf-age dependent responses to photosynthetic capacity and mesophyll conductance are critical to improve canopy photosynthesis models and will improve understanding of long-term responses to elevated CO2 in tree canopies.

Proteomic Responses to Ocean Acidification in the Brain of Juvenile Coral Reef Fish

Elevated CO2 levels predicted to occur by the end of the century can affect the physiology and behaviour of marine fishes. For one important survival mechanism, the response to chemical alarm cues from conspecifics, substantial among-individual variation in the extent of behavioural impairment when exposed to elevated CO2 has been observed in previous studies. Whole brain transcriptomic data has further emphasized the importance of parental phenotypic variation in the response of juvenile fish to elevated CO2. In this study, we investigate the genome-wide proteomic responses of this variation in the brain of 5-week old spiny damselfish, Acanthochromis polyacanthus. We compared the accumulation of proteins in the brains of juvenile A. polyacanthus from two different parental behavioural phenotypes (sensitive and tolerant) that had been experimentally exposed to short-term, long-term and inter-generational elevated CO2. Our results show differential accumulation of key proteins related to stress response and epigenetic markers with elevated CO2 exposure. Proteins related to neurological development and glucose metabolism were also differentially accumulated particularly in the long-term developmental treatment, which might be critical for juvenile development. By contrast, exposure to elevated CO2 in the parental generation resulted in only three differentially accumulated proteins in the offspring, revealing potential for inter-generational acclimation. Lastly, we found a distinct proteomic pattern in juveniles due to the behavioural sensitivity of parents to elevated CO2, even though the behaviour of the juvenile fish was impaired regardless of parental phenotype. Our data shows that developing juveniles are affected in their brain protein accumulation by elevated CO2, but the effect varies with the length of exposure as well as due to variation of parental phenotypes in the population.

Different physiological and molecular responses of the green algae Chlorella variabilis to long-term and short-term elevated CO2

Chlorella variabilis is a high-efficiency photosynthetic green plant, and its photosynthesis capacity is considerably higher than other plants. With increasing CO2 concentration in the atmosphere C. variabilis may change the metabolism to respond to elevated CO2. To investigate the response of C. variabilis when exposed to increased CO2 concentrations, physiological and transcriptome analyses were carried out on C. variabilis that had been cultivated at 1000 ppm CO2 for long (LT) and short (ST) periods of time and these were compared with samples from the control group (CT). The common responses of the ST and LT C. variabilis included increased pigment levels, and higher Fv/Fm and carbon fixation rates. These results indicated that current CO2 levels could limit C. variabilis growth and photosynthesis. Transcriptome changes in the ST samples were coincident with the observed physiological responses, which suggested that carbon flow to the acetyl-CoA pool from photosynthesis enhanced the production of lipids and fatty acids. This was confirmed by the depressed β-oxidation levels, upregulation of fatty acids and triglyceride synthesis pathways, lower soluble carbohydrate contents, and lipid accumulation. However, we found no change or downregulation of genes involved in cellular metabolism in the LT samples, such as the genes participating in protein, fatty acid, and triglyceride synthesis. The results demonstrated that C. variabilis was generally sensitive to the ST condition, but hardly responded to the LT treatment and failed to evolve any specific adaptations.

Impacts of long-term elevated atmospheric CO2 concentrations on communities of arbuscular mycorrhizal fungi.

The ecological impacts of long‐term elevated atmospheric CO2 (eCO2) levels on soil microbiota remain largely unknown. This is particularly true for the arbuscular mycorrhizal (AM) fungi, which form mutualistic associations with over two‐thirds of terrestrial plant species and are entirely dependent on their plant hosts for carbon. Here, we use high‐resolution amplicon sequencing (Illumina, HiSeq) to quantify the response of AM fungal communities to the longest running (>15 years) free‐air carbon dioxide enrichment (FACE) experiment in the Northern Hemisphere (GiFACE); providing the first evaluation of these responses from old‐growth (>100 years) semi‐natural grasslands subjected to a 20% increase in atmospheric CO2. eCO2 significantly increased AM fungal richness but had a less‐pronounced impact on the composition of their communities. However, while broader changes in community composition were not observed, more subtle responses of specific AM fungal taxa were with populations both increasing and decreasing in abundance in response to eCO2. Most population‐level responses to eCO2 were not consistent through time, with a significant interaction between sampling time and eCO2 treatment being observed. This suggests that the temporal dynamics of AM fungal populations may be disturbed by anthropogenic stressors. As AM fungi are functionally differentiated, with different taxa providing different benefits to host plants, changes in population densities in response to eCO2 may significantly impact terrestrial plant communities and their productivity. Thus, predictions regarding future terrestrial ecosystems must consider changes both aboveground and belowground, but avoid relying on broad‐scale community‐level responses of soil microbes observed on single occasions.

Nitrogen and Phosphorus Retranslocation of Leaves and Stemwood in a Mature Eucalyptus Forest Exposed to 5 Years of Elevated CO2.

Elevated CO2 affects C cycling processes which in turn can influence the nitrogen (N) and phosphorus (P) concentrations of plant tissues. Given differences in how N and P are used by plants, we asked if their stoichiometry in leaves and wood was maintained or altered in a long-term elevated CO2 experiment in a mature Eucalyptus forest on a low P soil (EucFACE). We measured N and P concentrations in green leaves at different ages at the top of mature trees across 6 years including 5 years in elevated CO2. N and P concentrations in green and senesced leaves and wood were determined to evaluate both spatial and temporal variation of leaf N and P concentrations, including the N and P retranslocation in leaves and wood. Leaf P concentrations were 32% lower in old mature leaves compared to newly flushed leaves with no effect of elevated CO2 on leaf P. By contrast, elevated CO2 significantly decreased leaf N concentrations in newly flushed leaves but this effect disappeared as leaves matured. As such, newly flushed leaves had 9% lower N:P ratios in elevated CO2 and N:P ratios were not different in mature green leaves (CO2 by Age effect, P = 0.02). Over time, leaf N and P concentrations in the upper canopy slightly declined in both CO2 treatments compared to before the start of the experiment. P retranslocation in leaves was 50%, almost double that of N retranslocation (29%), indicating that this site was P-limited and that P retranslocation was an important mechanism in this ecosystem to retain P in plants. As P-limited trees tend to store relatively more N than P, we found an increased N:P ratio in sapwood in response to elevated CO2 (P < 0.01), implying N accumulation in live wood. The flexible stoichiometric ratios we observed can have important implications for how plants adjust to variable environmental conditions including climate change. Hence, variable nutrient stoichiometry should be accounted for in large-scale Earth Systems models invoking biogeochemical processes.

Legacy effects of long-term CO2 enrichment on plant biomass recovery from fire seven years after return to ambient CO2 levels1

We sought to determine if elevated CO2 resulted in long-lasting changes to plant structure and function, particularly disturbance response patterns. Minimal legacy effects were observed 1 yr after the termination of a previous long-term experiment; however, those observations were based on 1 yr of data and only focused on fine root growth, herbivory, and arthropod diversity. We examined above- and belowground biomass inside the footprints of chambers from a former long-term elevated CO2 experiment in a scrub-oak community 7 yr after CO2 enrichment ceased and 2 yr after fire. Aboveground biomass was 40.6% higher in the previously elevated plots compared with ambient plots, suggesting that there are legacy effects in the form of more rapid aboveground recovery from fire on previously elevated CO2 plots. Belowground biomass exhibited minimal change since the historic CO2 enrichment, suggesting that differences were a result of the original treatment. After disturbance, regrowth occurs via sprouting from large belowground structures; thus changes in aboveground recovery are likely due to changes to belowground biomass caused by altered atmospheric CO2. Elevated CO2 has significantly affected disturbance recovery patterns in this community. These changes have the potential to alter plant carbon allocation patterns in frequently disturbed systems.