Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles

We tested a conceptual model describing the influence of elevated atmospheric CO₂ on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C₃) grown in an elevated CO₂ atmosphere often increase production and allocation to belowground structures. We predicted that greater belowground C inputs at elevated CO₂ should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO₂ concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO₂ with no open top chamber, ii) at ambient CO₂ in an open top chamber, and iii) at twice-ambient CO₂ in an open top chamber. Plants were fertilized with 4.5 g N m⁻² over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO₂ effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO₂ treatments. Rates of photosynthesis in plants grown at elevated CO₂ were significantly greater than those measured under ambient conditions. The number of roots, root length, and root length increment were also substantially greater at elevated CO₂. Total and belowground biomass were significantly greater at elevated CO₂. Under N-limited conditions, plants allocated 50–70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient treatments; there were no significant differences between labile C pools in the bulk soil of ambient and elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of plants grown at elevated CO₂ compared to that in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO₂ concentrations can have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

Effects of elevated atmospheric CO2 on protozoan abundance in soil planted with wheat and on decomposition of wheat roots

The effect of elevated CO₂ on growth of wheat plants (Triticum aestivum cv. Minaret) and soil protozoan and bacterial populations was investigated in soil pots placed in open top chambers fumigated with ambient air or air enriched with CO₂ (ambient + 320 l L^–1 CO₂). We harvested plants two times during the growing season and measured the biomass and the C and N content of roots and shoots. The soil was divided into bulk and rhizosphere soil and the number of bacteria (colony-forming units, CFU) and protozoa was determined. There was no effect of atmospheric CO₂ content on the number of bacteria, but the total number of bacterivorous protozoa was higher in pots from the elevated CO₂ treatment. This increase was mainly due to an increase in the number of protozoa in the bulk soil. Density of protozoa in the rhizosphere was not affected by elevated CO₂. This suggests that the increase in protozoan numbers was a result of a general increase in rhizodeposition, presumably caused by increased root production, and not to an increased root exudation per root mass. After harvest, soil from the two treatments was incubated with and without roots and the respiration rate was estimated at intervals for 200 days. During the first 55 days, the specific root induced respiration rate was not affected by the CO₂ level at which the plants had been grown, indicating that the quality of the easily decomposable components of the roots was not affected by CO₂ level.     

Up-regulation of sucrose phosphate synthase in rice grown under elevated CO2 and temperature

Rice (Oryza sativa L. cv. IR-30) was grown season-long in outdoor, controlled-environment chambers at 33 Pa CO₂ with day/night/paddy-water temperatures of 28/21/25 °C, and at 66 Pa CO₂ with five different day/night/paddy-water temperature regimes (25/18/21, 28/21/25, 31/24/28, 34/27/31 and 37/30/34 °C). Sucrose phosphate synthase (SPS) activities in leaf extracts at 21, 48 and 81 days after planting (DAP) were assayed under saturating and selective (limiting) conditions. Diel SPS activity data indicated that rice SPS was light regulated; with up to 2.2-fold higher rates during the day. Throughout the growth season, leaf SPS activities were up-regulated in the CO₂-enriched plants, averaging 20 and 12% higher than in ambient-CO₂ grown plants in selective and saturating assays, respectively. Similarly, SPS activities increased 2.4% for each 1 °C rise in growth temperature from 25 to 34 °C, but de creased 11.5% at 37 °C. Leaf sucrose content was higher, and mirrored SPS activity better, than starch, although starch was more responsive to CO₂ treatment. Leaf sucrose and starch contents were significantly higher throughout the season in plants at elevated CO₂, but the N content averaged 6.5% lower. Increasing growth temperatures from 25 to 37 °C caused a linear decrease (62%) in leaf starch content, but not in sucrose. Consequently, the starch:sucrose ratio declined with growth temperature. The data are consistent with the hypothesis that the up-regulation of leaf SPS may be an acclimation response of rice to optimize the utilization and export of organic-C with the increased rates of inorganic-C fixation in elevated CO₂ or temperature growth regimes.     

Photosynthesis of Cockspur [Echinochloa crus-galli (L.) Beauv.] at Sites of Naturally Elevated CO2 Concentration

High abundance of cockspur (Echinochloa crus-galli) at the geothermal carbon dioxide spring area in Staveinci indicates that this species is able to grow under widely varying CO₂ concentrations. Living cockspur plants can even be found very close to gas-releasing vents where growth is significantly reduced. Plant height correlated well with CO₂ exposure. The ¹³C value of the CO₂ spring air was –3.9 and ¹³C values of high-, medium-, and low-CO₂ plants were –10.14, –10.44, and –11.95 , respectively. Stomatal response directly followed the prevailing CO₂ concentrations, with the highest reduction of stomatal conductance in high CO₂ concentration grown plants. Analysis of the curves relating net photosynthetic rate to intercellular CO₂ concentration (P _N-C_i curves) revealed higher CO₂ compensation concentration in plants growing at higher CO₂ concentration. This indicates adjustment of respiration and photosynthetic carbon assimilation according to the prevailing CO₂ concentrations during germination and growth. There was no difference in other photosynthetic parameters measured.     

Ilyobacter delafieldii sp. nov., a metabolically restricted anaerobic bacterium fermenting PHB

Enrichments from an estuarine sediment with crotonate as substrate resulted in the isolation of a motile, gram-negative, obligately anaerobic rod with pointed ends, designated strain 10cr1. The organism was asporogenous, did not reduce sulfur, sulfate, thiosulfate, nitrate, oxygen or fumarate, and had a mol %G+C ratio of 29. Strain 10cr1 was able to ferment crotonate, 3-hydroxybutyrate, lactate, pyruvate, and poly--hydroxybutyric acid (PHB). Acetate, propionate, butyrate, CO₂ and H₂ were the fermentation products. When grown on PHB there was accumulation of 3-hydroxybutyrate once growth had ceased, indicating degradation of PHB to the monomer. The 3-hydroxybutyrate formed during growth of the culture was fermented to acetate, butyrate and H₂. Experimental evidence suggested the production of an extracellular PHB depolymerase. The cells were not attached to the PHB granules. This is the first isolation of an anaerobic bacterium capable of degrading exogenous PHB. This strain is described as a new species, Ilyobacter delafieldii sp. nov., and strain 10cr1 (=DSM 5704) is designated as the type (and at present, only) strain.Abbreviations G+C guanine plus cytosine - OD optical density - PHB poly--hydroxybutyric acid - specific growth rate - HPLC high-performance liquid chromatography - YE yeast extract     

The impact of elevated carbon dioxide on the phosphorus nutrition of plants: a review

Background Increasing attention is being focused on the influence of rapid increases in atmospheric CO₂ concentration on nutrient cycling in ecosystems. An understanding of how elevated CO₂ affects plant utilization and acquisition of phosphorus (P) will be critical for P management to maintain ecosystem sustainability in P-deficient regions.Scope This review focuses on the impact of elevated CO₂ on plant P demand, utilization in plants and P acquisition from soil. Several knowledge gaps on elevated CO₂-P associations are highlighted.Conclusions Significant increases in P demand by plants are likely to happen under elevated CO₂ due to the stimulation of photosynthesis, and subsequent growth responses. Elevated CO₂ alters P acquisition through changes in root morphology and increases in rooting depth. Moreover, the quantity and composition of root exudates are likely to change under elevated CO₂, due to the changes in carbon fluxes along the glycolytic pathway and the tricarboxylic acid cycle. As a consequence, these root exudates may lead to P mobilization by the chelation of P from sparingly soluble P complexes, by the alteration of the biochemical environment and by changes to microbial activity in the rhizosphere. Future research on chemical, molecular, microbiological and physiological aspects is needed to improve understanding of how elevated CO₂ might affect the use and acquisition of P by plants.     

Selection of soybean plant leaves which yield mesophyll cell isolates with maximal rates of CO2 and NO inf2 sup− photoassimilation

A problem often encountered when assaying mesophyll cell isolates prepared from mature soybean leaves, was that of poor reproducibility in rates of net ¹⁴CO₂ photoassimilation and NO₂ ^– photoreduction. It was known that soybean source leaves repeatedly displayed their most active net CO₂ photoassimilation in the period from attainment of maximal leaf area to approximately two to five days subsequent to that point. Advantage was taken of the fact that when soybean leaflets of each leaf reach their maximal area they also have reached their maximal leaf length from base to tip. This facilitates a more rapid determination of the point in time in which leaflet areas had reached A_max. Soybean plants (Glycine max cv. Williams) were propagated in the growth chamber with a 12 h light-12 h dark cycle, 25C, 65% RH, and 700 microeinsteins per meter squared per second. At 24 d post-emergence, the third leaf (numbered acropetally from the unifoliates) of each plant had just attained maximum leaflet areas (110 cm²) and lengths (13 cm). For this study, leaf mesophyll cells were enzymatically isolated, using commercially prepared pectinase, from leaflet sets of leaves selected from each of the second, third, and fourth leaf positions. Maximal rates of net ¹⁴CO₂ photoassimilation (with 5 mM HCO₃ ^–) for the second, third and fourth leaf (leaflet) isolates were, respectively, 27.0, 57.0, and 41.7 mol ¹⁴CO₂ assimilated per milligram chlorophyll per hour; simultaneously maximal rates of NO _inf2 ^sup– photoreduction (1 mM NO _inf2 ^sup– ) were, respectively, 4.4, 8.1, and 0.0 mol NO _inf2 ^sup– reduced per milligram chlorophyll per hour. These studies made it clear that in order repeatedly to attain reproducible maximal rates of leaf cell isolate net ¹⁴CO₂ photoassimilation and NO _inf2 ^sup– photoreduction, it always was necessary to select the newest, fully expanded leaves (e.g. leaf number 3) for cell isolation. Leaves from several plants only were pooled if they were excised from identically the same node on each of the plants.Abbreviations A_max - maximum leaflet (trifoliolate) area attained during ontogeny - CO₂ - CO₂ gas dissolved in solution - HCO _inf3 ^sup– - bicarbonate - L_max - maximum leaf blade length (midvein) attained during ontogeny - NiRase - chloroplast nitrite reductase (reduced ferredoxin) - NiPR - nitrite photoreduction - PE - post-emergence - Pn - net CO₂ photoassimilation (for leaflets and mesophyll cell isolates) - PPRC - pentose phosphate reductive cycle     

Induction of crassulacean acid metabolism in Mesembryanthemum crystallinum increases reproductive success under conditions of drought and salinity stress

Summary Mesembryanthemum crystallinum L., an inducible crassulacean acid metabolism (CAM) plant, was grown for approximately 5 weeks following germination in well-watered, non-saline soil in a controlled-environment chamber. During this time, plants were characterized by C₃ photosynthetic carbon metabolism. After the initial 5 weeks, CAM was induced by a combination of high soil salinity and reduced soil water content. One group of plants was allowed to engage in CAM by being continuously exposed to normal CO₂-containing air (about 350–400 ppm). A second group of plants was deprived of ambient CO₂ each night (12 h dark period) until completion of their life cycle, thereby minimizing potential carbon gain via dark CO₂ fixation. The capacity to express CAM under conditions of drought and salinity stress markedly improved reproductive success: plants kept in normal CO₂-containing air produced about 10 times more seeds than plants kept in CO₂-free air during dark periods. Seeds from plants deprived of ambient CO₂ overnight had more negative ¹³C values than seeds from plants kept in normal air.     

Fungal Inoculation and Elevated CO2 Mediate Growth of Lolium Mutiforum and Phytolacca Americana,Metal Uptake,and Metal Bioavailability in Metal-Contaminated Soil: Evidence from DGT Measurement

Fungal inoculation and elevated CO₂ may mediate plant growth and uptake of heavy metals, but little evidence from Diffusive Gradients in Thin-films (DGT) measurement has been obtained to characterize the process. Lolium mutiforum and Phytolacca americana were grown at ambient and elevated CO₂ on naturally Cd and Pb contaminated soils inoculated with and without Trichoderma asperellum strain C3 or Penicillium chrysogenum strain D4, to investigate plant growth, metal uptake, and metal bioavailability responses. Fungal inoculation increased plant biomass and shoot/root Cd and Pb concentrations. Elevated CO₂ significantly increased plants biomass, but decreased Cd and Pb concentrations in shoot/root to various extents, leading to a metal dilution phenomenon. Total Cd and Pb uptake by plants, and DGT-measured Cd and Pb concentrations in rhizosphere soils, were higher in all fungal inoculation and elevated CO₂ treatments than control treatments, with the combined treatments having more influence than either treatment alone. Metal dilution phenomenon occurred because the increase in DGT-measured bioavailable metal pools in plant rhizosphere due to elevated CO₂ was unable to match the increase in requirement for plant uptake of metals due to plant biomass increase.     

Changes of bacterial communities in the rhizosphere of sugarcane under elevated concentration of atmospheric CO2

It is believed that climate change will influence most of interactions that sustain life on Earth. Among these, the recruitment exerted by plants in their roots vicinity can change, leading to differential assemblages of microbiomes in the rhizosphere. We approached this issue analyzing the variations in the composition of bacterial communities in the rhizosphere of sugarcane cultivated under two concentrations of atmospheric CO₂ (350 or 700 ppm). In addition to the analysis of bacterial community, the use of DNA‐SIP allowed the comparison of bacterial groups assimilating roots exudates (based on ¹³C‐labeled DNA) in both conditions, in a period of 8 days after the CO₂ pulse. The separation of ¹³C‐DNA indicated the low but increasing frequency of labeling in the rhizosphere, as averages of 0.6, 2.4 and 5.0% of total DNA were labeled after 2, 4, and 8 days after the ¹³CO₂ pulse, respectively. Based on large‐scale sequencing of the V6 region in the gene 16S rRNA, we found an increase in the bacterial diversity in the ¹³C‐DNA along the sampling period. We also describe the occurrence of distinct bacterial groups assimilating roots exudates from sugarcane cultivated under each CO₂ concentration. Bacilli, Gammaproteobacteria, and Clostridia showed high affinity for the C‐sources released by sugarcane under 350 ppm of CO₂, while under elevated concentration of CO₂, the assimilation of roots exudates was prevalently made by members of Bacilli and Betaproteobacteria. The communities became more similar along time (4 and 8 days after CO₂ pulse), in both concentrations of CO₂, electing Actinobacteria, Sphingobacteriia, and Alphaproteobacteria as the major cross‐feeders on sugarcane exudates. In summary, we described the bacterial groups with higher affinity to assimilate roots exudates in the rhizosphere of sugarcane, and also demonstrated that the rhizosphere community can be differentially assembled in a future scenario with increased contents of CO₂.     

Time-dependent responses of soil CO2 efflux components to elevated atmospheric [CO2] and temperature in experimental forest mesocosms

We previously used dual stable isotope techniques to partition soil CO₂ efflux into three source components (rhizosphere respiration, litter decomposition, and soil organic matter (SOM) oxidation) using experimental chambers planted with Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings. The components responded differently to elevated CO₂ (ambient + 200 mol mol^–1) and elevated temperature (ambient + 4 °C) treatments during the first year. Rhizosphere respiration increased most under elevated CO₂, and SOM oxidation increased most under elevated temperature. However, many studies show that plants and soil processes can respond to altered climates in a transient way. Herein, we extend our analysis to 2 years to evaluate the stability of the responses of the source components. Total soil CO₂ efflux increased significantly under elevated CO₂ and elevated temperature in both years (1994 and 1995), but the enhancement was much less in 1995. Rhizosphere respiration increased less under elevated temperature in 1995 compared with 1994. Litter decomposition also tended to increase comparatively less in 1995 under elevated CO₂, but was unresponsive to elevated temperature between years. In contrast, SOM oxidation was similar under elevated CO₂ in the 2 years. Less SOM oxidation occurred under elevated temperature in 1995 compared with 1994. Our results indicate that temporal variations can occur in CO₂ production by the sources. The variations likely involve responses to antecedent physical disruption of the soil and physiological processes.     

Carbon fluxes in the rhizosphere of sweet chestnut seedlings (Castanea sativa) grown under two atmospheric CO2 concentrations: 14C partitioning after pulse labelling

Partitioning of ¹⁴C was assessed in sweet chestnut seedlings (Castanea sativa Mill.) grown in ambient and elevated atmospheric [CO₂] environments during two vegetative cycles. The seedlings were exposed to ¹⁴CO₂ atmosphere in both high and low [CO₂] environments for a 6-day pulse period under controlled laboratory conditions. Six days after exposure to ¹⁴CO₂, the plants were harvested, their dry mass and the radioactivity were evaluated. ¹⁴C concentration in plant tissues, root-soil system respiratory outputs and soil residues (rhizodeposition) were measured. Root production and rhizodeposition were increased in plants growing in elevated atmospheric [CO₂]. When measuring total respiration, i.e. CO₂ released from the root/soil system, it is difficult to separate CO₂ originating from roots and that coming from the rhizospheric microflora. For this reason a model accounting for kinetics of exudate mineralization was used to estimate respiration of rhizospheric microflora and roots separately. Root activity (respiration and exudation) was increased at the higher atmospheric CO₂ concentration. The proportion attributed to root respiration accounted for 70 to 90% of the total respiration. Microbial respiration was related to the amount of organic carbon available in the rhizosphere and showed a seasonal variation dependent upon the balance of root exudation and respiration. The increased carbon assimilated by plants grown under elevated atmospheric [CO₂] stayed equally distributed between these increased root activities. ei]H Lambers     

Influence of elevated CO2 and O3 on IAA, IAA oxidase and peroxidase in the leaves of ginkgo trees

This study examined the impacts of elevated CO₂ or O₃ on indole-3-acetic acid (IAA) content, activities of IAA oxidase (IAAO) and peroxidase (POD) in Ginkgo biloba leaves. Plants grown in open-top chambers were exposed to ambient atmosphere (control; C), elevated CO₂ and elevated O₃ from 1 June to 30 September. An increase in IAA content and decrease in IAAO and POD activities were observed in plants exposed to elevated CO₂ compared with C. Elevated O₃ had no significant effect on IAA content and IAAO activity, but increased POD activity during the early days. When trees pre-exposed to elevated CO₂ were transferred to elevated O₃ or C, the increase in IAAO activity resulted in the decrease in IAA content. When trees pre-exposed to elevated O₃ were transferred to elevated CO₂ or C, IAA content, IAAO and POD activities showed no significant changes. The influence of POD activity on the IAA activity was low.     

Temperature effects on the photosynthetic response of C3 plants to long-term CO2 enrichment

To assess the long-term effect of increased CO₂ and temperature on plants possessing the C₃ photosynthetic pathway, Chenopodium album plants were grown at one of three treatment conditions: (1) 23 °C mean day temperature and a mean ambient partial pressure of CO₂ equal to 350 bar; (2) 34 °C and 350 bar CO₂; and (3) 34 °C and 750 bar CO₂. No effect of the growth treatments was observed on the CO₂ reponse of photosynthesis, the temperature response of photosynthesis, the content of Ribulose-1,5-bisphosphate carboxylase (Rubisco), or the activity of whole chain electron transport when measurements were made under identical conditions. This indicated a lack of photosynthetic acclimation in C. album to the range of temperature and CO₂ used in the growth treatments. Plants from every treatment exhibited similar interactions between temperature and CO₂ on photosynthetic activity. At low CO₂ (< 300 bar), an increase in temperature from 25 to 35 °C was inhibitory for photosynthesis, while at elevated CO₂ (> 400 bar), the same increase in temperature enhanced photosynthesis by up to 40%. In turn, the stimulation of photosynthesis by CO₂ enrichment increased as temperature increased. Rubisco capacity was the primary limitation on photosynthetic activity at low CO₂ (195 bar). As a consequence, the temperature response of A was relatively flat, reflecting a low temperature response of Rubisco at CO₂ levels below its k_m for CO₂. At elevated CO₂ (750 bar), the temperature response of electron transport appeared to control the temperature dependency of photosynthesis above 18 °C. These results indicate that increasing CO₂ and temperature could substantially enhance the carbon gain potential in tropical and subtropical habitats, unless feedbacks at the whole plant or ecosystem level limit the long-term response of photosynthesis to an increase in CO₂ and temperature.Abbreviations A net CO₂ assimilation rate - C _a ambient partial pressure of CO₂ - C _i intercellular partial pressure of CO₂ - Rubisco Ribulose-1,5-bisphosphate carboxylase - VPD vapor pressure difference between leaf and air     

Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2 for 8 years

Alterations in microbial mineralization and nutrient cycling may control the long-term response of ecosystems to elevated CO₂. Because micro-organisms constitute a labile fraction of potentially available N and are regulators of decomposition, an understanding of microbial activity and microbial biomass is crucial. Tallgrass prairie was exposed to twice ambient CO₂ for 8 years beginning in 1989. Starting in 1991 and ending in 1996, soil samples from 0 to 5 and 5 to 15 cm depths were taken for measurement of microbial biomass C and N, total C and N, microbial activity, inorganic N and soil water content. Because of increased water-use-efficiency by plants, soil water content was consistently and significantly greater in elevated CO₂ compared to ambient treatments. Soil microbial biomass C and N tended to be greater under elevated CO₂ than ambient CO₂ in the 5–15 cm depth during most years, and in the month of October, when analyzed over the entire study period. Microbial activity was significantly greater at both depths in elevated CO₂ than ambient conditions for most years. During dry periods, the greater water content of the surface 5 cm soil in the elevated CO₂ treatments increased microbial activity relative to the ambient CO₂ conditions. The increase in microbial activity under elevated CO₂ in the 5–15 cm layer was not correlated with differences in soil water contents, but may have been related to increases in soil C inputs from enhanced root growth and possibly greater root exudation. Total soil C and N in the surface 15 cm were, after 8 years, significantly greater under elevated CO₂ than ambient CO₂. Our results suggest that decomposition is enhanced under elevated CO₂ compared with ambient CO₂, but that inputs of C are greater than the decomposition rates. Soil C sequestration in tallgrass prairie and other drought-prone grassland systems is, therefore, considered plausible as atmospheric CO₂ increases. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon Balance of a Winter Wheat-Root Microbiota System Under Elevated CO2

We examined the carbon budget of young winter wheat plants and their associated microorganisms as affected by a doubling of the atmospheric CO₂ concentration (700 μmol mol^-1). Plants were grown hydroponically in pre-sterilised sand at a controlled irradiance and temperature regime. Net photosynthesis (P_N) and respiration (R_D) rates of roots and shoots were measured continuously, plant growth and carbon distribution in the plant-root medium-associated microorganism system were determined destructively in interval-based analyses. P_N in elevated CO₂ grown plants (EC) was 123% of that in the control (AC) plants when averaged over the whole life span (39-d-old plants, 34 d in EC), but the percentage varied with the developmental stage being 115, 88, and 167% in the pretillering, tillering, and posttillering phase, respectively. There was a transient depression of P_N, higher amplitude of day/night fluctuations of the chloroplast starch content, and depression of carbon content in rhizosphere of EC plants during the period of tillering. After 34 d in EC, carbon content in shoots, roots, and in rhizodepositions was enhanced by the factors 1.05, 1.28, and 1.96, respectively. Carbon partitioning between above and belowground biomass was not affected by EC, however, proportionally more C in the belowground partitioning was allocated into the root biomass. Carbon flow from roots to rhizodepositions and rhizosphere microflora was proportional to P_N; its fraction in daily assimilated carbon decreased from young (17%) to order (3-4%) plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Production of poly-β-hydroxybutyrate by thermophilic cyanobacterium, Synechococcus sp. MA19, under phosphate-limited conditions

Synechococcus sp. MA19, grown autotrophically under phosphate-limited conditions at 50 °C, produced poly--hydroxybutyrate (PHB) when intracellular phosphate content was 0.043–0.076mmol per g of cellular components. In the culture for 260h using Ca₃(PO₄)₂ as a phosphate source, strain MA19 accumulated PHB at 55% (w/w) of the dry cells and the amount of PHB produced was 2.4gl^–1 which was almost twice that without Ca₃(PO₄)₂ addition.     

Contrasting leaf and ‘ecosystem’ CO2 and H2O exchange in Avena fatua monoculture: Growth at ambient and elevated CO2

Elevated CO₂ (ambient + 35 Pa) increased shoot dry mass production in Avena fatua by 68% at maturity. This increase in shoot biomass was paralleled by an 81% increase in average net CO₂ uptake (A) per unit of leaf area and a 65% increase in average A at the ecosystem level per unit of ground area. Elevated CO₂ also increased ecosystem A per unit of biomass. However, the products of total leaf area and light-saturated leaf A divided by the ground surface area over time appeared to lie on a single response curve for both CO₂ treatments. The approximate slope of the response suggests that the integrated light saturated capacity for leaf photosynthesis is 10-fold greater than the ecosystem rate. Ecosystem respiration (night) per unit of ground area, which includes soil and plant respiration, ranged from-20 (at day 19) to-18 (at day 40) mol m^-2 s^-1 for both elevated and ambient CO₂ Avena. Ecosystem below-ground respiration at the time of seedling emergence was -10 mol m^-2 s^-1, while that occuring after shoot removal at the termination of the experiment ranged from -5 to-6 mol m^-2 s^-1. Hence, no significant differences between elevated and ambient CO₂ treatments were found in any respiration measure on a ground area basis, though ecosystem respiration on a shoot biomass basis was clearly reduced by elevated CO₂. Significant differences existed between leaf and ecosystem water flux. In general, leaf transpiration (E) decreased over the course of the experiment, possibly in response to leaf aging, while ecosystem rates of evapotranspiration (ET) remained constant, probably because falling leaf rates were offset by an increasing total leaf biomass. Transpiration was lower in plants grown at elevated CO₂, though variation was high because of variability in leaf age and ambient light conditions and differences were not significant. In contrast, ecosystem evapotranspiration (ET) was significantly decreased by elevated CO₂ on 5 out of 8 measurement dates. Photosynthetic water use efficiencies (A/E at the leaf level, A/ET at the ecosystem level) were increased by elevated CO₂. Increases were due to both increased A at leaf and ecosystem level and decreased leaf E and ecosystem ET.     

Interactive effects of atmospheric CO2 enrichment,salinity and flooding on growth of C3 (Elymus athericus) and C4 (Spartina anglica) salt marsh species

The growth response of Dutch salt marsh species (C₃ and C₄) to atmospheric CO₂ enrichment was investigated. Tillers of the C₃ speciesElymus athericus were grown in combinations of 380 and 720 11^-1 CO₂ and low (O) and high (300 mM NaCl) soil salinity. CO₂ enrichment increased dry matter production and leaf area development while both parameters were reduced at high salinity. The relative growth response to CO₂ enrichment was higher under saline conditions. Growth increase at elevated CO₂ was higher after 34 than 71 days. A lower response to CO₂ enrichment after 71 days was associated with a decreased specific leaf area (SLA). In two other experiments the effect of CO₂ (380 and 720 11^-1) on growth of the C₄ speciesSpartina anglica was studied. In the first experiment total plant dry weight was reduced by 20% at elevated CO₂. SLA also decreased at high CO₂. The effect of elevated CO₂ was also studied in combination with soil salinity (50 and 400 mM NaCl) and flooding. Again plant weight was reduced (10%) at elevated CO₂, except under the combined treatment high salinity/non-flooded. But these effects were not significant. High salinity reduced total plant weight while flooding had no effect. Causes of the salinity-dependent effect of CO₂ enrichment on growth and consequences of elevated CO₂ for competition between C₃ and C₄ species are discussed.     

Stimulation of microbial extracellular enzyme activities by elevated CO2 depends on soil aggregate size

Increased belowground carbon (C) transfer by plant roots at elevated CO₂ may change properties of the microbial community in the rhizosphere. Previous investigations that focused on total soil organic C or total microbial C showed contrasting results: small increase, small decrease or no changes. We evaluated the effect of 5 years of elevated CO₂ (550 ppm) on four extracellular enzymes: β‐glucosidase, chitinase, phosphatase, and sulfatase. We expected microorganisms to be differently localized in aggregates of various sizes and, therefore analyzed microbial biomass (C_mic by SIR) and enzyme activities in three aggregate‐size classes: large macro‐ (> 2 mm), small macro‐ (0.25–2 mm), and microaggregates (< 0.25 mm). To estimate the potential enzyme production, we activated microorganisms by substrate (glucose and nutrients) amendment. Although C_total and C_mic as well as the activities of β‐glucosidase, phosphatase, and sulfatase were unaffected in bulk soil and in aggregate‐size classes by elevated CO₂, significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO₂ were 1.2–1.9‐fold higher than under ambient CO₂. This indicates the increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO₂. Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO₂ revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungal hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO₂ affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools.