全球增温与碳循环

全球变暖是地球系统科学领域的热点问题 .全面深入地理解全球碳循环的规律 ,将有助于客观地认识全球增温的原因 .系统地分析了全球碳循环的基本规律 ,评述了各个方面的研究进展 ,指出了研究当中应当注意的关键性问题 ,并且提出了目前探索和研究的重点课题 .     

Uncovering the Neoproterozoic carbon cycle

Interpretations of major climatic and biological events in Earth history are, in large part, derived from the stable carbon isotope records of carbonate rocks and sedimentary organic matter. Neoproterozoic carbonate records contain unusual and large negative isotopic anomalies within long periods (10-100 million years) characterized by δ(13)C in carbonate (δ(13)C(carb)) enriched to more than +5 per mil. Classically, δ(13)C(carb) is interpreted as a metric of the relative fraction of carbon buried as organic matter in marine sediments, which can be linked to oxygen accumulation through the stoichiometry of primary production. If a change in the isotopic composition of marine dissolved inorganic carbon is responsible for these excursions, it is expected that records of δ(13)C(carb) and δ(13)C in organic carbon (δ(13)C(org)) will covary, offset by the fractionation imparted by primary production. The documentation of several Neoproterozoic δ(13)C(carb) excursions that are decoupled from δ(13)C(org), however, indicates that other mechanisms may account for these excursions. Here we present δ(13)C data from Mongolia, northwest Canada and Namibia that capture multiple large-amplitude (over 10 per mil) negative carbon isotope anomalies, and use these data in a new quantitative mixing model to examine the behaviour of the Neoproterozoic carbon cycle. We find that carbonate and organic carbon isotope data from Mongolia and Canada are tightly coupled through multiple δ(13)C(carb) excursions, quantitatively ruling out previously suggested alternative explanations, such as diagenesis or the presence and terminal oxidation of a large marine dissolved organic carbon reservoir. Our data from Namibia, which do not record isotopic covariance, can be explained by simple mixing with a detrital flux of organic matter. We thus interpret δ(13)C(carb) anomalies as recording a primary perturbation to the surface carbon cycle. This interpretation requires the revisiting of models linking drastic isotope excursions to deep ocean oxygenation and the opening of environments capable of supporting animals.     

Inside out:deep carbon linked to deep-time carbon cycle

The Ediacaran Shuram excursion (SE) is widely regarded as one of the largest carbon isotope (δ13C) negative excursions in Earth history,with carbonate δ13C valu...     

Global metabolic impacts of recent climate warming

Documented shifts in geographical ranges, seasonal phenology, community interactions, genetics and extinctions have been attributed to recent global warming. Many such biotic shifts have been detected at mid- to high latitudes in the Northern Hemisphere-a latitudinal pattern that is expected because warming is fastest in these regions. In contrast, shifts in tropical regions are expected to be less marked because warming is less pronounced there. However, biotic impacts of warming are mediated through physiology, and metabolic rate, which is a fundamental measure of physiological activity and ecological impact, increases exponentially rather than linearly with temperature in ectotherms. Therefore, tropical ectotherms (with warm baseline temperatures) should experience larger absolute shifts in metabolic rate than the magnitude of tropical temperature change itself would suggest, but the impact of climate warming on metabolic rate has never been quantified on a global scale. Here we show that estimated changes in terrestrial metabolic rates in the tropics are large, are equivalent in magnitude to those in the north temperate-zone regions, and are in fact far greater than those in the Arctic, even though tropical temperature change has been relatively small. Because of temperature's nonlinear effects on metabolism, tropical organisms, which constitute much of Earth's biodiversity, should be profoundly affected by recent and projected climate warming.     

Global organization of metabolic fluxes in the bacterium Escherichia coli

Cellular metabolism, the integrated interconversion of thousands of metabolic substrates through enzyme-catalysed biochemical reactions, is the most investigated complex intracellular web of molecular interactions. Although the topological organization of individual reactions into metabolic networks is well understood, the principles that govern their global functional use under different growth conditions raise many unanswered questions. By implementing a flux balance analysis of the metabolism of Escherichia coli strain MG1655, here we show that network use is highly uneven. Whereas most metabolic reactions have low fluxes, the overall activity of the metabolism is dominated by several reactions with very high fluxes. E. coli responds to changes in growth conditions by reorganizing the rates of selected fluxes predominantly within this high-flux backbone. This behaviour probably represents a universal feature of metabolic activity in all cells, with potential implications for metabolic engineering.     

Evolution and metabolic significance of the urea cycle in photosynthetic diatoms

Diatoms dominate the biomass of phytoplankton in nutrient-rich conditions and form the basis of some of the world's most productive marine food webs. The diatom nuclear genome contains genes with bacterial and plastid origins as well as genes of the secondary endosymbiotic host (the exosymbiont), yet little is known about the relative contribution of each gene group to diatom metabolism. Here we show that the exosymbiont-derived ornithine-urea cycle, which is similar to that of metazoans but is absent in green algae and plants, facilitates rapid recovery from prolonged nitrogen limitation. RNA-interference-mediated knockdown of a mitochondrial carbamoyl phosphate synthase impairs the response of nitrogen-limited diatoms to nitrogen addition. Metabolomic analyses indicate that intermediates in the ornithine-urea cycle are particularly depleted and that both the tricarboxylic acid cycle and the glutamine synthetase/glutamate synthase cycles are linked directly with the ornithine-urea cycle. Several other depleted metabolites are generated from ornithine-urea cycle intermediates by the products of genes laterally acquired from bacteria. This metabolic coupling of bacterial- and exosymbiont-derived proteins seems to be fundamental to diatom physiology because the compounds affected include the major diatom osmolyte proline and the precursors for long-chain polyamines required for silica precipitation during cell wall formation. So far, the ornithine-urea cycle is only known for its essential role in the removal of fixed nitrogen in metazoans. In diatoms, this cycle serves as a distribution and repackaging hub for inorganic carbon and nitrogen and contributes significantly to the metabolic response of diatoms to episodic nitrogen availability. The diatom ornithine-urea cycle therefore represents a key pathway for anaplerotic carbon fixation into nitrogenous compounds that are essential for diatom growth and for the contribution of diatoms to marine productivity.     

Respiration as the main determinant of carbon balance in European forests

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha(-1) yr(-1), with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.     

三沙湾纳潮量及湾内外的水交换

根据三沙湾多年实测潮位资料和2个临时潮位站的实测资料,分析了三沙湾潮汐特征,计算了纳潮量、海水的平均交换率和海水半更换期.结果表明:三沙湾潮差较大,有较强的纳潮能力.但由于其水域面积较大,地形也比较复杂,其整个区域海水的平均“更新能力”一般.当潮差累积率为20%(大潮)时,三沙湾的海水半更换期约为36个潮周期;当潮差累积率为50%(中潮)时,海水半更换期约为75个潮周期;当潮差累积率为80%(小潮)时,海水半更换期达到114个潮周期.     

Dual modes of the carbon cycle since the Last Glacial Maximum

The most conspicuous feature of the record of past climate contained in polar ice is the rapid warming which occurs after long intervals of gradual cooling. During the last four transitions from glacial to interglacial conditions, over which such abrupt warmings occur, ice records indicate that the CO2 concentration of the atmosphere increased by roughly 80 to 100 parts per million by volume. But the causes of the atmospheric CO2 concentration increases are unclear. Here we present the stable-carbon-isotope composition (delta 13 CO2) of CO2 extracted from air trapped in ice at Taylor Dome, Antarctica, from the Last Glacial Maximum to the onset of Holocene times. The global carbon cycle is shown to have operated in two distinct primary modes on the timescale of thousands of years, one when climate was changing relatively slowly and another when warming was rapid, each with a characteristic average stable-carbon-isotope composition of the net CO2 exchanged by the atmosphere with the land and oceans. delta 13 CO2 increased between 16.5 and 9 thousand years ago by slightly more than would be estimated to be caused by the physical effects of a 5 degrees C rise in global average sea surface temperature driving a CO2 efflux from the ocean, but our data do not allow specific causes to be constrained.     

胡杨异胚发育叶片碳平衡研究

胡杨成熟个体植株上的叶片可大概分为3种叶形,即阔卵圆形(BO)、过渡形(O)及披针形(L).叶片尺度的碳平衡取决于光合作用和呼吸作用,但机理尚不明.作者研究了胡杨异形叶的气体交换、形态学和生化特性.结果显示:① 胡杨3种叶形比叶面积(SLA)存在显著差异,阔卵圆形叶比叶面积SLA=(6.69±0.24)m2·kg-1显著低于其余两者(过渡形叶SLA=(7.36±0.09)m2·kg-1,披针形叶SLA=(8.09±0.16)m2·kg-1),叶片单位质量氮含量(Nmass)存在差异但不具显著差异;② 阔卵圆形叶单位面积碳同化速率(Aarea=(11.92±1.22)μmol·m-2·s-1)显著高于过渡形叶(Aarea=(10.21±1.43)μmol·m-2·s-1)和披针形叶(Aarea=(9.98±1.04)μmol·m-2·s-1),然而,单位质量(Amass)和单位氮碳同化速率(AN)在3种叶形中并不具显著差异,披针形叶的Amass=(80.37±7.07)μmol·kg-1·s-1(DW)和AN=(80.37±7.07)μmol·kg-1·s-1(DW)略高于其它2种叶形;③ 阔卵圆形叶单位面积及单位质量暗呼吸速率(Rn-area=(1.90±0.17)μmol·m-2·s-1和Rn-mass=(12.58±0.77)μmol·kg-1·s-1(DW))显著高于过渡形叶(Rn-area=(1.60±0.19)μmol·m-2·s-1和Rn-mass=(11.85±0.15)μmol·kg-1·s-1(DW))及披针形叶(Rn-area=(1.57±0.29)μmol·m-2·s-1和Rn-mass=(11.25±0.92)μmol·kg-1·s-1(DW)),另外,3种叶形叶片光下线粒体呼吸速率(RL)均大于暗呼吸速率,说明光对暗呼吸有抑制作用,且不同叶形抑制程度也不同,从披针叶形50.37%到阔卵圆形叶的80.67%;④ 不同叶形叶片水平碳平衡(Ag/R)差异不显著,但披针形叶的Ag/R=7.90±1.39高于其余2种叶形(阔卵圆形叶Ag/R=6.19±0.87,过渡形叶Ag/R=7.65±1.71).由此推测,阔卵圆形叶片可能具更强结构支持能力和更高呼吸速率,利于个体的生理维持,而披针形叶具更高的碳同化能力则用于支持生长.     

Fire as the dominant driver of central Canadian boreal forest carbon balance

Changes in climate, atmospheric carbon dioxide concentration and fire regimes have been occurring for decades in the global boreal forest, with future climate change likely to increase fire frequency--the primary disturbance agent in most boreal forests. Previous attempts to assess quantitatively the effect of changing environmental conditions on the net boreal forest carbon balance have not taken into account the competition between different vegetation types on a large scale. Here we use a process model with three competing vascular and non-vascular vegetation types to examine the effects of climate, carbon dioxide concentrations and fire disturbance on net biome production, net primary production and vegetation dominance in 100 Mha of Canadian boreal forest. We find that the carbon balance of this region was driven by changes in fire disturbance from 1948 to 2005. Climate changes affected the variability, but not the mean, of the landscape carbon balance, with precipitation exerting a more significant effect than temperature. We show that more frequent and larger fires in the late twentieth century resulted in deciduous trees and mosses increasing production at the expense of coniferous trees. Our model did not however exhibit the increases in total forest net primary production that have been inferred from satellite data. We find that poor soil drainage decreased the variability of the landscape carbon balance, which suggests that increased climate and hydrological changes have the potential to affect disproportionately the carbon dynamics of these areas. Overall, we conclude that direct ecophysiological changes resulting from global climate change have not yet been felt in this large boreal region. Variations in the landscape carbon balance and vegetation dominance have so far been driven largely by increases in fire frequency.