Process-based isotope partitioning of winter soil respiration in a subalpine ecosystem reveals importance of rhizospheric respiration

Deep snow in sub-alpine ecosystems may reduce or eliminate soil freezing, thus contributing to the potential for winter soil respiration to account for a significant fraction of annual CO₂ efflux to the atmosphere. Quantification of carbon loss from soils requires separation of respiration produced by roots and rhizosphere organisms from that produced by heterotrophic, decomposer organisms because the former does not result in a net loss of stored carbon. Our objective was to quantify winter soil respiration rates in a sub-alpine forest and meadow, and to partition that flux into its rhizosphere and heterotrophic components. We were particularly interested in comparing early winter soil respiration to late winter/early spring soil respiration of each component because previous work has shown a consistent increase in soil respiration of subalpine systems from early winter to late winter/spring. Field data on the total soil CO₂ flux and its carbon isotope composition were coupled with data from laboratory incubations using a novel process-based stable isotope mixing model implemented in a hierarchical Bayesian framework. We found that soil respiration generally increased from early to later winter and was greatest mid-summer. After correcting for the effect of wind on snowpack δ¹³C–CO₂, the δ¹³C of soil-respired CO₂ varied little over winter, and the contributions of rhizospheric (~35 %) and heterotrophic (~65 %) respiration were relatively constant. The significance of winter respiration from the rhizosphere and apparent coupling of increases in rhizospheric and heterotrophic respiration in late winter are likely to be important for predicting changes in soil carbon in sub-alpine ecosystems.     

Ecosystem-atmosphere CO(2) exchange: interpreting signals of change using stable isotope ratios

Changes in the concentration and stable isotope ratio of atmospheric CO(2) can be used to study variations in the net exchange of carbon dioxide in terrestrial ecosystems (net difference between total photosynthesis and respiration). Changes in the timing of seasonal fluctuations in atmospheric CO(2) concentration have suggested that net uptake of carbon dioxide has been increasing in northern latitude ecosystems in association with warmer temperatures and a lengthening of the growing season. Stable isotope techniques allow a more detailed separation of differences between ecosystem photosynthesis and respiration because these two processes have contrasting effects on both the carbon and oxygen isotope ratio of atmospheric CO(2). Future applications of stable isotope analyses include documenting and monitoring the influence of global environmental change on ecosystem CO(2) exchange at regional scales (10-1000km(2)).     

Respiratory carbon metabolism following illumination in intact French bean leaves using (13)C/(12)C isotope labeling

The origin of the carbon atoms in the CO(2) respired by French bean (Phaseolus vulgaris) leaves in the dark has been studied using (13)C/(12)C isotopes as tracers. The stable isotope labeling was achieved through a technical device that uses an open gas-exchange system coupled online to an elemental analyzer and linked to an isotope ratio mass spectrometer. The isotopic analysis of the CO(2) respired in the dark after a light period revealed that the CO(2) was labeled, but the labeling level decreased progressively as the dark period increased. The pattern of disappearance depended on the amount of carbon fixed during the labeling and indicated that there were several pools of respiratory metabolites with distinct turnover rates. We demonstrate that the carbon recently assimilated during photosynthesis accounts for less than 50% of the carbon in the CO(2) lost by dark respiration and that the proportion is not influenced by leaf starvation in darkness before the labeling. Therefore, most of the carbon released by dark respiration after illumination does not come from new photosynthates.     

Respiration of the external mycelium in the arbuscular mycorrhizal symbiosis shows strong dependence on recent photosynthates and acclimation to temperature

* Although arbuscular mycorrhizal (AM) fungi are a major pathway in the global carbon cycle, their basic biology and, in particular, their respiratory response to temperature remain obscure. * A pulse label of the stable isotope (13)C was applied to Plantago lanceolata, either uninoculated or inoculated with the AM fungus Glomus mosseae. The extra-radical mycelium (ERM) of the fungus was allowed to grow into a separate hyphal compartment excluding roots. We determined the carbon costs of the ERM and tested for a direct temperature effect on its respiration by measuring total carbon and the (13)C:(12)C ratio of respired CO(2). With a second pulse we tested for acclimation of ERM respiration after 2 wk of soil warming. * Root colonization remained unchanged between the two pulses but warming the hyphal compartment increased ERM length. delta(13)C signals peaked within the first 10 h and were higher in mycorrhizal treatments. The concentration of CO(2) in the gas samples fluctuated diurnally and was highest in the mycorrhizal treatments but was unaffected by temperature. Heating increased ERM respiration only after the first pulse and reduced specific ERM respiration rates after the second pulse; however, both pulses strongly depended on radiation flux. * The results indicate a fast ERM acclimation to temperature, and that light is the key factor controlling carbon allocation to the fungus.     

In vivo respiratory metabolism of illuminated leaves

Day respiration of illuminated C(3) leaves is not well understood and particularly, the metabolic origin of the day respiratory CO(2) production is poorly known. This issue was addressed in leaves of French bean (Phaseolus vulgaris) using (12)C/(13)C stable isotope techniques on illuminated leaves fed with (13)C-enriched glucose or pyruvate. The (13)CO(2) production in light was measured using the deviation of the photosynthetic carbon isotope discrimination induced by the decarboxylation of the (13)C-enriched compounds. Using different positional (13)C-enrichments, it is shown that the Krebs cycle is reduced by 95% in the light and that the pyruvate dehydrogenase reaction is much less reduced, by 27% or less. Glucose molecules are scarcely metabolized to liberate CO(2) in the light, simply suggesting that they can rarely enter glycolysis. Nuclear magnetic resonance analysis confirmed this view; when leaves are fed with (13)C-glucose, leaf sucrose and glucose represent nearly 90% of the leaf (13)C content, demonstrating that glucose is mainly directed to sucrose synthesis. Taken together, these data indicate that several metabolic down-regulations (glycolysis, Krebs cycle) accompany the light/dark transition and emphasize the decrease of the Krebs cycle decarboxylations as a metabolic basis of the light-dependent inhibition of mitochondrial respiration.     

Carbon Isotopic Fractionation Does Not Occur during Dark Respiration in C3 and C4 Plants

The magnitude of possible carbon isotopic fractionation during dark respiration was investigated with isolated mesophyll cells from mature leaves of common bean (Phaseolus vulgaris L.), a C3 plant, and corn (Zea mays L.), a C4 plant. Mesophyll protoplasts were extracted from greenhouse-grown leaves and incubated in culture solutions containing different carbohydrate substrates (fructose, glucose, and sucrose) with known [delta]13C values. The CO2 produced by protoplasts after incubation in the dark was collected, purified, and analyzed for its carbon isotope ratio. From observations of the isotope ratios of the substrate and respired CO2, we calculated the carbon isotope discrimination associated with metabolism of each of these substrates. In eight of the 10 treatment combinations, the carbon isotope ratio discrimination was not significantly different from 0. In the remaining two treatment combinations, the carbon isotope ratio discrimination was 1[per mille (thousand) sign]. From these results, we conclude that there is no significant carbon isotopic discrimination during mitochondrial dark respiration when fructase, glucose, or sucrose are used as respiratory substrates.     

Carbon-13 isotope effects on reactions involving carbamate and carbamoyl phosphate

Several 13C isotope effects of relevance to reactions involving carbamate and carbamoyl phosphate have been determined. The fractionation factor of carbamate relative to aqueous CO2 is 1.011; the equilibrium isotope effect on the reaction catalyzed by carbamate kinase is 0.9983. From these data we can calculate that the fractionation factor of carbamoyl phosphate relative to aqueous CO2 is 1.013. The kinetic 13C isotope effect on the decomposition of carbamoyl phosphate to cyanate and phosphate is 1.058. The environment of the carbon atom in carbamate and carbamoyl phosphate and the mechanism of carbamoyl phosphate decomposition are discussed in light of these values.     

Effect of Oxygen on the Rate of Photorespiration in Detached Tobacco Leaves

The rate of carbon dioxide exchange in both light and darkness by detached tobacco leaves placed at various oxygen concentrations was measured by an Infra-Red CO₂ Analyzer and a Clark oxygen electrode. It was observed that during illumination oxygen had two different effects. One was to stimulate carbon dioxide evolution and the other to inhibit carbon dioxide absorption. Concentration of carbon dioxide at compensation point was found to be a linear function of oxygen concentration and this has been explained as due mainly to an increased evolution of carbon dioxide. Such an evolution during illumination has been called photorespiration. Increased concentrations of oxygen also had a stimulating effect on the magnitude of the initial post-illumination burst of carbon dioxide in darkness, but no effect on the subsequent steady rates. These data have been explained as due to the suspension of regular respiration in darkness and its replacement by a different process, tentatively called photorespiration. A second effect of oxygen was to reduce the efficiency (called “carboxylation efficiency”) with which a leaf was able to remove carbon dioxide from the atmosphere.     

Seasonal net carbon dioxide exchange of a beech forest with the atmosphere

The seasonal carbon dioxide exchange of a beech forest of Central Italy was studied by means of the eddy covariance technique. Additional measurements of biomass respiration with cuvettes and relationship of carbon dioxide exchanges with temperature and light were used to interpolate missing data during the dormant and part of the growing season. The net ecosystem production of the forest equals 472 g C m^?2 y^?1 while the gross ecosystem production 1016 g C m^?2 y^?1 and respiration 544 g C m^?2 y^?1. These estimates are compared with the net primary production determined by direct biomass sampling which amounts to 802 g C m^?2 y^?1.     

The use of the [13C]/[12C] ratio for the assay of the microbial oxidation of hydrocarbons

The study deals with a comparative analysis of the relative abundances of the carbon isotopes 12C and 13C in the metabolites and biomass of the Burkholderia sp. BS3702 and Pseudomonas putida BS202-p strains capable of utilizing aliphatic (n-hexadecane) and aromatic (naphthalene) hydrocarbons as sources of carbon and energy. The isotope composition of the carbon dioxide, biomass, and exometabolites produced during the growth of Burkholderia sp. BS3702 on n-hexadecane (delta 13C = -44.6 +/- 0.2@1000) were characterized by the isotope effects delta 13CCO2 = -50.2 +/- 0.4@1000, delta 13Cbiom = -46.6 +/- 0.4@1000 and delta 13Cexo = -41.5 +/- 0.4@1000, respectively. The isotope composition of the carbon dioxide, biomass, and exometabolites produced during the growth of the same bacterial strain on naphthalene (delta 13C = -21 +/- 0.4@1000) were characterized by the isotope effects delta 13CCO2 = -24.1 +/- 0.4@1000, delta 13Cbiom = -19.2 +/- 0.4@1000 and delta 13Cexo = -19.1 +/- 0.4@1000, respectively. The possibility of using the isotope composition of metabolic carbon dioxide for the rapid monitoring of the microbial degradation of petroleum hydrocarbons in the enviroment is discussed.     

Ternary effects on the gas exchange of isotopologues of carbon dioxide

The ternary effects of transpiration rate on the rate of assimilation of carbon dioxide through stomata, and on the calculation of the intercellular concentration of carbon dioxide, are now included in standard gas exchange studies. However, the equations for carbon isotope discrimination and for the exchange of oxygen isotopologues of carbon dioxide ignore ternary effects. Here we introduce equations to take them into account. The ternary effect is greatest when the leaf-to-air vapour mole fraction difference is greatest, and its impact is greatest on parameters derived by difference, such as the mesophyll resistance to CO(2) assimilation, r(m) . We show that the mesophyll resistance to CO(2) assimilation has been underestimated in the past. The impact is also large when there is a large difference in isotopic composition between the CO(2) inside the leaf and that in the air. We show that this partially reconciles estimates of the oxygen isotopic composition of CO(2) in the chloroplast and mitochondria in the light and in the dark, with values close to equilibrium with the estimated oxygen isotopic composition of water at the sites of evaporation within the leaf.     

The synthesis of amino acids by Methanobacterium omelianskii

1. Methanobacterium omelianskii was grown on (14)CO(2) and unlabelled ethanol, or on [1-(14)C]- or [2-(14)C]-ethanol and unlabelled carbon dioxide. The cell protein was hydrolysed and certain of the amino acids were isolated and degraded. 2. Carbon from both carbon dioxide and ethanol is used for biosynthesis of amino acids, and in most cases ethanol is incorporated as a C(2) unit. Ethanol carbon atoms and carbon dioxide carbon atoms apparently enter the same range of compounds. Ethanol and carbon dioxide are equally important as sources of cell carbon. 3. The origins of carbon atoms of aspartate, alanine, glycine, serine and threonine are consistent with the synthesis of these amino acids, by pathways known to exist in aerobic organisms, from pyruvate arising by a C(2)+C(1) condensation. The proportion of total radioactivity found in C-1 of lysine, proline, methionine and valine is consistent with synthesis of these amino acids by pathways similar to those found in Escherichia coli. Isoleucine is probably formed by carboxylation of a C(5) precursor formed entirely from ethanol. Glutamate is formed by an unknown pathway.     

Responses of respiration to increasing atmospheric carbon dioxide concentrations

It has been recently recognized that increases in carbon dioxide concentration such as are anticipated for the earth's atmosphere in the next century often reduce plant respiration. There can be both a short-term reversible effect of unknown cause, and long-term acclimation, which may reflect the synthesis and maintenance of less metabolically expensive materials in plants grown at elevated carbon dioxide concentrations. Because respiration provides energy and carbon intermediates for growth and maintenance, reductions in respiration by increasing carbon dioxide concentrations may have effects on physiology beyond an improvement in plant carbon balance. As atmospheric carbon dioxide concentration increases, reduced respiration could be as important as increased photosynthesis in improving the ability of terrestrial vegetation to act as a sink for carbon, but it could also have other consequences.     

环境条件对植物稳定碳同位素组成的影响

植物稳定碳同位素技术是近年兴起的一项快速、可靠的技术。利用稳定碳同位素技术可以揭示碳同化的过程的许多方面的信息。1 3C和1 2 C同位素效应 ,使它们在进行碳循环时发生稳定碳同位素的分馏。植物光合作用过程中CO2 经气孔扩散分差和RUBPCase及PEPCase羧化分馏是造成植物稳定碳同位素比率 (R =1 3C/ 1 2 C)不同于源CO2 中碳同位素比率的主要原因。遗传因素和环境因子同时决定植物碳同位素组成。植物稳定碳同位素技术同时还是古气候重建和预测未来环境变化的理论基础。本文综述了光照、温度、水分、二氧化碳、矿质营养、盐分和大气污染物等环境因素对植物稳定碳同位素组成影响方面的研究进展。     

(12)C/(13)C fractionations in plant primary metabolism

Natural (13)C abundance is now an unavoidable tool to study ecosystem and plant carbon economies. A growing number of studies take advantage of isotopic fractionation between carbon pools or (13)C abundance in respiratory CO(2) to examine the carbon source of respiration, plant biomass production or organic matter sequestration in soils. (12)C/(13)C isotope effects associated with plant metabolism are thus essential to understand natural isotopic signals. However, isotope effects of enzymes do not influence metabolites separately, but combine to yield a (12)C/(13)C isotopologue redistribution orchestrated by metabolic flux patterns. In this review, we summarise key metabolic isotope effects and integrate them into the corpus of plant primary carbon metabolism.     

马占相思人工林净二氧化碳交换和碳同位素通量

应用稳定碳同位素技术,对马占相思人工林冠层受光和遮荫叶片的碳同化率(A_net)和叶面积指数(L)进行加权,将叶片水平的¹³C甄别率(Δ_i)扩展至冠层光合甄别率(Δ_canopy),测定光合固定和呼吸释放的碳同位素通量及其净交换通量.结果表明：Δ_canopy的日变化明显,日出前和中午出现较低值(18.47‰和19.87‰),而日落前达到最大(21.21‰);秋季末期(11月)至翌年夏季,Δ_canopy逐步升高,年平均为(20.37±0.29)‰.不同季节自养呼吸(日间叶片呼吸除外)和异养呼吸释放CO₂的碳同位素比率(δ¹³C)平均值分别为(-28.70±0.75)‰和(-26.75±1.3)‰,春季林冠夜间呼吸CO₂的δ¹³C最低(-30.14‰),秋季末期最高(-28.01‰).马占相思林与大气的CO₂碳同位素通量在春季和夏季中午时峰值分别为178.5和217 μmol·m^-2 ·s^-1·‰,日均值分别为638.4 和873.2 μmol·m^-2·s^-1·‰.冠层叶片吸收CO₂的碳同位素通量较呼吸释出CO₂的碳同位素通量高1.6～2.5倍,表明马占相思林日间吸收大量CO₂,降低空气CO₂浓度,具有改善环境的良好生态服务功能.     

THE CHLOROPHYLL-CARBON DIOXIDE RATIO DURING PHOTOSYNTHESIS

Using a rapid spectrographic method of carbon dioxide measurement previously described by McAlister (1937) further studies on the time course of photosynthesis in the higher plant, wheat, variety Marquis, are herein reported. Of major importance in this work is the discovery of a pick-up of carbon dioxide in darkness immediately following a high rate of photosynthesis (see Figs. 3 and 4). This pick-up is believed to be due to the action of a carbon dioxide-combining intermediate; i.e., the "acceptor molecule" for carbon dioxide in photosynthesis. The conditions under which this phenomenon has so far been observed indicate that the intermediate is formed in relatively large quantities during the actual process of photosynthesis and not before. That the intermediate is chlorophyllous in nature is suggested by a simple stoichiometry of the order of unity that is found to exist between the number of carbon dioxide molecules taken up and the total number of chlorophyll molecules present in the plant. This is in opposition to the idea of a large photosynthetic unit of some 2000 chlorophyll molecules operating together in the reduction of 1 carbon dioxide molecule. Further studies of the induction phase under various conditions of previous dark rest and of carbon dioxide and light limitation are herein described. Employing the simple hypothesis that the number of carbon dioxide molecules not reduced during the induction period (induction loss) gives a measure of the number of elementary photosynthetic cycles unoperative or compensated for during induction together with the experimental fact that this induction loss is of the order of the total number of chlorophyll molecules present, these latter studies also indicate, in a less direct manner, that chlorophyll participates in photosynthesis as an individual molecule and not as part of a very large multimolecular chlorophyll unit. The fast dark reaction lasting about 1 minute (Fig. 7) required to reproduce both (a) the phenomena of induction in carbon dioxide assimilation and (b) the recovery of fluorescence of chlorophyll in leaves in darkness as observed by Franck and Wood (1936), demonstrates a close relationship between the fluorescence of chlorophyll and induction in photosynthesis. The rate of respiration (carbon dioxide production) of the higher plant, wheat, was measured under intense illumination and in the absence of carbon dioxide (to suppress assimilation). This value was found to be identical with the dark respirational rate measured before and after the light period, indicating very positively the absence of any direct effect of light on respiration.     

火烧对森林土壤有机碳的影响研究进展

对国内外火烧影响森林土壤有机碳动态的研究成果进行了综合述评。较多研究表明低强度火烧不会造成土壤有机碳贮量的明显变化,但火烧非常强烈而彻底,土壤有机碳明显减少。有限研究表明火烧对森林土壤呼吸的影响结果有增加、降低或无影响,因火烧强度、火后观测时间、森林类型、火烧迹地上植被恢复进程和气候条件等而异。同时,火烧对土壤有机碳组分(活性有机碳和黑碳)也具有不同程度的影响。随着全球变化研究的深入,火烧作为森林主要管理措施对大气CO2浓度影响亦愈来愈受重视,今后应着重开展以下几方面研究:(1)扩大气候和经营管理的变化对森林土壤有机碳贮量时空动态影响研究;(2)深入探讨火烧影响土壤CO2释放的过程及机理;(3)加强火烧历史和频率对黑碳影响的研究;(4)从广度和深度上加强火烧等经营措施对亚热带森林土壤碳动态影响的研究。     

A comparison of the effects of carbon dioxide concentration and temperature on respiration, translocation and nitrate reduction in darkened soybean leaves

BACKGROUND AND AIMS: Respiration of autotrophs is an important component of their carbon balance as well as the global carbon dioxide budget. How autotrophic respiration may respond to increasing carbon dioxide concentrations, [CO(2)], in the atmosphere remains uncertain. The existence of short-term responses of respiration rates of plant leaves to [CO(2)] is controversial. Short-term responses of respiration to temperature are not disputed. This work compared responses of dark respiration and two processes dependent on the energy and reductant supplied by dark respiration, translocation and nitrate reduction, to changes in [CO(2)] and temperature. METHODS: Mature soybean leaves were exposed for a single 8-h dark period to one of five combinations of air temperature and [CO(2)], and rates of respiration, translocation and nitrate reduction were determined for each treatment. KEY RESULTS: Low temperature and elevated [CO(2)] reduced rates of respiration, translocation and nitrate reduction, while increased temperature and low [CO(2)] increased rates of all three processes. A given change in the rate of respiration was accompanied by the same change in the rate of translocation or nitrate reduction, regardless of whether the altered respiration was caused by a change in temperature or by a change in [CO(2)]. CONCLUSIONS: These results make it highly unlikely that the observed responses of respiration rate to [CO(2)] were artefacts due to errors in the measurement of carbon dioxide exchange rates in this case, and indicate that elevated [CO(2)] at night can affect translocation and nitrate reduction through its effect on respiration.