Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment

Leaf photosynthesis (Ps), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free‐air CO₂ enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO₂, respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloud‐free days (PPFD_day,sat) around the lowermost branches was 16·8 ± 0·8 and 8·7 ± 0·2% of values at the top for the ambient and elevated CO₂ canopies, respectively. Elevated CO₂ significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD_day,sat throughout the canopies, and elevated CO₂ did not affect that relationship. Leaf Ps light‐response curves responded differently to elevated CO₂, depending upon canopy position. Elevated CO₂ increased Ps_sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO₂. Consequently, estimated daily integrated Ps on cloud‐free days (Ps_day,sat) was stimulated by elevated CO₂ only in the upper canopy. Ps_day,sat of the lowermost branches was actually lower with elevated CO₂ because of the darker light environment. The lack of CO₂ stimulation at the mid‐ and lower canopy was probably related to significant down‐regulation of photosynthetic capacity; there was no down‐regulation of Ps in the upper canopy. The relationship between Ps_day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole‐canopy Ps or photosynthetic N use efficiency. Elevated CO₂ had no effect on the optimization of canopy N allocation.     

Looking inside the box: using Raman microspectroscopy to deconstruct microbial biomass stoichiometry one cell at a time

Stoichiometry of microbial biomass is a key determinant of nutrient recycling in a wide variety of ecosystems. However, little is known about the underlying causes of variance in microbial biomass stoichiometry. This is primarily because of technological constraints limiting the analysis of macromolecular composition to large quantities of microbial biomass. Here, we use Raman microspectroscopy (MS), to analyze the macromolecular composition of single cells of two species of bacteria grown on minimal media over a wide range of resource stoichiometry. We show that macromolecular composition, determined from a subset of identified peaks within the Raman spectra, was consistent with macromolecular composition determined using traditional analytical methods. In addition, macromolecular composition determined by Raman MS correlated with total biomass stoichiometry, indicating that analysis with Raman MS included a large proportion of a cell''s total macromolecular composition. Growth phase (logarithmic or stationary), resource stoichiometry and species identity each influenced each organism''s macromolecular composition and thus biomass stoichiometry. Interestingly, the least variable peaks in the Raman spectra were those responsible for differentiation between species, suggesting a phylogenetically specific cellular architecture. As Raman MS has been previously shown to be applicable to cells sampled directly from complex environments, our results suggest Raman MS is an extremely useful application for evaluating the biomass stoichiometry of environmental microorganisms. This includes the ability to partition microbial biomass into its constituent macromolecules and increase our understanding of how microorganisms in the environment respond to resource heterogeneity.     

氮肥添加对青藏高原高寒草甸6个群落优势种生态化学计量学特征的影响

以青藏高原高寒草甸为研究对象, 通过人工氮肥添加试验, 研究6个群落优势种在不同施氮(N)水平下叶片碳(C)、N、磷(P)元素含量的变化以及生态化学计量学特征。结果表明: 自然条件下, 6个物种叶片N、P质量浓度存在显著的差异, 表现为: 黄花棘豆(Oxytropis ochrocephala)最高, 为24.5和2.51 g·kg^-1, 其叶片N含量低于而P含量高于我国其他草地的豆科植物; 其余5个物种叶片N、P质量浓度分别为11.5-18.1和1.49-1.72 g·kg^-1, 嵩草(Kobresia myosuroides)叶片N含量最低, 垂穗披碱草(Elymus nutans)叶片P含量最低, 与我国其他区域的研究结果相比, 其叶片N和P含量均低于我国其他草地非豆科植物。随氮素添加量的增大, 6种群落优势种叶片的C和P含量保持不变; 其他5种植物叶片N含量显著增加, 黄花棘豆叶片N含量保持不变。未添加氮肥时, 6种植物叶片N:P为7.3-11.2, 说明该区植物生长更多地受N限制。随N添加量的增加, 除黄花棘豆外, 其他5种植物叶片N:P大于16, 表现为植物生长受P限制。综合研究表明, 青藏草原高寒草甸植物叶片N含量较低, 植物受N影响显著, 但不同物种对N的添加反应不同, 豆科植物黄花棘豆叶片对N添加不敏感, 其他5个物种叶片全N含量随着N添加量的升高而增加, 该研究结果可为高寒草甸科学施肥提供理论依据。     

Seasonal correlations of elemental and ω3 PUFA composition of seston and dominant phytoplankton species in a eutrophic Siberian Reservoir

The elemental and fatty acid composition of seston was studied for 3 years, from May to October, in a small Reservoir. Under comparatively low C:P ratio, multivariate canonical analysis revealed no straightforward simple correlations between phosphorus and single ω3 PUFA species, but complex significant interaction between elemental composition (stoichiometry) of seston and total sestonic ω3 PUFA as a whole. Since sestonic C, P and N were found to originate mostly from phytoplankton, the contents of particulate elements and PUFA were attributed to single species in periods of their pronounced dominance. Phytoplankton species of genera of Stephanodiscus, Peridinium, Gomphosphaeria, Planktothrix and Anabaena in periods of their pronounced dominance had relatively constant species-specific elemental and PUFA composition. Phytoplankton species significantly differed in their elemental and PUFA composition, as well as in ratios of C:N, N:P, PUFA:P and partly C:P that indicate food quality for zooplankton. Hence, there were no phytoplankton species of clearly high or low nutritional value. All of phytoplankters, or at least detritus, that originated from them, may meet specific elemental and biochemical requirements of specific groups of zooplankton. Dividing phytoplankton on basis of their elemental and biochemical composition, i.e., nutrition quality, into large taxa (cyanobacteria, diatoms, etc.) appeared to be too coarse for assessing nutritional value for zooplankton.     

全球变化背景下陆地植物N/P生态化学计量学研究进展

生态化学计量学理论最早应用在水生生态系统的研究中,但最近20年来在陆地生态系统中也开展了大量的相关研究,特别是关于全球变化背景下陆地植物N/P生态化学计量学方面的研究得到很大的发展,极大地丰富和提升了我们对陆地植物包括生态系统生态过程的认识。就全球变化背景下陆地植物生态化学计量学的研究现状进行了综述,同时以中国科学院华南植物园90周庆为契机,总结我们关于南亚热带森林植物生态化学计量学的研究工作,进而分析当前存在的一些问题并提出今后研究的发展重点,以期促进和推动我园和我国生态化学计量学相关领域的研究。     

施氮和降水格局改变对土壤CH4和CO2通量的影响

氮沉降增加和降水格局改变是全球变化的两项重要内容,但是同时考虑上述两因素对温室气体CH4和CO2通量影响的原位双因子模拟研究还相当有限.本研究以长白山温带阔叶红松林土壤为研究对象,采用静态箱法研究了外施氮源(50 kg N·hm-2·a-1)和增减30％降水对土壤CH4和CO2通量的影响.结果表明:施氮能抑制土壤CH4吸收,有时甚至能将土壤对CH4的吸收转为释放,但这种抑制效应只能维持5d左右,且能在一定程度上改变CH4通量和环境因子(温度、土壤pH、粘粒含量)的相关关系.降水改变未能显著影响土壤CH4通量.对CO2通量而言,施氮能降低土壤CO2排放,长白山阔叶红松林连续施氮第4年的平均抑制效应为27.4％.长期连续施氮的平均抑制效应随施氮时间延长而逐渐增大,一定年限后达到最大值.单次施氮的抑制效应随时间延长逐渐减弱,并在1个月的施氮周期末期基本消失.施氮的抑制效应和土壤充水孔隙度(WFPS)呈显著负相关关系,且升温能增强施氮对CO2释放的抑制效应并延长抑制时间.施氮、降水有可能改变土壤呼吸的温度敏感性.本研究表明,长白山森林土壤氮素尚未达到一定阈值,未来氮沉降增加将抑制CO2的释放和CH4的吸收,因此总体来看施氮抑制土壤碳排放.     

植物的臭氧污染胁迫效应研究进展

近地面空气中的臭氧(O₃)属于二次污染物,是由氮氧化物(NO_x)和挥发性有机物(VOCs)等前体物在一定的环境条件下形成的。近年来,全球受O₃污染的区域增加,污染程度也日趋严重。O₃污染对植物的危害引起了国内外研究人员的广泛关注。众多研究发现,不同植物对O₃的敏感性不同,其大小主要取决于植物自身的特性及环境因素;O₃污染降低植物的净同化作用,减缓植物生长,改变同化物的分配,可对物种间的相互关系以及生态系统结构产生深远影响。该文在综述了国内外研究进展的基础上,提出我国在O₃污染研究领域应深入研究以下几个方面:1)选育具有对O₃污染抗性较强的植物尤其是作物品种;2)深入研究减轻O₃污染对植物危害的栽培管理措施;3)加强研究O₃污染对我国自然生态系统的影响;4)研究植被在治理O₃污染中的积极作用。     

湘潭锰矿栾树叶片和土壤N、P化学计量特征

湘潭锰矿区废弃地是一个典型的退化生态系统,针对矿区废弃地植被恢复中3个不同林龄的栾树林(3年生、5年生和9年生),测定了林木叶片以及相应土壤的N、P含量,综合分析了不同林龄土壤和林木叶片的化学计量特征。结果表明,随着年龄的增长土壤中N含量呈递增趋势,而P含量为递减趋势,在3个林龄中均表现出显著性差异;3个林龄叶片的N、P含量以及N∶P比值差异显著,N、P含量在年龄增长梯度上表现为降低,而N∶P比值却表现为升高;3个林龄叶片中N含量与N∶P比值之间表现出显著正相关,而P含量与N∶P比值之间却为显著负相关关系;土壤中N、P含量与叶片N∶P比值之间分别存在显著正相关和负相关关系。通过对3个林龄叶片和土壤N、P含量及化学计量特征研究,发现P很有可能成为湘潭锰矿退化生态系统植被恢复过程中植物生长的限制性因子。研究结果可为矿区废弃地植被恢复和经营管理以及森林可持续发展提供科学依据。     

Global change effects on a mechanistic decomposer food web model

Global change may affect the structure and functioning of decomposer food webs through qualitative changes in freshly fallen litter. We analyzed the predicted effects of a changing environment on a dynamic model of a donor‐controlled natural decomposer ecosystem near Wekerom, the Netherlands. This system consists of fungi, bacteria, fungivores, bacterivores and omnivores feeding on microbiota and litter as well. The model concentrates on carbon and nitrogen flows through the trophic niches that define this decomposer system, and is designed to predict litter masses and abundances of soil biota. For modeling purposes, the quality of freshly fallen leaf litter is defined in terms of nitrogenous and non‐nitrogenous components, of which refractory and labile forms are present. The environmental impacts of elevated CO₂, enhanced UV‐B and eutrophication, each with their own influence on leaf litter quality, are studied. The model predicts steady‐state dynamics exclusively, for all three scenarios. Environmental changes impact most demonstratively on the highest trophic niches, and affect microbiotic abundances and litter decomposition rates to a lesser extent. We conclude that the absence of trophic cascade effects may be attributed to weak trophic links, and that non‐equilibrium dynamics occurring in the system are generally because of encounter rates based on fractional substrate densities in the litter. We set out a number of experimentally testable hypotheses that may improve understanding of ecosystem dynamics.