The effects of elevated atmospheric CO<Subscript>2</Subscript> and nitrogen amendments on subsurface CO<Subscript>2</Subscript> production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO₂ concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO₂ enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO₂ concentration levels (ambient [CO₂]^a, ambient + 200 ppmv, [CO₂]^e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g_N m⁻² year⁻¹). The combination of these measurements permitted indirect estimation of belowground CO₂ production and flux profiles in the mineral soil. Adjacent to the soil CO₂ profiles, direct (chamber-based) measurements of CO₂ fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO₂ profiles, neither [CO₂]^e nor N fertilization had a statistically significant effect on seasonal soil CO₂, CO₂ production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO₂ concentration depending on the depth. Variations in CO₂ were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO₂ pulses were induced by rainfall events. The soil effluxes were equal to the CO₂ production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO₂ profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO₂ diffusivity is required for these soil moisture extremes.     

Leaf dynamics of a deciduous forest canopy: no response to elevated CO<Subscript>2</Subscript>

Leaf area index (LAI) and its seasonal dynamics are key determinants of terrestrial productivity and, therefore, of the response of ecosystems to a rising atmospheric CO₂ concentration. Despite the central importance of LAI, there is very little evidence from which to assess how forest LAI will respond to increasing [CO₂]. We assessed LAI and related leaf indices of a closed-canopy deciduous forest for 4 years in 25-m-diameter plots that were exposed to ambient or elevated CO₂ (542 ppm) in a free-air CO₂ enrichment (FACE) experiment. LAI of this Liquidambar styraciflua (sweetgum) stand was about 6 and was relatively constant year-to-year, including the 2 years prior to the onset of CO₂ treatment. LAI throughout the 1999–2002 growing seasons was assessed through a combination of data on photosynthetically active radiation (PAR) transmittance, mass of litter collected in traps, and leaf mass per unit area (LMA). There was no effect of [CO₂] on any expression of leaf area, including peak LAI, average LAI, or leaf area duration. Canopy mass and LMA, however, were significantly increased by CO₂ enrichment. The hypothesized connection between light compensation point (LCP) and LAI was rejected because LCP was reduced by [CO₂] enrichment only in leaves under full sun, but not in shaded leaves. Data on PAR interception also permitted calculation of absorbed PAR (APAR) and light use efficiency (LUE), which are key parameters connecting satellite assessments of terrestrial productivity with ecosystem models of future productivity. There was no effect of [CO₂] on APAR, and the observed increase in net primary productivity in elevated [CO₂] was ascribed to an increase in LUE, which ranged from 1.4 to 2.4 g MJ^–1. The current evidence seems convincing that LAI of non-expanding forest stands will not be different in a future CO₂-enriched atmosphere and that increases in LUE and productivity in elevated [CO₂] are driven primarily by functional responses rather than by structural changes. Ecosystem or regional models that incorporate feedbacks on resource use through LAI should not assume that LAI will increase with CO₂ enrichment of the atmosphere.     

A new viewpoint to understand the response of leaf dark respiration to elevated CO<Subscript>2</Subscript> concentration

By investigating the R _D-C _a (dark respiration rate-atmospheric CO₂ concentration) and P _N (net photosynthetic rate)-C _a curves of bamboo (Fargesia denudata) and poplar (Populus cathayanna), we found that: (1) the minimal R _D was close to ambient CO₂ concentration, and the elevated or decreased atmospheric CO₂ concentration enhanced the R _D of both species; (2) the response curves of R _D-C _a were simulated well by quadratic function. This phenomenon might be an inherent property of leaf R _D of F. denudata and P. cathayanna. If this was true, it implies that effect of CO₂ on R _D could be interpreted with the relationship of R _D-C _a curves and the quadratic function.     

Physiological and biochemical aspects and molecular mechanisms of plant adaptation to the elevated concentration of atmospheric CO<Subscript>2</Subscript>

The review of publications concerning the impact of increasing CO₂ concentration in the Earths atmosphere (C_a) on higher terrestrial plants. The physiological changes in plants induced by increasing C_a, including growth and biochemical composition, the characteristics of photosynthesis and respiration, as well as the molecular mechanisms of the regulation of the activity of most important biosynthetic enzymes at early and late stages of the exposure to elevated C_a are under consideration. Various concepts of metabolic regulation during acclimation to increasing CO₂ concentration are critically reviewed. The pathways of possible involvement of carbonic anhydrase-mediated systems of CO₂ transport and concentration during C₃ photosynthesis of higher plants, the metabolic and signal mechanisms of photosynthesis inhibition by carbohydrates and the role of ethylene at elevated C_a are presented. The effect of elevated C_a on plant development and source-sink relations, as well as its interaction with other environmental factors, such as mineral, primarily nitrogen nutrition, light, temperature, and water regime, are discussed in with the context of potential forecasting of the consequences of increase in C_a and temperature for the activities of various higher plant forms in the rapidly changing climate.Translated from Fiziologiya Rastenii, Vol. 52, No. 1, 2005, pp. 129–145.Original Russian Text Copyright © 2005 by Romanova.     

Decades-long effects of high CO<Subscript>2</Subscript> concentration on soil nitrogen dynamics at a natural CO<Subscript>2</Subscript> spring

The effects of high atmospheric CO₂ concentration ([CO₂]) on ecosystem processes have been explored using temporal facilities such as open-top-chambers and free-air CO₂ enrichment. However, the effects of high [CO₂] on soil properties takes decades and may not be captured by short-term experiments. Natural CO₂ springs provide a unique opportunity to study the long-term effects of high [CO₂]. In this study, we investigated soil properties at a natural CO₂ spring. We found that the amounts of total carbon (C) and nitrogen (N) stored in the soil at the high [CO₂] site exceeded those in the reference site by 60 and 30%, respectively. The effects of high [CO₂] were large in the upper slope position where the canopy openness was high and plants grew faster, but no effects were detected in the lowest position where the canopy openness was lower (half of that at the upper slope position). In contrast, effects of high [CO₂] on soil N dynamics, such as N mineralization and nitrification rates, did not exhibit a slope gradient. This suggests that effects of high [CO₂] differed among soil stoichiometric characteristics and N dynamics. These complicated effects of high [CO₂] imply that the future effects of high [CO₂] on ecosystems could vary widely in conjunction with environmental conditions such as light availability and/or topographic conditions.     

Effects of elevated CO<Subscript>2</Subscript> and nitrogen on wheat growth and photosynthesis

The effects of nitrogen [75 and 150 kg (N) ha⁻¹] and elevated CO₂ on growth, photosynthetic rate, contents of soluble leaf proteins and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and nitrate reductase (NR) were studied on wheat (Triticum aestivum L. cv. HD-2285) grown in open top chambers under either ambient (AC) or elevated (EC) CO₂ concentration (350 ± 50, 600 ± 50 μmol mol⁻¹) and analyzed at 40, 60 and 90 d after sowing. Plants grown under EC showed greater photosynthetic rate and were taller and attained greater leaf area along with higher total plant dry mass at all growth stages than those grown under AC. Total soluble and Rubisco protein contents decreased under EC but the activation of Rubisco was higher at EC with higher N supply. Nitrogen increased the NR activity whereas EC reduced it. Thus, EC causes increased growth and P_N ability per unit uptake of N in wheat plants, even if N is limiting.     

Nitrate supply and plant development influence nitrogen uptake and allocation under elevated CO<Subscript>2</Subscript> in durum wheat grown hydroponically

Growth in elevated CO₂ often leads to decreased plant nitrogen contents and down-regulation of photosynthetic capacity. Here, we investigated whether elevated CO₂ limits nitrogen uptake when nutrient movement to roots is unrestricted, and the dependence of this limitation on nitrogen supply and plant development in durum wheat (Triticum durum Desf.). Plants were grown hydroponically at two N supplies and ambient and elevated CO₂ concentrations. Elevated CO₂ decreased nitrate uptake per unit root mass with low N supply at early grain filling, but not at anthesis. This decrease was not associated with higher nitrate or amino acid, or lower non-structural carbohydrate contents in roots. At anthesis, elevated CO₂ decreased the nitrogen content of roots with both levels of N and that of aboveground organs with high N. With low N, elevated CO₂ increased N allocation to aboveground plant organs and nitrogen concentration per unit flag leaf area at anthesis, and per unit aboveground dry mass at both growth stages. The results from the hydroponic experiment suggest that elevated CO₂ restricts nitrate uptake late in development, high N supply overriding this restriction. Increased nitrogen allocation to young leaves at low N supply could alleviate photosynthetic acclimation to elevated CO₂.     

Photosynthetic CO<Subscript>2</Subscript> uptake in seedlings of two tropical tree species exposed to oscillating elevated concentrations of CO<Subscript>2</Subscript>

Do short-term fluctuations in CO₂ concentrations at elevated CO₂ levels affect net CO₂ uptake rates of plants? When exposed to 600 μl CO₂ l^?1, net CO₂ uptake rates in shoots or leaves of seedlings of two tropical C₃ tree species, teak (Tectona grandis L. f.) and barrigon [Pseudobombax septenatum (Jacq.) Dug.], increased by 28 and 52% respectively. In the presence of oscillations with half-cycles of 20 s, amplitude of ca. 170 μl CO₂ l^?1 and mean of 600 μl CO₂ l^?1, the stimulation in net CO₂ uptake by the two species was reduced to 19 and 36%, respectively, i.e. the CO₂ stimulation in photosynthesis associated with a change in exposure from 370 to 600 μl CO₂ l^?1 was reduced by a third in both species. Similar reductions in CO₂-stimulated net CO₂ uptake were observed in T. grandis exposed to 40-s oscillations. Rates of CO₂ efflux in the dark by whole shoots of T. grandis decreased by 4.8% upon exposure of plants grown at 370 μl CO₂ l^?1 to 600 μl CO₂ l^?1. The potential implications of the observations on CO₂ oscillations and dark respiration are discussed in the context of free-air CO₂ enrichment (FACE) systems in which short-term fluctuations of CO₂ concentration are a common feature.     

Long-term effects of elevated atmospheric CO<Subscript>2</Subscript> on species composition and productivity of a southern African C<Subscript>4</Subscript> dominated grassland in the vicinity of a CO<Subscript>2</Subscript> exhalation

We describe the long-term effects of a CO₂ exhalation, created more than 70 years ago, on a natural C₄ dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO₂ enrichment changes the competitive balance between plants with C₃ and C₄ photosynthetic pathways and how CO₂ enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO₂ on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO₂ alone. Species compositional data and soil δ¹³C isotopic evidence suggest no detectable effect of CO₂ enrichment on C₃:C₄ plant mixtures and individual species dominance. Contrary to many general predictions C₃ grasses did not become more abundant and C₃ shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO₂ concentrations averaging 610 μmol mol⁻¹. Leaf properties such as total N decreased in the C₃ but not C₄ grass under elevated CO₂ while total non-structural carbohydrate accumulation was not affected. Elevated CO₂ possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO₂ enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO₂ source. The conservative response of this grassland suggests that many of the reported effects of elevated CO₂ on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO₂ conditions.     

Effect of elevated CO<Subscript>2</Subscript> levels on leaf starch,nitrogen and photosynthesis of plants growing at three natural CO<Subscript>2</Subscript> springs in Japan

Plant communities around natural CO₂ springs have been exposed to elevated CO₂ levels over many generations and give us a unique opportunity to investigate the effects of long-term elevated CO₂ levels on wild plants. We searched for natural CO₂ springs in cool temperate climate regions in Japan and found three springs that were suitable for studying long-term responses of plants to elevated levels of CO₂: Ryuzin-numa, Yuno-kawa and Nyuu. At these CO₂ springs, the surrounding air was at high CO₂ concentration with no toxic gas emissions throughout the growth season, and there was natural vegetation around the springs. At each site, high-CO₂ (HC) and low-CO₂ (LC) plots were established, and three dominant species at the shrub layers were used for physiological analyses. Although the microenvironments were different among the springs, dicotyledonous species growing at the HC plots tended to have more starch and less nitrogen per unit dry mass in the leaves than those growing at the LC plots. In contrast, monocotyledonous species growing in the HC and LC plots had similar starch and nitrogen concentrations. Photosynthetic rates at the mean growth CO₂ concentration were higher in HC plants than LC plants, but photosynthetic rates at a common CO₂ concentration were lower in HC plants. Efficiency of water and nitrogen use of leaves at growth CO₂ concentration was greatly increased in HC plants. These results suggest that natural plants growing in elevated CO₂ levels under cool temperate climate conditions have down-regulated their photosynthetic capacity but that they increased photosynthetic rates and resource use efficiencies due to the direct effect of elevated CO₂ concentration.     

Effects of elevated atmospheric CO<Subscript>2</Subscript> on the nutritional ecology of C<Subscript>3</Subscript> and C<Subscript>4</Subscript> grass-feeding caterpillars

It is plausible that the nutritional quality of C₃ plants will decline more under elevated atmospheric CO₂ than will the nutritional quality of C₄ plants, causing herbivorous insects to increase their feeding on C₃ plants relative to C₄ plants. We tested this hypothesis with a C₃ and C₄ grass and two caterpillar species with different diet breadths. Lolium multiflorum (C₃) and Bouteloua curtipendula (C₄) were grown in outdoor open top chambers at ambient (370 ppm) or elevated (740 ppm) CO₂. Bioassays compared the performance and digestive efficiencies of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid). As expected, the nutritional quality of L. multiflorum changed to a greater extent than did that of B. curtipendula when grown in elevated CO₂; levels of protein (considered growth limiting) declined in the C₃ grass, while levels of carbohydrates (sugar, starch and fructan) increased. However, neither insect species increased its feeding rate on the C₃ grass to compensate for its lower nutritional quality when grown in an elevated CO₂ atmosphere. Consumption rates of P. unipuncta and S. frugiperda were higher on the C₃ grass than the C₄ grass, the opposite of the result expected for a compensatory response to the lower nutritional quality of the C₄ grass. Although our results do not support the hypothesis that grass-specialist insects compensate for lower nutritional quality by increasing their consumption rates more than do generalist insects, the performance of the specialist was greater than that of the generalist on each grass species and at both CO₂ levels. Mechanisms other than compensatory feeding, such as increased nutrient assimilation efficiency, appear to determine the relative performance of these herbivores. Our results also provide further evidence against the hypothesis that C₄ grasses would be avoided by insect herbivores because a large fraction of their nutrients is unavailable to herbivores. Instead, our results are consistent with the hypothesis that C₄ grasses are poorer host plants primarily because of their lower nutrient levels, higher fiber levels, and greater toughness.     

The effects of elevated atmospheric [CO<Subscript>2</Subscript>] on Norway spruce needle parameters

Studies of selected morphological needle parameters were carried out on young (17–19 year old) Norway spruce trees cultivated inside glass domes at ambient (A, 370 μmol (CO₂) mol⁻¹) and elevated (E, 700 μmol (CO₂) mol⁻¹) atmospheric CO₂ concentrations [CO₂] beginning in 1997. Annual analyses performed from 2002 to 2004 revealed higher values for needle length (especially for current needles, up to 18%) and projected needle area (up to 13%) accompanied by lower values for specific needle area (up to 15% lower, as quantified by needle mass to projected area ratio) in the E treatment compared to the A treatment. Statistically significant differences for most of the investigated morphological parameters were found in young needles in the well irradiated sun-adapted crown parts, particularly under water-limiting soil conditions in 2003. This was likely a result of different water relations in E compared to A trees as investigated under temperate water stress (Kuper et al. in Biol Plantarum 50:603–609, 2006). Furthermore, E trees had much higher absorbing root area, which modified and enhanced root:shoot as well as root:conductive stem area proportions. These hydraulic properties and early seasonal stimulation of photosynthesis forced advanced needle development in E trees, particularly under limited soil water conditions. The number of needles per unit shoot length was found to be unaffected by elevated [CO₂].     

The interacting effects of elevated atmospheric CO<Subscript>2</Subscript> concentration,drought and leaf-to-air vapour pressure deficit on ecosystem isoprene fluxes

Isoprene is the most abundant biogenic hydrocarbon released from vegetation and it plays a major role in tropospheric chemistry. Because of its link to climate change, there is interest in understanding the relationship between CO₂, water availability and isoprene emission. We explored the effect of atmospheric elevated CO₂ concentration and its interaction with vapour pressure deficit (VPD) and water stress, on gross isoprene production (GIP) and net ecosystem exchange of CO₂ (NEE) in two Populus deltoides plantations grown at ambient and elevated atmospheric CO₂ concentration in the Biosphere 2 Laboratory facility. Although GIP and NEE showed a similar response to light and temperature, their responses to CO₂ and VPD were opposite; NEE was stimulated by elevated CO₂ and depressed by high VPD, while GIP was inhibited by elevated CO₂ and stimulated by high VPD. The difference in response between isoprene production and photosynthesis was also evident during water stress. GIP was stimulated in the short term and declined only when the stress was severe, whereas NEE started to decrease from the beginning of the experiment. This contrasting response led the carbon lost as isoprene in both the ambient and the elevated CO₂ treatments to increase as water stress progressed. Our results suggest that water limitation can override the inhibitory effect of elevated CO₂ leading to increased global isoprene emissions in a climate change scenario with warmer and drier climate.     

Responses of tropical native and invader C<Subscript>4</Subscript> grasses to water stress,clipping and increased atmospheric CO<Subscript>2</Subscript> concentration

The invasion of African grasses into Neotropical savannas has altered savanna composition, structure and function. The projected increase in atmospheric CO₂ concentration has the potential to further alter the competitive relationship between native and invader grasses. The objective of this study was to quantify the responses of two populations of a widespread native C₄ grass (Trachypogon plumosus) and two African C₄ grass invaders (Hyparrhenia rufa and Melinis minutiflora) to high CO₂ concentration interacting with two primary savanna stressors: drought and herbivory. Elevated CO₂ increased the competitive potential of invader grasses in several ways. Germination and seedling size was promoted in introduced grasses. Under high CO₂, the relative growth rate of young introduced grasses was twice that of native grass (0.58 g g⁻¹ week⁻¹ vs 0.25 g g⁻¹ week⁻¹). This initial growth advantage was maintained throughout the course of the study. Well-watered and unstressed African grasses also responded more to high CO₂ than did the native grass (biomass increases of 21–47% compared with decreases of 13–51%). Observed higher water and nitrogen use efficiency of invader grasses may aid their establishment and competitive strength in unfertile sites, specially if the climate becomes drier. In addition, high CO₂ promoted lower leaf N content more in the invader grasses. The more intensive land use, predicted to occur in this region, may interact with high CO₂ to fincreasesavor the African grasses, as they generally recovered faster after simulated herbivory. The superiority of invader grasses under high CO₂ suggests further in their competitive strength and a potential increased rate of displacement of the native savannas in the future by grasslands dominated by introduced African species.     

Impacts of increased atmospheric CO<Subscript>2</Subscript> concentration on photosynthesis and growth of micro- and macro-algae

Marine photosynthesis drives the oceanic biological CO₂ pump to absorb CO₂ from the atmosphere, which sinks more than one third of the industry-originated CO₂ into the ocean. The increasing atmospheric CO₂ and subsequent rise of pCO₂ in seawater, which alters the carbonate system and related chemical reactions and results in lower pH and higher HCO₃ ⁻ concentration, affect photosynthetic CO₂ fixation processes of phytoplanktonic and macroalgal species in direct and/or indirect ways. Although many unicellular and multicellular species can operate CO₂-concentrating mechanisms (CCMs) to utilize the large HCO₃ ⁻ pool in seawater, enriched CO₂ up to several times the present atmospheric level has been shown to enhance photosynthesis and growth of both phytoplanktonic and macro-species that have less capacity of CCMs. Even for species that operate active CCMs and those whose photosynthesis is not limited by CO₂ in seawater, increased CO₂ levels can down-regulate their CCMs and therefore enhance their growth under light-limiting conditions (at higher CO₂ levels, less light energy is required to drive CCM). Altered physiological performances under high-CO₂ conditions may cause genetic alteration in view of adaptation over long time scale. Marine algae may adapt to a high CO₂ oceanic environment so that the evolved communities in future are likely to be genetically different from the contemporary communities. However, most of the previous studies have been carried out under indoor conditions without considering the acidifying effects on seawater by increased CO₂ and other interacting environmental factors, and little has been documented so far to explain how physiology of marine primary producers performs in a high-CO₂ and low-pH ocean.     

Stomatal conductance in mature deciduous forest trees exposed to elevated CO<Subscript>2</Subscript>

Stomatal conductance (g _s) of mature trees exposed to elevated CO₂ concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g _s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO₂ compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO₂ treatment. During the 2003 drought in the third treatment year, the CO₂ effect became reversed in Carpinus, resulting in higher g _s in trees exposed to elevated CO₂ compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO₂ enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO₂-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO₂. An erratum to this article can be found at     

Stomatal conductance in mature deciduous forest trees exposed to elevated CO<Subscript>2</Subscript>

Stomatal conductance (g _s) of mature trees exposed to elevated CO₂ concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g _s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO₂ compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO₂ treatment. During the 2003 drought in the third treatment year, the CO₂ effect became reversed in Carpinus, resulting in higher g _s in trees exposed to elevated CO₂ compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO₂ enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO₂-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO₂.     

Arbuscular mycorrhiza improve growth,nitrogen uptake,and nitrogen use efficiency in wheat grown under elevated CO<Subscript>2</Subscript>

Effects of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis on plant growth, carbon (C) and nitrogen (N) accumulation, and partitioning was investigated in Triticum aestivum L. plants grown under elevated CO₂ in a pot experiment. Wheat plants inoculated or not inoculated with the AM fungus were grown in two glasshouse cells with different CO₂ concentrations (400 and 700 ppm) for 10 weeks. A ¹⁵N isotope labeling technique was used to trace plant N uptake. Results showed that elevated CO₂ increased AM fungal colonization. Under CO₂ elevation, AM plants had higher C concentration and higher plant biomass than the non-AM plants. CO₂ elevation did not affect C and N partitioning in plant organs, while AM symbiosis increased C and N allocation into the roots. In addition, plant C and N accumulation, ¹⁵N recovery rate, and N use efficiency (NUE) were significantly higher in AM plants than in non-AM controls under CO₂ enrichment. It is concluded that AM symbiosis favors C and N partitioning in roots, increases C accumulation and N uptake, and leads to greater NUE in wheat plants grown at elevated CO₂.     

Rice (<Emphasis Type=Italic>Oryza sativa</Emphasis> L.) growth and nitrogen distribution under elevated CO<Subscript>2</Subscript> concentration and air temperature

A field experiment was conducted to investigate the effects of elevated atmospheric CO₂ concentration and temperature, singly and in combination, on grain yield and the distribution of nitrogen (N) in different rice organs. The rice ‘Wuyunjing 23’ was planted under four treatments: ambient CO₂ and temperature (ACT), elevated CO₂ (200 μmol mol^?1 higher than ambient CO₂) (EC), elevated temperature (1 °C above the ambient temperature) (ET), and the combination of elevated CO₂ and temperature (ECT) under T-FACE (temperature and CO₂ free air controlled enrichment) system. CO₂-induced increment and temperature-induced reduction in grain yield was 6.0 and 25.2% in 2013, and that was 9.8 and 10.8% in 2014, respectively. Dry matter (DM) production in different organs increased under EC at vegetative stage but decreased under ET at reproductive stage. The negative effects of temperature on grain yield and DM was weakened when combined with CO₂ enrichment. And the trends of decrease for yield and DM under ET and ECT in 2013 were more obvious than those in 2014 due to the annual temperature differences. Furthermore, ET led to greater distribution of N in root and stem but not for panicle than that under ACT. These mainly demonstrated that the rice production would be suffered varying degree of loss under global warming in future although the CO₂ enrichment could alleviate the effects of high temperature on rice growth.     

Fine root chemistry and decomposition in model communities of north-temperate tree species show little response to elevated atmospheric CO<Subscript>2</Subscript> and varying soil resource availability

Rising atmospheric [CO₂] has the potential to alter soil carbon (C) cycling by increasing the content of recalcitrant constituents in plant litter, thereby decreasing rates of decomposition. Because fine root turnover constitutes a large fraction of annual NPP, changes in fine root decomposition are especially important. These responses will likely be affected by soil resource availability and the life history characteristics of the dominant tree species. We evaluated the effects of elevated atmospheric [CO₂] and soil resource availability on the production and chemistry, mycorrhizal colonization, and decomposition of fine roots in an early- and late-successional tree species that are economically and ecologically important in north temperate forests. Open-top chambers were used to expose young trembling aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees to ambient (36 Pa) and elevated (56 Pa) atmospheric CO₂. Soil resource availability was composed of two treatments that bracketed the range found in the Upper Lake States, USA. After 2.5 years of growth, sugar maple had greater fine root standing crop due to relatively greater allocation to fine roots (30% of total root biomass) relative to aspen (7% total root biomass). Relative to the low soil resources treatment, aspen fine root biomass increased 76% with increased soil resource availability, but only under elevated [CO₂]. Sugar maple fine root biomass increased 26% with increased soil resource availability (relative to the low soil resources treatment), and showed little response to elevated [CO₂]. Concentrations of N and soluble phenolics, and C/N ratio in roots were similar for the two species, but aspen had slightly higher lignin and lower condensed tannins contents compared to sugar maple. As predicted by source-sink models of carbon allocation, pooled constituents (C/N ratio, soluble phenolics) increased in response to increased relative carbon availability (elevated [CO₂]/low soil resource availability), however, biosynthetically distinct compounds (lignin, starch, condensed tannins) did not always respond as predicted. We found that mycorrhizal colonization of fine roots was not strongly affected by atmospheric [CO₂] or soil resource availability, as indicated by root ergosterol contents. Overall, absolute changes in root chemical composition in response to increases in C and soil resource availability were small and had no effect on soil fungal biomass or specific rates of fine root decomposition. We conclude that root contributions to soil carbon cycling will mainly be influenced by fine root production and turnover responses to rising atmospheric [CO₂], rather than changes in substrate chemistry.