Decomposition of Betula papyrifera leaf litter under the independent and interactive effects of elevated CO2 and O3

Litter decay dynamics of paper birch (Betula papyrifera) were assessed at the Aspen free‐air CO₂ enrichment (FACE) facility in northern Wisconsin, USA. Leaf litter was decomposed for 12 months under factorial combinations of 360 vs. 560 μL CO₂ L^?1, crossed with 36 vs. 55 nL O₃ L^?1. To differentiate between substrate quality and environment effects, litterbags were placed in their Native Plots of origin or transplanted into the other treatments. CO₂ enrichment, regardless of O₃ concentration, produced poorer quality litter (high C/N, lignin/N and condensed tannins) than did ambient CO₂ (low C/N, lignin/N and condensed tannins). Substrate quality differences were reflected in the mass loss rates (k‐values), which were high for litter generated under ambient CO₂ (0.887 year^?1) and low for litter generated under elevated CO₂ (0.674 year^?1). The rate‐retarding effects of CO₂ enrichment were neither alleviated nor exacerbated by O₃ exposure. Decay rates varied, however, depending on whether litter was placed back into its plot of origin or transplanted to Common Gardens. The results of this study are species specific, but they have important implications for understanding the processes regulating storage of fixed C and the release of CO₂ from northern forest ecosystems.     

Decomposition of soybean grown under elevated concentrations of CO2 and O3

A critical global climate change issue is how increasing concentrations of atmospheric CO₂ and ground‐level O₃ will affect agricultural productivity. This includes effects on decomposition of residues left in the field and availability of mineral nutrients to subsequent crops. To address questions about decomposition processes, a 2‐year experiment was conducted to determine the chemistry and decomposition rate of aboveground residues of soybean (Glycine max (L.) Merr.) grown under reciprocal combinations of low and high concentrations of CO₂ and O₃ in open‐top field chambers. The CO₂ treatments were ambient (370 μmol mol^?1) and elevated (714 μmol mol^?1) levels (daytime 12 h averages). Ozone treatments were charcoal‐filtered air (21 nmol mol^?1) and nonfiltered air plus 1.5 times ambient O₃ (74 nmol mol^?1) 12 h day^?1. Elevated CO₂ increased aboveground postharvest residue production by 28–56% while elevated O₃ suppressed it by 15–46%. In combination, inhibitory effects of added O₃ on biomass production were largely negated by elevated CO₂. Plant residue chemistry was generally unaffected by elevated CO₂, except for an increase in leaf residue lignin concentration. Leaf residues from the elevated O₃ treatments had lower concentrations of nonstructural carbohydrates, but higher N, fiber, and lignin levels. Chemical composition of petiole, stem, and pod husk residues was only marginally affected by the elevated gas treatments. Treatment effects on plant biomass production, however, influenced the content of chemical constituents on an areal basis. Elevated CO₂ increased the mass per square meter of nonstructural carbohydrates, phenolics, N, cellulose, and lignin by 24–46%. Elevated O₃ decreased the mass per square meter of these constituents by 30–48%, while elevated CO₂ largely ameliorated the added O₃ effect. Carbon mineralization rates of component residues from the elevated gas treatments were not significantly different from the control. However, N immobilization increased in soils containing petiole and stem residues from the elevated CO₂, O₃, and combined gas treatments. Mass loss of decomposing leaf residue from the added O₃ and combined gas treatments was 48% less than the control treatment after 20 weeks, while differences in decomposition of petiole, stem, and husk residues among treatments were minor. Decreased decomposition of leaf residues was correlated with lower starch and higher lignin levels. However, leaf residues only comprised about 20% of the total residue biomass assayed so treatment effects on mass loss of total aboveground residues were relatively small. The primary influence of elevated atmospheric CO₂ and O₃ concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO₂ and suppressed by O₃.     

Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

An experiment was carried out to determine the effects of elevated CO₂, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C₄ perennial grass, or Pascopyrum smithii, a C₃ perennial grass, were placed in growth chambers with 350 or 700 μL L^?1 atmospheric CO₂, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO₂, with no significant difference between the C₃ and C₄ grass. The effect of elevated CO₂ on biomass was greatest at the intermediate water level. The positive effect of elevated CO₂ on shoot biomass was greater at normal temperatures in B. gracilis, and greater at elevated temperatures in P. smithii. Neither root-to-shoot ratio nor production of seed heads was affected by elevated CO₂. Plant tissue N and soil inorganic N concentrations were lower under elevated Co₂, but no more so in the C₃ than the C₄ plant. Elevated CO₂ appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO₂ effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO₂, in spite of greater N limitation.     

Comparative ecophysiology of CAM and C3 bromeliads. III. Environmental influences on CO2 assimilation and transpiration

Abstract Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to compare carbon assimilation with water use in CAM and C₃ photosynthesis. The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO₂ uptake was restricted to the end of the dark period in plants found at drier habitats, and stomatal conductance in two CAM species was found to respond directly to humidity or temperature. Total night-time CO₂ uptake, when compared with malic-acid formation (measured as the dawn-dusk difference in acidity, ΔH⁺), could only account for 10–40% of the total ΔH⁺ accumulated. The remaining malic acid must have been derived from the refixation of respired CO₂ (recycling). Within the genus Aechmea (12 samples from four species), recycling was significantly correlated with night temperature at the six sample sites. Recycling was lowest in A. fendleri (54% of ΔH⁺ derived from respired CO₂), a CAM bromeliad with little water-storage parenchyma that is restricted to wetter, cooler regions of Trinidad. Gas-exchange rates of C₃ bromeliads were found to be similar to those of the CAM bromeliads, with CO₂ uptake from 1 to 3 μmol m^?2 s^?1 and stomatal conductances generally up to 100 mmol m^?2 s^?1. The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO₂ uptake of the C₃ bromeliad Guzmania lingulata was saturated at around 500 μmol m^?2 s^?1 PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade-tolerant rather than shade-demanding. Transpiration ratios (TR) during CO₂ fixation in CAM (Phase I and IV) and C₃ bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO₂, TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO₂ uptake in CAM (42 ± 41, n= 12), Phase IV of CAM (69 ± 36, n= 3) and for C₃ photosynthesis (99 ± 73, n= 4) in these plants. It appears that recycling of respired CO₂ by CAM bromeliads and efficient use of water in all phases of CO₂ uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics.     

Effects of chronic ozone and elevated atmospheric CO2 concentrations on ribulose-1,5-bisphosphate in soybean (Glycine max)

Ribulose-1,5-bisphosphate (RuBP) pool size was determined at regular intervals during the growing season to understand the effects of tropospheric ozone concentrations, elevated atmospheric carbon dioxide concentrations and their interactions on the photosynthetic limitation by RuBP regeneration. Soybean (Glycine max [L.] Merr. cv. Essex) was grown from seed to maturity in open-top field chambers in charcoal-filtered air (CF) either without (22 nmol O₃ mol^?1) or with added O₃ (83 nmol mol^?1) at ambient (AA, 369 μmol CO₂ mol^?1) or elevated CO₂ (710 μmol mol^?1). The RuBP pool size generally declined with plant age in all treatments when expressed on a unit leaf area and in all treatments but CF-AA when expressed per unit ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) binding site. Although O₃ in ambient CO₂ generally reduced the RuBP pool per unit leaf area, it did not change the RuBP pool per unit Rubisco binding site. Elevated CO₂, in CF or O₃-fumigated air, generally had no significant effect on RuBP pool size, thus mitigating the negative O₃ effect. The RuBP pools were below 2 mol mol^?1 binding site in all treatments for most of the season, indicating limiting RuBP regeneration capacity. These low RuBP pools resulted in increased RuBP regeneration via faster RuBP turnover, but only in CF air and during vegetative and flowering stages at elevated CO₂. Also, the low RuBP pool sizes did not always reflect RuBP consumption rates or the RuBP regeneration limitation relative to potential carboxylation (%RuBP). Rather, %RuBP increased linearly with decrease in the RuBP pool turnover time. These data suggest that amelioration of damage from O₃ by elevated atmospheric CO₂ to the RuBP regeneration may be in response to changes in the Rubisco carboxylation.     

C4 characteristics of photosynthesis in the C3 alga Hydrodictyon africanum

Abstract Results obtained with Hydrodictyon africanum, and data from the literature, show that most green algae of the chlorophyte type (e.g. Chlorella, Chlamydomonas, Hydrodictyon) differ in their photosynthetic C fixation characteristics from most green algae of the charophyte type (e.g. Spirogyra, Chara) and from C₃ higher plants. The chlorophyte algae fix inorganic carbon by the photosynthetic carbon reduction cycle pathway, but have a low CO₂ compensation point in 250 μM O₂, a low inhibition of CO₂ fixation from 10 μM CO₂/250 μM O₂ when compared with 10 μM CO₂/zero O₂, and a low half-saturation constant for CO₂. These three characteristics are different from those of charophytes and C₃ higher plants, and resemble those of C₄ higher plants. It is suggested that these characteristics of chlorophyte algae are the result of a ‘CO₂ concentrating mechanism’ which increases the CO₂/O₂ ratio at the site of ribulose bisphosphate carboxylase-oxygenase action in a similar way to that achieved by the C₄?C₃ acid cycle in C₄ plants. In the chlorophyte algae, however, CO₂ concentration probably involves active HCO₃^? transport at the inner membrane of the chloroplast envelope. Active HCO₃^? transport can occur at the plasmalemma of charophyte algae and submerged aquatic higher plants as well as chlorophyte algae, so it is unlikely to explain the differences between the two groups of aquatic green plants. Differences in the properties of ribulose bisphosphate carboxylase-oxygenase, and differences in CO₂ production in the light, also seem inadequate to account for the different photosynthetic characteristics. The chlorophyte type of ‘C0₂ concentrating mechanism’ appears to be common in other classes of eukaryotic algae, and in cyanophytes. Some of the ‘advanced’ members of these eukaryotic algal classes (including the chlorophytes) may lack the mechanism, while some ‘primitive’ charophytes may retain the mechanism which their ancestors presumably possessed.     

Interannual climatic variation mediates elevated CO2 and O3 effects on forest growth

We analyzed growth data from model aspen (Populus tremuloides Michx.) forest ecosystems grown in elevated atmospheric carbon dioxide ([CO₂]; 518 μL L^?1) and ozone concentrations ([O₃]; 1.5 × background of 30–40 nL L^?1 during daylight hours) for 7 years using free‐air CO₂ enrichment technology to determine how interannual variability in present‐day climate might affect growth responses to either gas. We also tested whether growth effects of those gasses were sustained over time. Elevated [CO₂] increased tree heights, diameters, and main stem volumes by 11%, 16%, and 20%, respectively, whereas elevated ozone [O₃] decreased them by 11%, 8%, and 29%, respectively. Responses similar to these were found for stand volume and basal area. There were no growth responses to the combination of elevated [CO₂+O₃]. The elevated [CO₂] growth stimulation was found to be decreasing, but relative growth rates varied considerably from year to year. Neither the variation in annual relative growth rates nor the apparent decline in CO₂ growth response could be explained in terms of nitrogen or water limitations. Instead, growth responses to elevated [CO₂] and [O₃] interacted strongly with present‐day interannual variability in climatic conditions. The amount of photosynthetically active radiation and temperature during specific times of the year coinciding with growth phenology explained 20–63% of the annual variation in growth response to elevated [CO₂] and [O₃]. Years with higher photosynthetic photon flux (PPF) during the month of July resulted in more positive growth responses to elevated [CO₂] and more negative growth responses to elevated [O₃]. Mean daily temperatures during the month of October affected growth in a similar fashion the following year. These results indicate that a several‐year trend of increasingly cloudy summers and cool autumns were responsible for the decrease in CO₂ growth response.     

Belowground competition and the response of developing forest communities to atmospheric CO2 and O3

As human activity continues to increase CO₂ and O₃, broad expanses of north temperate forests will be simultaneously exposed to elevated concentrations of these trace gases. Although both CO₂ and O₃ are potent modifiers of plant growth, we do not understand the extent to which they alter competition for limiting soil nutrients, like nitrogen (N). We quantified the acquisition of soil N in two 8‐year‐old communities composed of trembling aspen genotypes (n= 5) and trembling aspen–paper birch which were exposed to factorial combinations of CO₂ (ambient and 560 μL L⁻¹) and O₃ (ambient = 30–40 vs. 50–60 nL L⁻¹). Tracer amount of ¹⁵NH₄⁺ were applied to soil to determine how these trace gases altered the competitive ability of genotypes and species to acquire soil N. One year after isotope addition, we assessed N acquisition by measuring the amount of ¹⁵N tracer contained in the plant canopy (i.e. recent N acquisition), as well as the total amount of canopy N (i.e. cumulative N acquisition). Exposure to elevated CO₂ differentially altered recent and cumulative N acquisition among aspen genotypes, changing the rank order in which they obtained soil N. Elevated O₃ also altered the rank order in which aspen genotypes obtained soil N by eliciting increases, decreases and no response among genotypes. If aspen genotypes respond similarly under field conditions, then rising concentrations of CO₂ and O₃ could alter the structure of aspen populations. In the aspen–birch community, elevated CO₂ increased recent N (i.e. ¹⁵N) acquisition in birch (68%) to a greater extent than aspen (19%), suggesting that, over the course of this experiment, birch had gained a competitive advantage over aspen. The response of genotypes and species to rising CO₂ and O₃ concentrations, and how these responses are modified by competitive interactions, has the potential to change the future composition and productivity of northern temperate forests.     

Lack of C4 photosynthetic metabolism in ears of C3 cereals

There is continuing controversy over whether a degree of C₄ photosynthetic metabolism exists in ears of C₃ cereals. In this context, CO₂ exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat (Triticum durum Desf.) and two six-rowed barley (Hordeum vulgare L.) cultivars. Three weeks after anthesis, the CO₂ compensation concentration at 210 mmol mol^?1 O₂ in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O₂ dependence of the CO₂ compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C₃ photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with ¹⁴CO₂ during a 10s pulse, and the initial products of fixation were studied in various parts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade. All tissues assimilated CO₂ mainly by the Calvin (C₃) cycle, with little fixation of ¹⁴CO₂ into the C₄ acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C₃ cereals C₄ photosynthetic metabolism is nil.     

Oxygen sensitivity of C4 photosynthesis: evidence from gas exchange and chlorophyll fluorescence analyses with different C4 subtypes

Because photosynthetic rates in C₄ plants are the same at normal levels of O₂ (c, 20 kPa) and at c, 2 kPa O₂ (a conventional test for evaluating photorespiration in C₃ plants) it has been thought that C₄ photosynthesis is O₂ insensitive. However, we have found a dual effect of O₂ on the net rate of CO₂ assimilation among species representing all three C₄ subtypes from both monocots and dicots. The optimum O₂ partial pressure for C₄ photosynthesis at 30 °C, atmospheric CO₂ level, and half full sunlight (1000 μmol quanta m^?2 s^?1) was about 5–10 kPa. Photosynthesis was inhibited by O₂ below or above the optimum partial pressure. Decreasing CO₂ levels from ambient levels (32.6 Pa) to 9.3 Pa caused a substantial increase in the degree of inhibition of photosynthesis by supra-optimum levels of O₂ and a large decrease in the ratio of quantum yield of CO₂ fixation/quantum yield of photosystem II (PSII) measured by chlorophyll a fluorescence. Photosystem II activity, measured from chlorophyll a fluorescence analysis, was not inhibited at levels of O₂ that were above the optimum for CO₂ assimilation, which is consistent with a compensating, alternative electron How as net CO₂ assimilation is inhibited. At suboptimum levels of O₂, however, the inhibition of photosynthesis was paralleled by an inhibition of PSII quantum yield, increased state of reduction of quinone A, and decreased efficiency of open PSII centres. These results with different C₄ types suggest that inhibition of net CO₂ assimilation with increasing O₂ partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O₂ may be caused by a reduced supply of ATP to the C₄ cycle as a result of inhibition of its production photochemically.     

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C₄ species can be stimulated by increasing CO₂ concentration, [CO₂], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO₂ leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO₂]. Three C₄ species that differed in the reported degree of bundle sheath leakiness to CO₂, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO₂] of 350 μl l^?1 (ambient) or 700 μl l^?1 (elevated). Assimilation as a function of increasing [CO₂] at high photosynthetic photon flux density (PPFD, 1 600 μmol m^?2 s^?1) indicated that leaf photosynthesis was not saturated under current ambient [CO₂] for any of the three C₄ species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO₂] was light dependent for all three C₄ species. The stimulation of leaf photosynthesis at elevated [CO₂] was not associated with previously published values of CO₂ leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO₂] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C₄ species can respond directly to increased [CO₂] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.     

Effects of elevated CO2 and/or O3 on growth, development and physiology of wheat (Triticum aestivum L.)

Two cultivars of spring wheat (Triticum aestivum L. cvs. Alexandria and Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO₂ [ambient (355 pmol mol^?1) and elevated (708 μmol mol^?1)] under two O₃ regimes [clean air (< 5 nmol mol^?1 O₃) and polluted air (15 nmol mol^?1 O₃ at night rising to a midday maximum of 75 nmol mol^?1)] in a phytotron at the University of Newcastle-upon-Tyne. Between the two-leaf stage and anthesis, measurements of leaf gas-exchange, non-structural carbohydrate content, visible O₃ damage, growth, dry matter partitioning, yield components and root development were made in order to examine responses to elevated CO₂ and/or O₃. Growth at elevated CO₂ resulted in a sustained increase in the rate of CO₂ assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosynthetic rate (c.-15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO₂ assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO₂ enrichment stimulated shoot and root growth to an equivalent extent, and increased tillering and yield components, however, non-structural carbohydrates still accumulated in source leaves. In contrast, long-term exposure to O₃ resulted in a decreased CO₂ assimilation rate (c. -13%), partial stomatal closure, and the accumulation of fructan and starch in leaves in the light. These effects were manifested in decreased rates of shoot and root growth, with root growth more severely affected than shoot growth. In the combined treatment growth of O₃-treated plants was enhanced by elevated CO₂, but there was little evidence that CO₂ enrichment afforded additional protection against O₃ damage. The reduction in growth induced by O₃ at elevated CO₂ was similar to that induced by O₃ at ambient CO₂ despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O₃ flux by 20%, and also CO₂-induced increases in the provision of substrates for detoxification and repair processes. These observations suggest that CO₂ enrichment may render plants more susceptible to O₃ damage at the cellular level. Possible mechanisms are discussed.     

Soil nitrogen transformations under Populus tremuloides, Betula papyrifera and Acer saccharum following 3 years exposure to elevated CO2 and O3

Increases in atmospheric CO₂ and tropospheric O₃ may affect forest N cycling by altering plant litter production and the availability of substrates for microbial metabolism. Three years following the establishment of our free‐air CO₂–O₃ enrichment experiment, plant growth has been stimulated by elevated CO₂ resulting in greater substrate input to soil; elevated O₃ has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO₂, and that CO₂ effects would be dampened by O₃. We found that elevated CO₂ did not alter gross N transformation rates. Elevated O₃ significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO₂ and O₃: (i) gross N mineralization was greater under elevated CO₂ (1.0 mg N kg^?1 day^?1) than in the presence of both CO₂ and O₃ (0.5 mg N kg^?1 day^?1) and (ii) gross NH₄⁺ immobilization was also greater under elevated CO₂ (0.8 mg N kg^?1 day^?1) than under CO₂ plus O₃ (0.4 mg N kg^?1 day^?1). We used a laboratory ¹⁵N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO₂ led to greater recovery of NH₄⁺‐¹⁵N in microbial biomass and corresponding lower recovery in the extractable NO₃^? pool. Elevated CO₂ resulted in a substantial increase in NO₃^?‐¹⁵N recovery in soil organic matter. We observed no O₃ main effect and no CO₂ by O₃ interaction effect on ¹⁵N recovery in any soil pool. All of the above responses were most pronounced beneath Betula papyrifera and Populus tremuloides, which have grown more rapidly than Acer saccharum. Although elevated CO₂ has increased plant productivity, the resulting increase in plant litter production has yet to overcome the influence of the pre‐existing pool of soil organic matter on soil microbial activity and rates of N cycling. Ozone reduces plant litter inputs and also appears to affect the composition of plant litter in a way that reduces microbial biomass and activity.     

Interaction between atmospheric CO2 concentration and water deficit on gas exchange and crop growth: testing of ecosys with data from the Free Air CO2 Enrichment (FACE) experiment

Soil water deficits are likely to influence the response of crop growth and yield to changes in atmospheric CO₂ concentrations (C_a), but the extent of this influence is uncertain. To study the interaction of water deficits and C_a on crop growth, the ecosystem simulation model ecosys was tested with data for diurnal gas exchange and seasonal wheat growth measured during 1993 under high and low irrigation at C_a= 370 and 550 μmol mol^?1 in the Free Air CO₂ Enrichment (FACE) experiment near Phoenix, AZ. The model, supported by the data from canopy gas exchange enclosures, indicated that under high irrigation canopy conductance (g_c) at C_a= 550 μmol mol^?1 was reduced to about 0.75 that at C_a= 370 μmol mol^?1, but that under low irrigation, g_c was reduced less. Consequently when C_a was increased from 370 to 550 μmol mol^?1, canopy transpiration was reduced less, and net CO₂ fixation was increased more, under low irrigation than under high irrigation. The simulated effects of C_a and irrigation on diurnal gas exchange were also apparent on seasonal water use and grain yield. Simulated vs. measured seasonal water use by wheat under high irrigation was reduced by 6% vs. 4% at C_a= 550 vs. 370 μmol mol^?1 but that under low irrigation was increased by 3% vs. 5%. Simulated vs. measured grain yield of wheat under high irrigation was increased by 16% vs. 8%, but that under low irrigation was increased by 38% vs. 21%. In ecosys, the interaction between C_a and irrigation on diurnal gas exchange, and hence on seasonal crop growth and water use, was attributed to a convergence of simulated g_c towards common values under both C_a as canopy turgor declined. This convergence caused transpiration to decrease comparatively less, but CO₂ fixation to increase comparatively more, under high vs. low C_a. Convergence of g_c was in turn attributed to improved turgor maintenance under elevated C_a caused by greater storage C concentrations in the leaves, and by greater rooting density in the soil.     

Mass spectrometric monitoring of gases (CO2, CH4, O2) in a mesotrophic peat core from Kopparås Mire, Sweden

Membrane inlet mass spectrometry was used to monitor dissolved gas concentrations (CO₂, CH₄ and O₂) in a mesotrophic peat core from Kopparås, Sweden. 1 A comparison of depth profiles (down to 22 cm) with an ombrotrophic peat core (Ellergower, SW Scotland) investigated previously, revealed major differences in gas concentrations. Thus methane reached concentrations more than twice as high (800 μM) at depths greater than 12 cm in the Kopparås core. As shown previously, the primary determinant of the depth of the oxic zone is the level of the water table. Whereas in the Scottish cores, mass spectrometric detectability of O₂ was confined to the first 3 cm below this level, in the Swedish core penetration of O₂ was greater (7 cm). CO₂ profiles were similar in cores from both locations. 2 A thick layer of Sphagnum mosses dominated the plant cover of the Swedish peat core. A poorly developed deep root system, as distinct from that of the vascular plant cover in Scottish cores, diminished gas exchange rates, and presumably aerobic methane oxidation at depth around roots. These characteristics may contribute to the development of discontinuities in gas profiles at depths greater 15 cm as upward gas transport is established predominantly by diffusion and/or ebullition in the Swedish core. 3 Monitoring gas concentrations at the peat surface and at 2 cm depth after changing water tables showed a delayed response of approximately 4 days as a result of the high water content and moisture‐regulating capacity of mosses. 4 Recovery processes at 2 cm depth after raising the water table revealed final production rates of dissolved CO₂ and CH₄ in the peat pore water between 0.8 and 4.4 μmol h^?1 L^?1 and between 0.1 and 1.7 μmol h^?1 L^?1, respectively. Higher production rates were found during the day, indicating a diurnal rhythm due to plant photosynthetic activity even at the low values of photosynthetically active radiation (PAR: 110 μmol s^?1 m^?2) used in the experimental set‐up. 5 In the water‐logged mesotrophic Kopparås core changes of dissolved gas concentrations (DGC) at 3 and 14 cm depth were surface temperature‐dependent rather than light dependent. This suggests that changes of air temperature alters the covering vegetation to increase the conductivity for dissolved gases through vascular plants and to facilitate gas transport by diffusion and/or ebullition.     

Photophosphorylation in Scenedesmus in vivo: O2 evolution, ATP pools and transients, and phosphate binding during photoreduction of NO3−, NO2− and CO2

The relation between light-induced electron transport with NO₃^?, NO₂^? or CO₂ as acceptors, ATP pools and transients in dark-light-dark transitions, and phosphate uptake was examined in phosphorus-starved cells of Scenedesmus obtusiusculus Chod. Net O₂ evolution at saturating light was around 6 μmol × (mg chlorophyll × h)^?1 in the absence of any acceptor, but reached average rates of 21, 65 and 145 μmol × (mg chlorophyll × h)^?1 upon additions of 5 mM KNO₃, KNO₂ and KHCO₃, respectively. The apparent rate of photophosphorylation in transition experiments was only a few percent of the rate calculated from CO₂-dependent O₂ evolution. Blocking non-cyclic electron transport with DCMU inhibited phosphate assimilation, but acceleration of non-cyclic electron flow by addition of NO₃^? or NO₂^? did not stimulate phosphate assimilation as compared to the situation without an acceptor. A functional non-cyclic system might primarily be needed for an efficient shuttle transfer of ATP from the chloroplast to the cytoplasm. An inhibition of the non-cyclic system due to lack of reducible substrates accelerates the cyclic system and thus indicates a regulation mechanism between the two systems.     

CO2 uptake and fluorescence responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO2 concentrations

Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO₂ mol^?1 air showed a Crassulacean acid metabolism (CAM) pattern of CO₂ uptake, with 34% more total daily CO₂ uptake under the doubled CO₂ concentration and most of the increase occurring in the late afternoon. For both CO₂ concentrations, 90% of the maximal daily CO₂ uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m^?2 day^?1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO₂ uptake by doubling the CO₂ concentration was greater under the highest PPFD (30 mol m^?2 day^?1) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO₂ uptake under 370 μmol CO₂ mol^?1 was 25% of that under 740 μmol CO₂ mol^?1. The ratio of variable to maximal chlorophyll fluorescence (F_y/F_m) decreased as the PPFD was raised above 5 mol m^?2 day^?1, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F_v/F_m was higher under the doubled CO₂ concentration, indicating that the current CO₂ concentration was apparently limiting for photosynthesis. Thus net CO₂ uptake by the shade-tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO₂ concentration, especially under stressful environmental conditions.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Initial cultivation of a temperate-region soil immediately accelerates aggregate turnover and CO2 and N2O fluxes

The immediate effects of tillage on protected soil C and N pools and on trace gas emissions from soils at precultivation levels of native C remain largely unknown. We measured the response to cultivation of CO₂ and N₂O emissions and associated environmental factors in a previously uncultivated U.S. Midwest Alfisol with C concentrations that were indistinguishable from those in adjacent late successional forests on the same soil type (3.2%). Within 2 days of initial cultivation in 2002, tillage significantly (P=0.001, n=4) increased CO₂ fluxes from 91 to 196 mg CO₂‐C m^?2 h^?1 and within the first 30 days higher fluxes because of cultivation were responsible for losses of 85 g CO₂‐C m^?2. Additional daily C losses were sustained during a second and third year of cultivation of the same plots at rates of 1.9 and 1.0 g C m^?2 day^?1, respectively. Associated with the CO₂ responses were increased soil temperature, substantially reduced soil aggregate size (mean weight diameter decreased 35% within 60 days), and a reduction in the proportion of intraaggregate, physically protected light fraction organic matter. Nitrous oxide fluxes in cultivated plots increased 7.7‐fold in 2002, 3.1‐fold in 2003, and 6.7‐fold in 2004 and were associated with increased soil NO₃^? concentrations, which approached 15 μg N g^?1. Decreased plant N uptake immediately after tillage, plus increased mineralization rates and fivefold greater nitrifier enzyme activity, likely contributed to increased NO₃^? concentrations. Our results demonstrate that initial cultivation of a soil at precultivation levels of native soil C immediately destabilizes physical and microbial processes related to C and N retention in soils and accelerates trace gas fluxes. Policies designed to promote long‐term C sequestration may thus need to protect soils from even occasional cultivation in order to preserve sequestered C.