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.     

干旱对土壤剖面不同深度土壤CO2通量的影响

由于全球气候变化,预计未来我国亚热带地区干旱频率和持续时间将会增加。森林土壤CO₂的释放是陆地生态系统碳循环的重要组成部分,然而,有关不同深度土壤CO₂通量对干旱响应的理解仍相当有限。选择武夷山针叶林（Coniferous Forest,CF）和常绿阔叶林（Evergreen Broadleaved Forest,EBF）为研究对象,于2014年6月至2015年12月,采用梯度法计算10、30 cm和50 cm深度各层土壤CO₂通量,探讨模拟干旱对其影响。结果表明：CF和EBF样地土壤CO₂浓度均随土壤深度的增加而升高。CF和EBF样地对照（CK）处理10 cm深度土壤CO₂生产量分别占总CO₂生产量的53.5%和55.7%,表明土壤CO₂生产量主要来源于浅层土壤,这可能与浅层土壤有高的有机碳含量及细根生物量主要分布区有关。干旱处理使CF和EBF样地不同深度土壤CO₂通量均显著减少。在两个样地土壤CO₂通量的温度敏感性（Q₁₀）值均随着土壤深度的增加而减少。干旱处理显著减少了CF样地浅层土壤的Q₁₀值（P=0.02）,对深层土壤影响不显著（30 cm：P=0.30;50 cm：P=0.23）;而在EBF样地干旱处理显著减少了深层土壤的Q₁₀值（30 cm：P=0.02;50 cm：P=0.01）,对浅层土壤影响不显著（P=0.32）。     

High precision measurements of atmospheric concentrations and plant exchange rates of carbonyl sulfide using mid‐IR quantum cascade laser

Linking measurements of carbonyl sulfide (COS) with those of carbon dioxide (CO₂) has a potential in providing a powerful tracer of gross CO₂ fluxes between the atmosphere and land plants, a critical element in understanding the response of the land biosphere to global change. A new application of online COS, CO₂ and water vapor measurements based on newly designed mid‐infrared (IR) dual quantum cascade laser (QCL) spectrometer measures COS and CO₂ (at 2056 cm^?1) and water vapor (at 2190 cm^?1), with detectors cooled thermoelectrically (at ?43 °C) or with liquid nitrogen (?197 °C) for improved precision. Using the cryogenic detectors with averaging time of 1 s, precision was 50 pmol mol^?1, 0.4 μmol mol^?1 and 0.01 mmol mol^?1 for COS, CO₂ and water vapor, respectively (14, 0.2 and 0.003, respectively, for 60 s averaging time). We measured COS concentrations in ambient air, and changes in the rates of COS, CO₂ and water vapor exchange of attached leaves in response to changes in light intensity and ambient COS concentrations. The results were consistent with those of nononline gas chromatography–mass spectrometry for COS and IR gas analyzer for CO₂ and water vapor, with a high linear correlation for a broad range of concentrations (R²= 0.85 for COS and R²= 1.00 for CO₂ and water vapor). The new methodology opens the way for lab and field explorations of COS fluxes as a powerful new tracer for CO₂ exchange in the land biosphere.     

The effects of elevated CO2 concentrations on the root growth of Lolium perenne and Trifolium repens grown in a FACE* system

Lolium perenne and Trifolium repens were grown in a Free Air CO₂ Enrichment (FACE) system at elevated (600 μimol mol-¹) and ambient (340 μmol mol-¹) carbon dioxide concentrations during a whole growing season. Using a root ingrowth bag technique the extent to which CO₂ enrichment influenced the growth of L, perenne and T. repens roots under two contrasting nutrient regimes was examined. Root ingrowth bags were inserted for a fixed time into the soil in order to trap roots. It was also possible to follow the mortality of roots in bags inserted for different time intervals. Root ingrowth of both L. perenne and T. repens increased under elevated CO₂ conditions. In L. perenne, root ingrowth decreased with increasing nutrient fertilizer level, but for T. repens the root ingrowth was not affected by the nutrient application rate. Besides biomass measurements, root length estimates were made for T, repens. These showed an increase under elevated CO₂ concentrations. Root decomposition appeared to decrease under elevated CO₂ concentrations. A possible explanation for this effect is the observed changes in tissue composition, such as the increase in the carbon: nitrogen ratio in roots of L. perenne at elevated CO₂ concentrations.     

Nitrous oxide emissions from grass swards during the eighth year of elevated atmospheric pCO2 (Swiss FACE)

Emissions of N₂O were measured during the growth season over a year from grass swards under ambient (360 μL L^?1) and elevated (600 μL L^?1) CO₂ partial pressures at the Free Air Carbon dioxide Enrichment (FACE) experiment, Eschikon, Switzerland. Measurements were made following high (56 g N m^?2 yr^?1) and low (14 g N m^?2 yr^?1) rates of fertilizer application, split over 5 re‐growth periods, to Lolium perenne, Trifolium repens and mixed Lolium/Trifolium swards. Elevated pCO₂ increased annual emissions of N₂O from the high fertilized Lolium and mixed Lolium/Trifolium swards resulting in increases in GWP (N₂O emissions) of 179 and 111 g CO₂ equivalents m^?2, respectively, compared with the GWP of ambient pCO₂ swards, but had no significant effect on annual emissions from Trifolium monoculture swards. The greater emissions from the high fertilized elevated pCO₂Lolium swards were attributed to greater below‐ground C allocation under elevated pCO₂ providing the energy for denitrification in the presence of excess mineral N. An annual emission of 959 mg N₂O‐N m^?2 yr^?1 (1.7% of fertilizer N applied) was measured from the high fertilized Lolium sward under elevated pCO₂. The magnitude of emissions varied throughout the year with 84% of the total emission from the elevated pCO₂Lolium swards measured during the first two re‐growths (April–June 2001). This was associated with higher rainfall and soil water contents at this time of year. Trends in emissions varied between the first two re‐growths (April–June 2001) and the third, fourth and fifth re‐growths (late June–October 2000), with available soil NO₃^? and rainfall explaining 70%, and soil water content explaining 72% of the variability in N₂O in these periods, respectively. Caution is therefore required when extrapolating from short‐term measurements to predict long‐term responses to global climate change. Our findings are of global significance as increases in atmospheric concentrations of CO₂ may, depending on sward composition and fertilizer management, increase greenhouse gas emissions of N₂O, thereby exacerbating the forcing effect of elevated CO₂ on global climate. Our results suggest that when applying high rates of N fertilizer to grassland systems, Trifolium repens swards, or a greater component of Trifolium in mixed swards, may minimize the negative effect of continued increasing atmospheric CO₂ concentrations on global warming.     

An oxygen-mediated positive feedback between elevated carbon dioxide and soil organic matter decomposition in a simulated anaerobic wetland

We examined the effects of elevated atmospheric CO₂ on soil carbon decomposition in an experimental anaerobic wetland system. Pots containing either bare C₄‐derived soil or the C₃ sedge Scirpus olneyi planted in C₄‐derived soil were incubated in greenhouse chambers at either ambient or twice‐ambient atmospheric CO₂. We measured CO₂ flux from each pot, quantified soil organic matter (SOM) mineralization using δ¹³C, and determined root and shoot biomass. SOM mineralization increased in response to elevated CO₂ by 83–218% (P<0.0001). In addition, soil redox potential was significantly and positively correlated with root biomass (P= 0.003). Our results (1) show that there is a positive feedback between elevated atmospheric CO₂ concentrations and wetland SOM decomposition and (2) suggest that this process is mediated by the release of oxygen from the roots of wetland plants. Because this feedback may occur in any wetland system, including peatlands, these results suggest a limitation on the size of the carbon sink presented by anaerobic wetland soils in a future elevated‐CO₂ atmosphere.     

Effects of elevated atmospheric CO2 on fine root production and activity in an intact temperate forest ecosystem

We investigated the effects of elevated atmospheric CO₂ concentrations (ambient + 200 ppm) on fine root production and soil carbon dynamics in a loblolly pine (Pinus taeda) forest subject to free‐air CO₂ enrichment (FACE) near Durham, NC (USA). Live fine root mass (LFR) showed less seasonal variation than dead fine root mass (DFR), which was correlated with seasonal changes in soil moisture and soil temperature. LFR mass increased significantly (by 86%) in the elevated CO₂ treatment, with an increment of 37 g(dry weight) m^?2 above the control plots after two years of CO₂ fumigation. There was no long‐term increment in DFR associated with elevated CO₂, but significant seasonal accumulations of DFR mass occurred during the summer of the second year of fumigation. Overall, root net primary production (RNPP) was not significantly different, but annual carbon inputs were 21.7 gC m^?2 y^?1 (68%) higher in the elevated CO₂ treatment compared to controls. Specific root respiration was not altered by the CO₂ treatment during most of the year; however, it was significantly higher by 21% and 13% in September 1997 and May 1998, respectively, in elevated CO₂. We did not find statistically significant differences in the C/N ratio of the root tissue, root decomposition or phosphatase activity in soil and roots associated with the treatment. Our data show that the early response of a loblolly pine forest ecosystem subject to CO₂ enrichment is an increase in its fine root population and a trend towards higher total RNPP after two years of CO₂ fumigation.     

Effects of atmospheric CO2 enrichment and root restriction on leaf gas exchange and growth of banana (Musa)

The effects of atmospheric CO₂ enrichment and root restriction on photosynthetic characteristics and growth of banana (Musa sp. AAA cv. Gros Michel) plants were investigated. Plants were grown aeroponically in root chambers in controlled environment glasshouse rooms at CO₂ concentrations of 350 or 1 000 μmol CO₂ mol^-1. At each CO₂ concentration, plants were grown in large (2001) root chambers that did not restrict root growth or in small (20 1) root chambers that restricted root growth. Plants grown at 350 μmol CO₂ mol^-1 generally had a higher carboxylation efficiency than plants grown at 1 000 μmol CO₂ mol^-1 although actual net CO₂ assimilation (A) was higher at the higher ambient CO₂ concentration due to increased intercellular CO₂ concentrations (C_i resulting from CO₂ enrichment. Thus, plants grown at 1 000 μmol CO₂ mol^-1 accumulated more leaf area and dry weight than plants grown at 350 μmol CO₂ mol^-1. Plants grown in the large root chambers were more photosynthetically efficient than plants grown in the small root chambers. At 350 μmol CO₂ mol^-1, leaf area and dry weights of plant organs were generally greater for plants in the large root chambers compared to those in the small root chambers. Atmospheric CO₂ enrichment may have compensated for the effects of root restriction on plant growth since at 1 000 μmol CO₂ mol^-1 there was generally no effect of root chamber size on plant dry weight.     

Effect of CO2, CO2 and Diamox on photosynthesis and photorespiration in Chlamydomonas reinhardtii (green alga) and Anacystis nidulans (Cyanobacterium, blue-green alga)

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO₂ uptake and the CO₂ compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO₂, but CO₂ insensitive in cultures bubbled with air. Anacystis, even when cultured in 5% CO₂, exhibited an CO₂ insensitive net photosynthesis. The CO₂ compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO₂ enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO₂ 1^?1; while Chlamydomonas grown in 5% CO₂ enriched air for 4 days showed a high, but temporary CO₂ compensation point (60 μl CO₂ 1^?1). After a two hour adaptation in low CO₂, a stable, low CO₂ compensation point was reached. It seems that photorespiration can only be detected by the methods used in this study when the algae are cultured in high CO₂, but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO₂ concentrations. When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO₂ (air), the cells behaved as if they had been grown in high CO₂. They showed an oxygen sensitive net photosynthesis and a high CO₂ compensation point. This indicates that carbonic anhydrase plays an important role in the regulation of a measurable photorespiration in Chlamydomonas. The results are discussed in relation to previous observations of photorespiration measured by enzyme assay, metabolic products and gas exchange properties.     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

Sediment respiration and lake trophic state are important predictors of large CO2 evasion from small boreal lakes

We show that sediment respiration is one of the key factors contributing to the high CO₂ supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (<100 km²) and 32 lakes with the highest total phosphorus (P_tot) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO₂ concentrations plotted against depth and time demonstrate strong CO₂ accumulation in hypolimnetic waters during the stratification periods. The relationship between O₂ departure from the saturation and CO₂ departure from the saturation was strong in the entire data set (r²=0.79, n=2 740, P<0.0001). CO₂ concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO₂ concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake‐area‐weighted annual CO₂ evasion based on our 177 randomly selected lakes and all Finnish lakes >100 km² ( Rantakari & Kortelainen, 2005 ) was 42 g C m^?2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO₂ evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.     

Fine root dynamics in a loblolly pine forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study

Efforts to characterize carbon (C) cycling among atmosphere, forest canopy, and soil C pools are hindered by poorly quantified fine root dynamics. We characterized the influence of free‐air‐CO₂‐enrichment (ambient +200 ppm) on fine roots for a period of 6 years (Autumn 1998 through Autumn 2004) in an 18‐year‐old loblolly pine (Pinus taeda) plantation near Durham, NC, USA using minirhizotrons. Root production and mortality were synchronous processes that peaked most years during spring and early summer. Seasonality of fine root production and mortality was not influenced by atmospheric CO₂ availability. Averaged over all 6 years of the study, CO₂ enrichment increased average fine root standing crop (+23%), annual root length production (+25%), and annual root length mortality (+36%). Larger increase in mortality compared with production with CO₂ enrichment is explained by shorter average fine root lifespans in elevated plots (500 days) compared with controls (574 days). The effects of CO₂‐enrichment on fine root proliferation tended to shift from shallow (0–15 cm) to deeper soil depths (15–30) with increasing duration of the study. Diameters of fine roots were initially increased by CO₂‐enrichment but this effect diminished over time. Averaged over 6 years, annual fine root NPP was estimated to be 163 g dw m^?2 yr^?1 in CO₂‐enriched plots and 130 g dw m^?2 yr^?1 in control plots (P= 0.13) corresponding to an average annual additional input of fine root biomass to soil of 33 g m^?2 yr^?1 in CO₂‐enriched plots. A lack of consistent CO₂× year effects suggest that the positive effects of CO₂ enrichment on fine root growth persisted 6 years following minirhizotron tube installation (8 years following initiation of the CO₂ fumigation). Although CO₂‐enrichment contributed to extra flow of C into soil in this experiment, the magnitude of the effect was small suggesting only modest potential for fine root processes to directly contribute to soil C storage in south‐eastern pine forests.     

Elevated pCO2 Affects C and N Metabolism in Wild Type and Transgenic Tobacco Exhibiting Altered C/N Balance in Metabolite Analysis

Abstract: Diurnal changes in starch, sugar and amino acid concentrations in source leaves, sink leaves and roots of tobacco plants were determined. In addition to wild type tobacco, transformed plants deficient in root nitrate reductase and exhibiting decreased rates of growth were employed. Further, the growth rates of tobacco plants were modulated by exposure to elevated pCO₂. From the diurnal alterations in metabolite concentrations, the daily turnover of starch and amino N was estimated in order to: (i) elucidate whether turnover rates can be related to growth rates, and (ii) identify individual amino compounds with the potential to indicate nitrogen fluxes and the C/N status of plants. Elevated pCO₂ increased growth rates and daily turnover of starch in both wild type and transformed plants, indicating enhanced rates of photosynthesis. In wild type plants, elevated pCO₂ increased the turnover of amino N, notably glutamine and alanine, in mature source leaves, indicating enhanced nitrate reduction. By contrast, amino N turnover in source leaves of transformed plants was not affected by elevated pCO₂, although nitrate reduction was presumably enhanced. Apparently, export of amino N was increased from the source leaves of transformed plants. This assumption was supported by a significantly increased turnover of amino N in young sink leaves compared to mature source leaves, indicating a preference for acropetal amino N allocation and import into the young leaves of the transformed plants. Further, elevated pCO₂ increased the allocation of leaf‐derived amino N to the roots of transformed plants. This led to increased levels of amino compounds during the entire day, notably glutamate, but did not affect root growth of the transformed plants. The suitability of individual amino compounds as markers for major N fluxes, such as nitrate reduction, photorespiration, and amino N export and import is discussed.     

Soil CO2 concentration does not affect growth or root respiration in bean or citrus

Contrasting effects of soil CO₂ concentration on root respiration rates during short-term CO₂ exposure, and on plant growth during long-term CO₂ exposure, have been reported. Here we examine the effects of both short- and long-term exposure to soil CO₂ on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. & Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO₂. Respiration rates of citrus roots were unaffected by the CO₂ concentration used during the respiration measurements (200 and 2000 μmol mol⁻¹), regardless of the soil CO₂, concentration during the previous month (600 and 20 000 μmol mol⁻¹). Bean plants were grown with their roots exposed to either a natural CO₂ diffusion gradient, or to an artificially maintained CO₂ concentration of 600 or 20 000 μmol mol⁻¹. These treatments had no effect on shoot and root growth. Growth respiration and maintenance respiration of bean roots were also unaffected by CO₂ pretreatment and the CO₂ concentration used during the respiration measurements (200–2000 μmol mol⁻¹). We conclude that soil CO₂ concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean.     

Production, turnover and mycorrhizal colonization of root systems of three Populus species grown under elevated CO2 (POPFACE)

A fast growing high density Populus plantation located in central Italy was exposed to elevated carbon dioxide for a period of three years. An elevated CO₂ treatment (550 ppm), of 200 ppm over ambient (350 ppm) was provided using a FACE technique. Standing root biomass, fine root turnover and mycorrhizal colonization of the following Populus species was examined: Populus alba L., Populus nigra L., Populus x euramericana Dode (Guinier). Elevated CO₂ increased belowground allocation of biomass in all three species examined, standing root biomass increased by 47–76% as a result of FACE treatment. Similarly, fine root biomass present in the soil increased by 35–84%. The FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. x euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO₂. Response of the mycorrhizal community to elevated CO₂ was more varied, the rate of infection increased only in P. alba for both ectomycorrhizal (EM) and arbuscular mycorrhizas (AM). The roots of P. nigra showed greater infection only by AM and the colonization of the root system of P. x euramericana was not affected by FACE treatment. The results suggest that elevated atmospheric CO₂ conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This may have implications for C sequestration and must be taken into account when considering long‐term C storage in the soil.     

Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass

Continuing enrichment of atmospheric CO₂ may change plant community composition, in part by altering the availability of other limiting resources including soil water, nutrients, or light. The combined effects of CO₂ enrichment and altered resource availability on species flowering remain poorly understood. We quantified flowering culm and ramet production and biomass allocation to flowering culms/ramets for 10 years in C₄‐dominated grassland communities on contrasting soils along a CO₂ concentration gradient spanning pre‐industrial to expected mid‐21st century levels (250–500 μl/L). CO₂ enrichment explained up to 77% of the variation in flowering culm count across soils for three of the five species, and was correlated with flowering culm count on at least one soil for four of five species. In contrast, allocation to flowering culms was only weakly correlated with CO₂ enrichment for two species. Flowering culm counts were strongly correlated with species aboveground biomass (AGB; R² = .34–.74), a measure of species abundance. CO₂ enrichment also increased soil moisture and decreased light levels within the canopy but did not affect soil inorganic nitrogen availability. Structural equation models fit across the soils suggested species‐specific controls on flowering in two general forms: (1) CO₂ effects on flowering culm count mediated by canopy light level and relative species AGB (species AGB/total AGB) or by soil moisture effects on flowering culm count; (2) effects of canopy light level or soil inorganic nitrogen on flowering and/or relative species AGB, but with no significant CO₂ effect. Understanding the heterogeneity in species responses to CO₂ enrichment in plant communities across soils in edaphically variable landscapes is critical to predict CO₂ effects on flowering and other plant fitness components, and species potential to adapt to future environmental changes.     

Effect of CO2 on succinate production in dual-phase Escherichia coli fermentations

Succinate production under different concentrations of carbon dioxide (CO₂) was studied in Escherichia coli AFP111, which contains mutations in pyruvate formate lyase (pfl), lactate dehydrogenase (ldhA) and the phosphotransferase system glucosephosphotransferase enzyme II (ptsG). A series of two-phase fermentations were conducted in which an aerobic cell growth phase was followed by an anaerobic succinate production phase using several constant concentrations of CO₂. As the concentration of CO₂ in the gas phase increased from 0% to 50%, the succinate specific productivity increased from 1.9 mg/g h to 225 mg/g h, and the succinate yield increased from 0.04 g/g to 0.75 g/g. Above 50% CO₂, succinate production did not increase further. Intracellular fluxes were determined at three different CO₂ concentrations (3%, 10%, and 50%) using ¹³C-label tracing coupled with LC–MS analysis. The fraction of carbon flux into the pentose phosphate pathway increased from 0.04 at 3% CO₂ to 0.17 at 50% CO₂. Also, the fractional flux through anaplerotic carboxylation at the phosphoenolpyruvate (PEP) node increased slightly from 0.53 at 3% CO₂ to 0.63 at 50% CO₂. The increased flux into the pentose phosphate pathway is attributed to an increased demand for reduced cofactors with elevated CO₂. A four-process explicit model to describe the CO₂ transfer and utilization was proposed. The model predicted that at CO₂ concentrations below about 30–40% the system becomes limited by gas phase CO₂, while at higher CO₂ concentrations the system is limited by PEP carboxylase enzyme kinetics.     

Supersaturation of Dissolved H2 and CO2 During Fermentative Hydrogen Production with N2 Sparging

Dissolved H₂ and CO₂ were measured by an improved manual headspace-gas chromatographic method during fermentative H₂ production with N₂ sparging. Sparging increased the yield from 1.3 to 1.8 mol H₂/mol glucose converted, although H₂ and CO₂ were still supersaturated regardless of sparging. The common assumption that sparging increases the H₂ yield because of lower dissolved H₂ concentrations may be incorrect, because H₂ was not lowered into the range necessary to affect the relevant enzymes. More likely, N₂ sparging decreased the rate of H₂ consumption via lower substrate concentrations.     

Effects of enhanced atmospheric CO2 and nutrient supply on the quality and subsequent decomposition of fine roots of Betula pendula Roth. and Picea sitchensis (Bong.) Carr.

Fine root litter derived from birch (Betula pendula Roth.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) plants grown under two CO₂ atmospheric concentrations (350 ppm and 600 ppm) and two nutrient regimes was used for decomposition studies in laboratory microcosms. Although there were interactions between litter type, CO₂/fertiliser treatments and decomposition rates, in general, an increase in the C/N ratio of the root tissue was observed for roots of both species grown under elevated CO₂ in unfertilized soil. Both weight loss and respiration of decomposing birch roots were significantly reduced in materials derived from enriched CO₂, whilst the decomposition of spruce roots showed no such effect. A parallel experiment was performed using Betula pendula root litter grown under different N regimes, in order to test the relationship between C/N ratio of litter and root decomposition rate. A highly significant (p<0.001) negative correlation between C/N ratio and root litter respiration was found, with an r²=0.97. The results suggest that the increased C/N ratio of plant tissues induced by elevated CO₂ can result in a reduction of decomposition rate, with a resulting increase in forest soil C stores.     

Shoots, roots and ectomycorrhiza formation of pine seedlings at elevated atmospheric carbon dioxide

The effect of elevated atmospheric CO₂ concentration on the growth of shoots, roots, mycorrhizas and extraradical mycorrhizal mycelia of pine (Pinus silvestris L.) was examined. Two and a half-month-old seedlings were inoculated axenically with the mycorrhizal fungus Pisolithus tincto-rius (Pers.) by a method allowing rapid mycorrhiza formation in Petri dishes. The plants were then cultivated for 3 months in growth chambers with daily concentrations of 350 and 600 μmol mol^?1 CO₂ during the day. Whereas plants harvested after 1 and 2 months did not differ appreciably between ambient and increased CO₂ concentrations, after 3 months they developed a considerably higher root biomass (%57%) at elevated CO₂, but did not increase significantly in root length. The mycorrhizal fungus Pisolithus tinctorius, which depended entirely on the plant assimilates in the model system, grew much faster at increased CO₂: 3 times more mycorrhizal root clusters were formed and the extraradical mycelium produced had twice the biomass at elevated as at ambient CO₂. No difference in shoot biomass was found between the two treatments after 91 d. However, since the total water consumption of seedlings was similar in the two treatments, the water use efficiency was appreciably higher for the seedlings at increased CO₂ because of the higher below-ground biomass.