Symbiotic N2 fixation in a high Alpine grassland: effects of four growing seasons of elevated CO2

1. Increasing carbon dioxide concentration (E: 680 μl CO₂ litre^–1 vs ambient, A: 355 μl CO₂ litre^–1) around late-successional Alpine sedge communities of the Swiss Central Alps (2450 m) for four growing seasons (1992–1995) had no detectable effect on symbiotic N₂ fixation in Trifolium alpinum —the sole N₂-fixing plant species in these communities (74 ± 30 mg N m^–2 year^–1, A and E plots pooled).
2. This result is based on data collected in the fourth growing season showing that elevated CO₂ had no effect on Trifolium above-ground biomass (4·4 ± 1·7 g m^–2, A and E plots pooled, n = 24) or N content per unit land area (124 ± 51 mg N m^–2, A and E pooled), or on the percentage of N Trifolium derived from the atmosphere through symbiotic N₂ fixation (%Ndfa: 61·0 ± 4·1 across A and E plots) estimated using the ¹⁵N dilution method.
3. Thus, it appears that N inputs to this ecosystem via symbiotic N₂ fixation will not be dramatically affected in the foreseeable future even as atmospheric CO₂ continues to rise.     

Comparison of gas use efficiency and treatment uniformity in a forest ecosystem exposed to elevated [CO2] using pure and prediluted free-air CO2 enrichment technology

A direct comparison of treatment uniformity and CO₂ use of pure and prediluted free-air CO₂ enrichment (FACE) systems was conducted in a forest ecosystem. A vertical release pure CO₂ fumigation system was superimposed on an existing prediluted CO₂ fumigation system and operated on alternate days. The FACE system using prediluted CO₂ fumigation technology exhibited less temporal and spatial variability than the pure CO₂ fumigation system. The pure CO₂ fumigation system tended to over-fumigate the upwind portions of the plot and used 25% more CO₂ than the prediluted CO₂ fumigation system. The increased CO₂ use by the pure CO₂ system was exacerbated at low wind speeds. It is not clear if this phenomenon will also be observed in plots with smaller diameters and low-stature vegetation.     

Carbon gains by desiccation-tolerant plants at elevated CO2

1. There have been no reports of the long-term responses of the desiccation-tolerant (DT) plants to elevated CO₂. Xerophyta scabrida is a DT woody shrub, which loses chlorophylls and thylakoids during desiccation: a so-called poikilochlorophyllous desiccation-tolerant species (PDT). When the leaves of X. scabria are allowed to desiccate, the species shows many of the normal features of (P)DT plants.
2. However, the duration of photosynthesis in X. scabria is prolonged by 300% when the measurements are made at 700 as opposed to 350p.p.m. CO₂. The implication is that the carboxylating enzymes must still have been active at this time to enable appreciable photosynthetic activity. This response could have far-reaching implications for the success of such species in a future climate.
3. Lichens and mosses, representing the homoiochlorophyllous DTs (HDT), retain their chlorophyll content and photosynthetic apparatus during desiccation. We show the desiccation responses of two common HDT species ( Cladonia convoluta and Tortula ruralis ) to elevated CO₂ for comparison. Both HDT species showed increased net CO₂ uptake in the material grown at high CO₂ by more than 30% in moss and by more than 50% in lichen. It is concluded that desiccation-tolerant plants will be among the main beneficiaries of a high CO₂ future.     

Carbon and water fluxes in a calcareous grassland under elevated CO2

1. As part of a long-term study of the effects of elevated CO₂ on biodiversity and ecosystem function in a calcareous grassland, we measured ecosystem carbon dioxide and water-vapour fluxes over 24-h periods during the 1994 and 1995 growing seasons. Data were used to derive CO₂ and H₂O gas-exchange response functions to quantum flux density (QFD).
2. The relative increase in net ecosystem CO₂ flux (NEC) owing to CO₂ enrichment increased as QFD rose. Daytime NEC at high QFD under elevated CO₂ increased by 25% to 60%, with the greatest increases in the spring and after mowing in June when above-ground biomass was lowest. There was much less stimulation of NEC in early June and again in October when the canopy was fully developed. Night-time NEC was not significantly altered under elevated CO₂.
3. Short-term reversal of CO₂ concentrations between treatments after two seasons of CO₂ exposure provided evidence for a 50% downward adjustment of NEC expressed per unit above-ground plant dry weight. However, when expressed on a land area basis, this difference disappeared because of a c. 20% increase in above-ground biomass under elevated CO₂.
4. Ecosystem evapotranspiration (ET) was not significantly altered by elevated CO₂ when averaged over all measurement dates and positions. However, ET was reduced 3–18% at high QFD in plots at the top of the slope at our study site. In summary, CO₂ enrichment resulted in a large stimulation of ecosystem CO₂ capture, especially during periods of a large demand of carbon in relationship to its supply, and resulted in a relatively small and variable effect on ecosystem water consumption.     

Growth, CO2 consumption and H2 production of Anabaena variabilis ATCC 29413-U under different irradiances and CO2 concentrations

Aims:  The objective of this study is to develop kinetic models based on batch experiments describing the growth, CO₂ consumption, and H₂ production of Anabaena variabilis ATCC 29413-U^TM as functions of irradiance and CO₂ concentration.
Methods and Results:  A parametric experimental study is performed for irradiances from 1120 to 16100 lux and for initial CO₂ mole fractions from 0·03 to 0·20 in argon at pH 7·0 ± 0·4 with nitrate in the medium. Kinetic models are successfully developed based on the Monod model and on a novel scaling analysis employing the CO₂ consumption half-time as the time scale.
Conclusions:  Monod models predict the growth, CO₂ consumption and O₂ production within 30%. Moreover, the CO₂ consumption half-time is an appropriate time scale for analysing all experimental data. In addition, the optimum initial CO₂ mole fraction is 0·05 for maximum growth and CO₂ consumption rates. Finally, the saturation irradiance is determined to be 5170 lux for CO₂ consumption and growth whereas, the maximum H₂ production rate occurs around 10 000 lux.
Significance and Impact of the Study:  The study presents kinetic models predicting the growth, CO₂ consumption and H₂ production of A. variabilis . The experimental and scaling analysis methods can be generalized to other micro-organisms.     

Wood composition and energy content in a poplar short rotation plantation on fertilized agricultural land in a future CO2 atmosphere

To study the influence of elevated CO₂ and nitrogen (N) fertilization on wood properties and energy, Populus × euramericana trees were exposed to ambient CO₂ (about 370 μmol mol⁻¹ CO₂) or elevated CO₂ (about 550 μmol mol⁻¹ CO₂) using Free Air CO₂ Enrichment (FACE) technology in combination with two N levels. Elevated CO₂ was maintained for 5 years. After three growing seasons, the plantation was coppiced, one half of each experimental plot was fertilized and secondary sprouts were harvested after two growing seasons. Fourier transform infrared (FT-IR) spectra of wood revealed significant effects of both elevated CO₂ and N fertilization on wood chemistry, in particular, significant increases in lignin and decreases in N content. These results were corroborated by chemical analysis. Neither elevated CO₂ nor N fertilization affected the calorific value of wood, which was 19.3 MJ kg⁻¹. N fertilization enhanced the energy production per land area by 16–69% because of higher aboveground woody biomass production than on nonfertilized land. Estimates indicate that high yielding poplar short rotation cultivation may significantly contribute as an alternative feedstock for energy production.     

Inorganic carbon limitation of photosynthesis in lake phytoplankton

1. Inorganic carbon availability influences species composition of phytoplankton in acidic and highly alkaline lakes, whereas the overall influence on community photosynthesis and growth is subject to debate.
2. The influence of total dissolved inorganic carbon (DIC) and free CO₂ on community photosynthesis was studied in six Danish lakes during the summer of 1995. The lakes were selected to ensure a wide range of chlorophyll a concentrations (1–120 μg l^–1), pH (5.6–9.6) and DIC concentration (0.02–2.5 m m ). Photosynthesis experiments were performed using the ¹⁴C technique in CO₂-manipulated water samples, either by changing the pH or by adding/removing CO₂.
3. Lake waters were naturally CO₂ supersaturated during most of the experimental period and inorganic carbon limitation of photosynthetic rates did not occur under ambient conditions. However, photosynthesis by phytoplankton in lakes with low and intermediate DIC concentrations was seriously restricted when CO₂ concentrations declined. Similarly, photosynthesis was limited by low CO₂ concentrations during phytoplankton blooms in the hardwater alkaline lakes.     

Free Air CO2 Enrichment of potato (Solanum tuberosum, L.): design and performance of the CO2-fumigation system

Free Air CO₂ Enrichment (FACE) systems are used to fumigate unconfined field plots with CO₂. As these installations can treat a sufficiently large area without interfering with natural climatic conditions, they are considered important tools for global change research worldwide. However, there is general consensus that elevated capital costs of existing FACE systems as well as high running costs may prevent their application at the required level of scale. A new and small FACE system that was designed to reduce both capital costs and CO₂ use, is described in this paper. Due to its intermediate size (8 m diameter) between the smaller Mini-FACE systems that were developed in Italy and the larger systems designed by the Brookhaven National Laboratory in the USA, it was named Mid-FACE. The Mid-FACE was at first developed as a prototype and then used to enrich field grown potato crops in a CO₂ concentration gradient experimental design. Technical details of a Mid-FACE prototype and of the operational set-up are presented in this paper together with performance data in terms of temporal and spatial control of CO₂ concentrations within the experimental area.     

Elevated atmospheric CO2 affects soil microbial diversity associated with trembling aspen

The effects of elevated atmospheric CO₂ (560 p.p.m.) and subsequent plant responses on the soil microbial community composition associated with trembling aspen was assessed through the classification of 6996 complete ribosomal DNA sequences amplified from the Rhinelander WI free-air CO₂ and O₃ enrichment (FACE) experiments microbial community metagenome. This in-depth comparative analysis provides an unprecedented, detailed and deep branching profile of population changes incurred as a response to this environmental perturbation. Total bacterial and eukaryotic abundance does not change; however, an increase in heterotrophic decomposers and ectomycorrhizal fungi is observed. Nitrate reducers of the domain bacteria and archaea, of the phylum Crenarchaea , potentially implicated in ammonium oxidation, significantly decreased with elevated CO₂. These changes in soil biota are evidence for altered interactions between trembling aspen and the microorganisms in its surrounding soil, and support the theory that greater plant detritus production under elevated CO₂ significantly alters soil microbial community composition.     

In situ responses to elevated CO2 in tropical forest understorey plants

1. Plants growing in deep shade and high temperature, such as in the understorey of humid tropical forests, have been predicted to be particularly sensitive to rising atmospheric CO₂. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariable. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pendula ) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius ) of elevated CO₂ treatment ( c. 700 μl litre^–1) under mean QFD of less than 11 μmol m^–2 s^–1, all species produced more biomass (25–76%) under elevated CO₂.
2. Total plant biomass tended to increase with microhabitat QFD (daytime means varying from 5 to 11μmol m^–2 s^–1) but the relative stimulation by elevated CO₂ was higher at low QFD except in Pharus .
3. Non-structural carbohydrate concentrations in leaves increased significantly in Pharus (+ 27%) and Tachigalia (+ 40%).
4. The data support the hypothesis that tropical plants growing near the photosynthetic light compensation point are responsive to elevated CO₂. An improved plant carbon balance in deep shade is likely to influence understorey plant recruitment and competition as atmospheric CO₂ continues to rise.     

Effects of raised CO2 on potential CH4 production and oxidation in, and CH4 emission from, a boreal mire

1 In a glasshouse experiment we studied the effect of raised CO₂ concentration (720 p.p.m.) on CH₄ emission at natural boreal peat temperatures using intact cores of boreal peat with living vascular plants and Sphagnum mosses. After the end of the growing season half of the cores were kept unnaturally warm (17–20 °C). The potential for CH₄ production and oxidation was measured at the end of the emission experiment.
2 The vascular cores ('Sedge') consisted of a moss layer with sedges, and the moss cores (' Sphagnum ') of Sphagnum mosses (some sedge seedlings were removed by cutting). Methane efflux was 6–12 times higher from the Sedge cores than from the Sphagnum cores. The release of CH ₄ from Sedge cores increased with increasing temperature of the peat and decreased with decreasing temperature. Methane efflux from Sphagnum cores was quite stable independent of the peat temperatures.
3 In both Sedge and Sphagnum samples, CO₂ treatment doubled the potential CH₄ production but had no effect on the potential CH₄ oxidation. A raised concentration of CO₂ increased CH₄ efflux weakly and only at the highest peat temperatures (17–20 °C).
4 The results suggest that in cool regions, such as boreal wetlands, temperature would restrict decomposition of the extra substrates probably derived from enhanced primary production of mire vegetation under raised CO₂ concentrations, and would thus retard any consequent increase in CH₄ emission.     

Affinity for CO2 in relation to the ability of freshwater macrophytes to use HCO–3

1. The affinity of photosynthesis for CO₂ is calculated here as the initial slope of net-photosynthetic rate against concentration of CO₂. The affinity for CO₂ for pairs of freshwater macrophytes with similar leaf morphology but able or unable to use HCO^–₃ as a carbon source was compared.
2. Species restricted to CO₂ had a higher affinity for CO₂ than species that were also able to use HCO^–₃ when rates were expressed on the basis of area, dry mass and content of chlorophyll a .
3. Published values for the affinity for CO₂ and the concentration of CO₂ which half-saturated rate of photosynthesis were compiled and compared. Despite a large range of values, affinity for CO₂ was greater for species restricted to CO₂ than for those also able to use HCO^–₃ and statistically different when the slope was expressed on the basis of dry mass and chlorophyll a content.
4. The difference in affinity is consistent with predicted benefits of a high permeability to CO₂ for species relying on passive diffusion of CO₂ and a lower permeability for species able to use HCO^–₃ in order to reduce efflux of CO₂ from a high internal concentration generated by active transport.
5. The implications of the different affinities are discussed in terms of species distribution.     

Effects of Free-Air CO2 Enrichment (FACE) on experimental grassland communities

1. Experimental grassland communities (turves) were exposed to elevated (60 Pa) and ambient (35 Pa) CO₂ partial pressures (pCO₂) in a Free-Air Carbon Dioxide Enrichment (FACE) experiment between 30 March 1995 and 4 July 1996. The vegetation was cut once during the experiment prior to the final harvest (harvest 2).
2. No significant treatment effects on total plant biomass at the whole turf level were detected, although biomass was typically about 25% higher under fumigation in year 1 and about 15% higher in year 2.
3. Biomass for two of the six sown species was significantly higher at harvest 2 than at harvest 1. There were no significant differences between individual species' biomass under the two CO₂ treatments at either harvest 1 or 2 or in terms of overall cumulative biomass. However, in four of the five sown species in both years biomass tended to be higher in the fumigated than in the control rings ( Cerastium holosteiodes, Phleum pratense, Plantago lanceolata and Poa trivialis ). In contrast, Lolium perenne showed increased biomass under the control treatment relative to the fumigated treatment in both years. Owing to the high variance both within and between rings for each of the two treatments the statistical power of most, but not all, of the analyses carried out was poor.
4. The relative proportions of each species in the turves under fumigated and control treatments was broadly similar after the first summer, with differences in the second year being mainly owing to the negative response of L. perenne to CO₂ fumigation.     

A controlled-environment chamber for measurement of canopy photosynthesis by small stands of lettuce (Lactuca sativa L.)

Abstract. A controlled-environment chamber constructed from a standard chest freezer was used to grow and measure the CO₂ exchange of small stands of lettuce ( Lactuca sativa L. ev. Ambassador). The chamber, with horizontal air flow, provided good control of air temperature ( c. 6 to 16°C), irradiance (0–300 μmol PAR m ²S¹ and CO₂ (350–1000 μmol mol⁻¹). The photosynthetic response to changes in these variables was measured using an inexpensive CO₂ dosing system which recorded the input rate required to maintain a constant concentration of CO₂ (to ± 2.5%). Characteristis of the growth environment and the changes in response to temperature and irradiance are described.     

CO2 fertilization in temperate FACE experiments not representative of boreal and tropical forests

Results from free-air CO₂ enrichment (FACE) experiments in temperate climates indicate that the response of forest net primary productivity (NPP) to elevated CO₂ might be highly conserved across a broad range of productivities. In this study, we show that the LPJ-GUESS dynamic vegetation model reproduces the magnitude of the NPP enhancement at temperate forest FACE experiments. A global application of the model suggests that the response found in the experiments might also be representative of the average response of forests globally. However, the predicted NPP enhancement in tropical forests is more than twice as high as in boreal forests, suggesting that currently available FACE results are not applicable to these ecosystems. The modeled geographic pattern is to a large extent driven by the temperature dependence of the relative affinities of the primary assimilation enzyme (Rubisco) for CO₂ and O₂.     

Carbon Dioxide Exchange Between the Atmosphere and an Alpine Shrubland Meadow During the Growing Season on the Qinghai-Tibetan Plateau

Abstract: In the present study, we used the eddy covariance method to measure CO₂ exchange between the atmosphere and an alpine shrubland meadow ecosystem (37°36'N, 101°18'E; 3 250 m a.s.l.) on the Qinghai-Tibetan Plateau, China, during the growing season in 2003, from 20 April to 30 September. This meadow is dominated by formations of Potentilla fruticosa L. The soil is Mol-Cryic Cambisols. During the study period, the meadow was not grazed. The maximum rates of CO₂ uptake and release derived from the diurnal course of CO₂ flux were -9.38 and 5.02 μmol·m^-2·s^-1, respectively. The largest daily CO₂ uptake was 1.7 g C·m^-2·d^-1 on 14 July, which is less than half that of an alpine Kobresia meadow ecosystem at similar latitudes. Daily CO₂ uptake during the measurement period indicated that the alpine shrubland meadow ecosystem may behave as a sink of atmospheric CO₂ during the growing season. The daytime CO₂ uptake was correlated exponentially or linearly with the daily photo synthetic photon flux density each month. The daytime average water use efficiency of the ecosystem was 6.47 mg CO₂/g H₂O. The efficiency of the ecosystem increased with a decrease in vapor pressure deficit.
(Managing editor: Ya-Qin HAN)     

Effect of enhanced atmospheric CO2 on mycorrhizal colonization and phosphorus inflow in 10 herbaceous species of contrasting growth strategies

1. Ten herbaceous species were grown over a 4-month period under ambient (360 μmol mol^–1) and elevated (610 μmol mol^–1) atmospheric CO₂ conditions. Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus mosseae and given a phosphorus (P) supply which was not immediately available to the plants.
2. Multiple harvests were taken in order to determine whether the effect of elevated CO₂ on mycorrhizal colonization and phosphorus inflow was independent of its effect on plant growth.
3. All species grew faster under elevated CO₂ and carbon partitioning was altered, generally in favour of the shoots. All species responded similarly to elevated CO₂.
4. Elevated CO₂ did not affect the percentage of root length colonized by AM fungi, but the total amount of colonized root length was increased, because the plants were bigger.
5. Elevated CO₂ increased total P content, but had little or no effect on P concentration. At a given age, P inflow was stimulated by elevated CO₂, but when root length was taken into account the CO₂ effect disappeared.
6. In these host species there is no evidence for a direct effect of elevated CO₂ on mycorrhizal functioning, because both internal mycorrhizal colonization and P inflow are unaffected.
7. Future research should concentrate on the potential for carbon flow to the soil via the external mycelial network.     

Yield of wheat across a subambient carbon dioxide gradient

Yields and yield components of two cultivars of day-neutral spring wheat ( Triticum aestivum L.) were assessed along a gradient of daytime carbon dioxide (CO₂) concentrations from about 200 to near 350 μmol CO₂ (mol air)^–1 in a 38 m-long controlled environment chamber. The range in CO₂ concentration studied approximates that of Earth's atmosphere since the last ice age. This 75% rise in CO₂ concentration increased grain yields more than 200% under well-watered conditions and by 80–150% when wheat was grown without additions of water during the last half of the 100-day growing season. The 27% increase in CO₂ from the pre-industrial level of 150 years ago (275 μmol mol^–1) to near the current concentration (350 μmol mol^–1) increased grain yields of 'Yaqui 54' and 'Seri M82' spring wheats by 55% and 53%, respectively, under well-watered conditions. Yield increased because of greater numbers of grains per spike, rather than heavier grains or numbers of spikes per plant. Water use increased little with CO₂ concentration, resulting in improved water use efficiency as CO₂ rose. Data suggest that rising CO₂ concentration contributed to the substantial increase in average wheat yields in the U.S. during recent decades.     

Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO₂ concentrations (350 and 550 μmol mol⁻¹). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root:shoot ratios) and within tissues:infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO₂. Elevated CO₂ stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO₂ on growth, with infected plants showing a greater response, such that elevated CO₂ partly alleviated the effects of the parasite on host growth. Elevated CO₂ did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO₂ but was unaffected by O. minor . There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO₂ in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO₂ on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO₂.     

Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2

1. We tested the hypothesis that the net partitioning of dry mass and dry mass:area relationships is unaltered when plants are grown at elevated atmospheric CO₂ concentrations.
2. The total dry mass of Dactylis glomerata, Bellis perennis and Trifolium repens was higher for plants in 700 compared to 350 μmol CO₂ mol^–1 when grown hydroponically in controlled-environment cabinets.
3. Shoot:root ratios were higher and leaf area ratios and specific leaf areas lower in all species grown at elevated CO₂. Leaf mass ratio was higher in plants of B. perennis and D. glomerata grown at elevated CO₂.
4. Whilst these data suggest that CO₂ alters the net partitioning of dry mass and dry mass:leaf area relationships, allometric comparisons of the components of dry mass and leaf area suggest at most a small effect of CO₂. CO₂ changed only two of a total of 12 allometric coefficients we calculated for the three species: ν relating shoot to root dry mass was higher in D. glomerata , whilst ν relating leaf area to total dry mass was lower in T. repens .
5. CO₂ alone has very little effect on partitioning when the size of the plant is taken into account.