Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida

To evaluate whether leaf spot disease and related effects on photosynthesis are influenced by increased nitrogen (N) input and elevated atmospheric CO₂ concentration ([CO₂]), we examined disease incidence and photosynthetic rate of Solidago rigida grown in monoculture under ambient or elevated (560 μmol mol⁻¹) [CO₂] and ambient or elevated (+4 g N m⁻² year⁻¹) N conditions in a field experiment in Minnesota, USA. Disease incidence was lower in plots with either elevated [CO₂] or enriched N (−57 and −37%, respectively) than in plots with ambient conditions. Elevated [CO₂] had no significant effect on total plant biomass, or on photosynthetic rate, but reduced tissue%N by 13%. In contrast, N fertilization increased both biomass and total plant N by 70%, and as a consequence tissue%N was unaffected and photosynthetic rate was lower on N fertilized plants than on unfertilized plants. Regardless of treatment, photosynthetic rate was reduced on leaves with disease symptoms. On average across all treatments, asymptomatic leaf tissue on diseased leaves had 53% lower photosynthetic rate than non-diseased leaves, indicating that the negative effect from the disease extended beyond the visual lesion area. Our results show that, in this instance, indirect effects from elevated [CO₂], i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher [CO₂].     

Elevated CO2 alters belowground exoenzyme activities in tussock tundra

A three-year exposure to a CO2 concentration of 680 mol mol-1 altered the enzymic characteristics of root surfaces, associated ectomycorrhizae, and in soils surrounding roots in a tussock tundra ecosystem of north Alaska, USA. At elevated CO2, phosphatase activity was higher on Eriophorum vaginatum root surfaces, ectomycorrhizal rhizomorphs and mantles associated with Betula nana roots, and in Oe and Oi soil horizons associated with plant roots. Also, endocellulase and exocellulase activities at elevated CO2 were higher in ectomycorrhizal rhizomorphs and lower in Oe and Oi soil horizons associated with roots. These results suggest that arctic plants respond to raised CO2 by increasing activities associated with nutrient acquisition, e.g. higher phosphatase activities on surfaces of roots and ectomycorrhizae, and greater cellulase activity in ectomycorrhizae. Changes in enzyme activities of surrounding soils are consistent with an increase in carbon exudation from plant roots, which would be expected to inhibit cellulase activities and stimulate phosphatase activities of soil microflora. These data were used to modify existing simulation models describing tussock phosphatase activities and litter decay. Model projections suggest that observed increases in phosphatase activities at 680 mol mol-1 CO2 could augment total annual phosphorus release within tussocks by more than 40%, at present levels of root and ectomycorrhizae biomass. This includes a nearly three-fold increase in potential phosphatase activity of E. vaginatum roots, per unit of surface area. Observed reductions in cellulase activities could diminish cellulose turnover by 45% in soils within rooting zones, which could substantially increase mineral nitrogen availability in soils due to lowered microbial immobilization.     

13C discrimination during CO2 assimilation by the terrestrial biosphere

Estimates of the extent of the discrimination against¹³CO₂ during photosynthesis (_A) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO₂ mole fractions and then, in association with leaf internal conductances, to calculate _A. Weighted according to gross primary productivity (GPP, annual net CO₂ asimilation per unit ground area), estimated _A for C₃ biomes ranged from 12.9 for xerophytic woods and shrub to 19.6 for cool/cold deciduous forest, with an average value from C₃ plants of 17.8. This is slightly less than the commonly used values of 18–20. For C₄ plants the average modelled discrimination was 3.6, again slightly less than would be calculated from C₄ plant dry matter carbon isotopic composition (yielding around 5). From our model we estimate that, on a global basis, 21% of GPP is by C₄ plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8. There are large variations in _A across the globe, the largest of which are associated with the precence or absence of C₄ plants. Due to longitudinal variations in _A, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.     

A meta-analytical test of elevated CO2 effects on plant respiration

Contrasting results regarding elevated CO₂ effects on leafdark respiration (Rd) have hampered efforts to incorporate this importantcomponent of the plant carbon budget into long-term predictions of ecologicalresponses to rising atmospheric CO₂. To help resolve some of theseinconsistencies in the literature, we used meta-analysis to quantitativelysummarize 45 area-based leaf Rd (Rd_a) and 44 mass-based leaf Rd(Rd_m) observations from independent studies on 33 species. Ouranalysis showed that across all studies, leaf Rd_m was significantlyreduced (–18%, P < 0.05), while leaf Rd_a was marginallyincreased (+8%, P < 0.15), under elevated CO₂. There weresignificant differences among categorical groups in CO₂ effects onleaf Rd_a and Rd_m. For example, leaf Rd_a ofherbaceous species increased 28%, but leaf Rd_a of woody speciesremained unchanged under elevated CO₂. Plants exposed to elevatedCO₂ for < 60 days had significantly higher leaf Rd_a atelevated compared to ambient CO₂, while plants exposed to elevatedCO₂ for longer period of time showed no response. The magnitude ofreduction in leaf Rd_m for plants exposed to elevated CO₂for > 100 days was significantly greater than that for plants exposed toelevated CO₂ for < 100 days. Our meta-analysis of publishedresults suggest that the amount of carbon loss through leaf Rd will likelyincrease in a higher CO₂ environment because of higher leafRd_a and a proportionally greater leaf biomass increase than leafRd_m reduction at elevated CO₂. Our results alsodemonstrated the strong dependency of Rd responses to elevated CO₂onexperimental conditions.     

Crop responses to CO2 enrichment

Carbon dioxide is rising in the global atmosphere, and this increase can be expected to continue into the foreseeable future. This compound is an essential input to plant life. Crop function is affected across all scales from biochemical to agro-ecosystem. An array of methods (leaf cuvettes, field chambers, free-air release systems) are available for experimental studies of CO₂ effects. Carbon dioxide enrichment of the air in which crops grow usually stimulates their growth and yield. Plant structure and physiology are markedly altered. Interactions between CO₂ and environmental factors that influence plants are known to occur. Implications for crop growth and yield are enormous. Strategies designed to assure future global food security must include a consideration of crop responses to elevated atmospheric CO₂. Future research should include these targets: search for new insights, development of new techniques, construction of better simulation models, investigation of belowground processes, study of interactions, and the elimination of major discrepancies in the scientific knowledge base.     

C4 photosynthesis, atmospheric CO2, and climate

The objectives of this synthesis are (1) to review the factors that influence the ecological, geographical, and palaeoecological distributions of plants possessing C₄ photosynthesis and (2) to propose a hypothesis/model to explain both the distribution of C₄ plants with respect to temperature and CO₂ and why C₄ photosynthesis is relatively uncommon in dicotyledonous plants (hereafter dicots), especially in comparison with its widespread distribution in monocotyledonous species (hereafter monocots). Our goal is to stimulate discussion of the factors controlling distributions of C₄ plants today, historically, and under future elevated CO₂ environments. Understanding the distributions of C₃/C₄ plants impacts not only primary productivity, but also the distribution, evolution, and migration of both invertebrates and vertebrates that graze on these plants. Sixteen separate studies all indicate that the current distributions of C₄ monocots are tightly correlated with temperature: elevated temperatures during the growing season favor C₄ monocots. In contrast, the seven studies on C₄ dicot distributions suggest that a different environmental parameter, such as aridity (combination of temperature and evaporative potential), more closely describes their distributions. Differences in the temperature dependence of the quantum yield for CO₂ uptake (light-use efficiency) of C₃ and C₄ species relate well to observed plant distributions and light-use efficiency is the only mechanism that has been proposed to explain distributional differences in C₃/C₄ monocots. Modeling of C₃ and C₄ light-use efficiencies under different combinations of atmospheric CO₂ and temperature predicts that C₄-dominated ecosystems should not have expanded until atmospheric CO₂ concentrations reached the lower levels that are thought to have existed beginning near the end of the Miocene. At that time, palaeocarbonate and fossil data indicate a simultaneous, global expansion of C₄-dominated grasslands. The C₄ monocots generally have a higher quantum yield than C₄ dicots and it is proposed that leaf venation patterns play a role in increasing the light-use efficiency of most C₄ monocots. The reduced quantum yield of most C₄ dicots is consistent with their rarity, and it is suggested that C₄ dicots may not have been selected until CO₂ concentrations reached their lowest levels during glacial maxima in the Quaternary. Given the intrinsic light-use efficiency advantage of C₄ monocots, C₄ dicots may have been limited in their distributions to the warmest ecosystems, saline ecosystems, and/or to highly disturbed ecosystems. All C₄ plants have a significant advantage over C₃ plants under low atmospheric CO₂ conditions and are predicted to have expanded significantly on a global scale during full-glacial periods, especially in tropical regions. Bog and lake sediment cores as well as pedogenic carbonates support the hypothesis that C₄ ecosystems were more extensive during the last glacial maximum and then decreased in abundance following deglaciation as atmospheric CO₂ levels increased. Received: 12 February 1997 / Accepted: 20 June 1997     

兴安落叶松林碳平衡和全球变化影响研究

利用CENTURY模型模拟兴安落叶松林的C循环并探讨全球变化对其C循环的影响,结果表明,兴安落叶松林是一个C汇,年净吸收C2.65t.hm^-2,气候变化和大气CO2浓度增加将对北方森林的生长有利,使其净吸收C的能力增强,温度上升2℃时,兴安落叶松林的植物总生物量和生产力均增加,而土壤C含量降低,降水减少20％比降水增加20％时其植物总生物量,生产力和土壤C含量变化的幅度大,说明温度是大兴安岭地区森林生长的主要限制因子。     

Elevated carbon dioxide concentrations indirectly affect plant fitness by altering plant tolerance to herbivory

Global environmental changes, such as rising atmospheric CO₂ concentrations, have a wide range of direct effects on plant physiology, growth, and fecundity. These environmental changes also can affect plants indirectly by altering interactions with other species. Therefore, the effects of global changes on a particular species may depend on the presence and abundance of other community members. We experimentally manipulated atmospheric CO₂ concentration and amounts of herbivore damage (natural insect folivory and clipping to simulate browsing) to examine: (1) how herbivores mediate the effects of elevated CO₂ (eCO₂) on the growth and fitness of Arabidopsis thaliana; and (2) how predicted changes in CO₂ concentration affect plant resistance to herbivores, which influences the amount of damage plants receive, and plant tolerance of herbivory, or the fitness consequences of damage. We found no evidence that CO₂ altered resistance, but plants grown in eCO₂ were less tolerant of herbivory—clipping reduced aboveground biomass and fruit production by 13 and 22%, respectively, when plants were reared under eCO₂, but plants fully compensated for clipping in ambient CO₂ (aCO₂) environments. Costs of tolerance in the form of reduced fitness of undamaged plants were detected in eCO₂ but not aCO₂ environments. Increased costs could reduce selection on tolerance in eCO₂ environments, potentially resulting in even larger fitness effects of clipping in predicted future eCO₂ conditions. Thus, environmental perturbations can indirectly affect both the ecology and evolution of plant populations by altering both the intensity of species interactions as well as the fitness consequences of those interactions.     

Assimilation,respiration and allocation of carbon inPlantago major as affected by atmospheric CO2 levels

The response ofPlantago major ssp,pleiosperma plants, grown on nutrient solution in a climate chamber, to a doubling of the ambient atmospheric CO₂ concentration was investigated. Total dry matter production was increased by 30% after 3 weeks of exposure, due to a transient stimulation of the relative growth rate (RGR) during the first 10 days. Thereafter RGR returned to the level of control plants. Photosynthesis, expressed per unit leaf area, was stimulated during the first two weeks of the experiment, thereafter it dropped and nearly reached the level of the control plants. Root respiration was not affected by increased atmospheric CO₂ levels, whereas shoot, dark respiration was stimulated throughout the experimental period. Dry matter allocation over leaves stems and roots was not affected by the CO₂ level. SLA was reduced by 10%, which can partly be explained by an increased dry matter content of the leaves. Both in the early and later stages of the experiment, shoot respiration accounted for a larger part of the carbon budget in plants grown at elevated atmospheric CO₂. Shifts in the total carbon budget were mainly due to the effects on shoot respiration. Leaf growth accounted for nearly 50% of the C budget at all stages of the experiment and in both treatments.Abbreviations LAR leaf area ratio - LWR leaf weight ratio - RGR relative growth rate - R/S root to shoot ratio - RWR root weight ratio - SLA specific leaf area - SWR stem weight ratio     

Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands

Root, arbuscular-mycorrhizal (AM), soil faunal (protozoa and microarthropods), and microbial responses to field exposure to CO₂ for six growing seasons were measured in spring 1997 in two adjacent grassland communities. The grasslands showed contrasting root responses to CO₂ enrichment: whereas root length was not affected in the sandstone grassland, it was greater in the serpentine grassland, as was specific root length. AM fungal hyphal lengths were greater in the sandstone, but were unaffected in the serpentine community. This lent support to the hypothesis that there may be a tradeoff in resource allocation to more fine roots or greater mycorrhizal extraradical hyphal length. AM root infection was greater in both communities at elevated CO₂, as was the proportion of roots containing arbuscules. Our data on total hyphal lengths, culturable and active fungi, bacteria, and protozoa supported the hypothesis that the fungal food chain was more strongly stimulated than the bacterial chain. This study is one of the first to test these hypotheses in natural multi-species communities in the field. Received: 23 October 1998 / Accepted: 19 February 1999     

Elevated CO2 increases fungal-based micro-foodwebs in soils of contrasting plant species

Plant and Soil - As different plant species support different soil communities, assessment of climate change impacts on soil communities must consider how these impacts depend on the vegetation. We...     

开放系统中农作物对空气CO2浓度增加的响应

FACE试验（free-air CO2 enrichment)开展的10多年中,供试农作物主要有：C3禾本科作物小麦（Triticum aestivum L.）、多年生黑麦草（Lolium perenne)和水稻（Oryza sativaL.),C4禾本科类高梁（Sorghum bicolor(L.)Moench),C3豆科植物白三叶草（Trifolium repens ),C3非禾本科块茎状作物马铃薯（Solanum tuberosum L.）,以及多年生C3类木作物棉花（Gossypium hirsutum L.)和葡萄（Vitisvinifera l.)。本文系统整理和分析了以下各项参数的结果;光合作用、气孔导度、冠层温度、水分利用、水势、叶面积指数、根茎生物量累积、作物产量、辐射利用率,比叶面积、N含量、N收益、碳水化合物含量、物候变化、土壤微生物、土壤呼吸、痕量气体交换以及土壤碳固定,CO2浓度升高对农作物的影响作用主要表现在以下方面：（1）促进了植物光合作用,增加了其生物量累积;（2）显著提高C3作物产量,但对C4作物产量的影响很小;（3）降低了C3和C4作物气孔导度,非常显著地提高了所有作物的水分利用率;（4）对植物生长的促进作用在水分不足与水分充中时二者相当或前者大于后者;（6）对根系生长的促进作用要大于地上部分;（7）对多年生植物气孔导度的影响较小,但对其生长的促进作用仍很高;（8）降低了植物体内N含量,但作物体内碳水化合物及某些其他含碳化合物含量增加,且叶部含量要明显高于植物其他器官;（9）对大多数作物的物候略有加速;（10）对某些土壤微生物具显著影响,而对有些则无,但都增加了微生物活性;（11）综合多年、多地点的试验结果表明土壤对大气CO2的固定增加,但单独一个试验无法观测到SOC的显著性变化,对FACE和前期的熏气室试验结果都进行了尽可能的对比研究,除了二例以外,发现在大多数情况下二者的结果基本一致,其中,FACE使气孔导度降低的1.5倍,明显高于前期熏气室试验的结果;其二,相对于熏气室,FACE条件下CO2倍增对根的相对促进作用要高于地上部分,因此,我们对基于这二者的结论的准确性和可靠性是充满信心的,不过,更接近自然环境和具更大小区面积的FACE试验仍是必需的,它可以为我们提供在CO2升高条件下更具代表性的田间试验条件,从而为我们提供更多、更有益的多学科交叉的试验数据和研究结果。     

Modeled responses of terrestrial ecosystems to elevated atmospheric CO2: a comparison of simulations by the biogeochemistry models of the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP)

Although there is a great deal of information concerning responses to increases in atmospheric CO₂ at the tissue and plant levels, there are substantially fewer studies that have investigated ecosystem-level responses in the context of integrated carbon, water, and nutrient cycles. Because our understanding of ecosystem responses to elevated CO₂ is incomplete, modeling is a tool that can be used to investigate the role of plant and soil interactions in the response of terrestrial ecosystems to elevated CO₂. In this study, we analyze the responses of net primary production (NPP) to doubled CO₂ from 355 to 710 ppmv among three biogeochemistry models in the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP): BIOME-BGC (BioGeochemical Cycles), Century, and the Terrestrial Ecosystem Model (TEM). For the conterminous United States, doubled atmospheric CO₂ causes NPP to increase by 5% in Century, 8% in TEM, and 11% in BIOME-BGC. Multiple regression analyses between the NPP response to doubled CO₂ and the mean annual temperature and annual precipitation of biomes or grid cells indicate that there are negative relationships between precipitation and the response of NPP to doubled CO₂ for all three models. In contrast, there are different relationships between temperature and the response of NPP to doubled CO₂ for the three models: there is a negative relationship in the responses of BIOME-BGC, no relationship in the responses of Century, and a positive relationship in the responses of TEM. In BIOME-BGC, the NPP response to doubled CO₂ is controlled by the change in transpiration associated with reduced leaf conductance to water vapor. This change affects soil water, then leaf area development and, finally, NPP. In Century, the response of NPP to doubled CO₂ is controlled by changes in decomposition rates associated with increased soil moisture that results from reduced evapotranspiration. This change affects nitrogen availability for plants, which influences NPP. In TEM, the NPP response to doubled CO₂ is controlled by increased carboxylation which is modified by canopy conductance and the degree to which nitrogen constraints cause down-regulation of photosynthesis. The implementation of these different mechanisms has consequences for the spatial pattern of NPP responses, and represents, in part, conceptual uncertainty about controls over NPP responses. Progress in reducing these uncertainties requires research focused at the ecosystem level to understand how interactions between the carbon, nitrogen, and water cycles influence the response of NPP to elevated atmospheric CO₂. Received: 13 December 1996 / Accepted: 20 November 1997     

The impact of elevated CO2 on plant-herbivore interactions: experimental evidence of moderating effects at the community level

Surprisingly little research has been published on the responses to elevated [CO₂] at the community level, where herbivores can select their preferred food. We investigated the combined effects of atmospheric [CO₂] and herbivory on synthesised plant communities growing on soils of different fertility. Factorial combinations of two [CO₂] (350 or 700 l l⁻¹), two fertility (fertilised or non-fertilised), and two herbivory (herbivores present or absent) treatments were applied to a standard mixture of seven fast- and eight slow-growing plants in outdoor microcosms. The herbivores used were the grain aphid (Sitobion avenae) and the garden snail (Helix aspersa). We measured plant biomass, foliar nitrogen and soluble tannin concentration, aphid fecundity, and snail growth, fecundity, and feeding preferences over one growing season. Elevated [CO₂] did not have a significant impact on (1) the combined biomass of fast-growing or slow-growing plants, (2) herbivore feeding preferences, or (3) herbivore fitness. There was, however, a significant biomass increase of Carex flacca (which represented in all cases less than 5% of total live biomass), and some chemical changes in unpalatable plants under elevated [CO₂]. The herbivory treatment significantly increased the biomass of slow-growing plants over fast-growing plants, whereas fertilisation significantly increased the abundance of fast-growing plants over slow-growing plants. Predictions on the effects of elevated [CO₂] based on published single-species experiments were not supported by the results of this microcosm study. Received: 30 November 1997 / Accepted: 24 July 1998     

Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts

Despite predictions that both atmospheric CO₂ concentrations and air temperature will rise together, very limited data are currently available to assess the possible interactive effects of these two global change factors on temperate forest tree species. Using yellow birch (Betula alleghaniensis) as a model species, we studied how elevated CO₂ (800 vs. 400 μl l⁻¹) influences seedling growth and physiological responses to a 5°C increase in summer air temperatures (31/26 vs. 26/21°C day/night), and how both elevated CO₂ and air temperature during the growing season influence seedling ability to survive freezing stress during the winter dormant season. Our results show that while increased temperature decreases seedling growth, temperature-induced growth reductions are significantly lower at elevated CO₂ concentrations (43% vs. 73%). The amelioration of high-temperature stress was related to CO₂-induced reductions in both whole-shoot dark respiration and transpiration. Our results also show that increased summer air temperature, and to a lesser degree CO₂ concentration, make dormant winter buds less susceptible to freezing stress. We show the relevance of these results to models used to predict how climate change will influence future forest species distribution and productivity, without considering the direct or interactive effects of CO_2. Received: 5 June 1997 / Accepted: 16 December 1997     

Trophic interactions in a changing world: modelling aboveground–belowground interactions

The rate and scale of human-driven changes can exert profound impacts on ecosystems, the species that make them up and the services they provide that sustain humanity. Given the speed at which these changes are occurring, one of society's major challenges is to coexist within ecosystems and to manage ecosystem services in a sustainable way. The effect of possible scenarios of global change on ecosystem services can be explored using ecosystem models. Such models should adequately represent ecosystem processes above and below the soil surface (aboveground and belowground) and the interactions between them. We explore possibilities to include such interactions into ecosystem models at scales that range from global to local. At the regional to global scale we suggest to expand the plant functional type concept (aggregating plants into groups according to their physiological attributes) to include functional types of aboveground–belowground interactions. At the scale of discrete plant communities, process-based and organism-oriented models could be combined into “hybrid approaches” that include organism-oriented mechanistic representation of a limited number of trophic interactions in an otherwise process-oriented approach. Under global change the density and activity of organisms determining the processes may change non-linearly and therefore explicit knowledge of the organisms and their responses should ideally be included. At the individual plant scale a common organism-based conceptual model of aboveground–belowground interactions has emerged. This conceptual model facilitates the formulation of research questions to guide experiments aiming to identify patterns that are common within, but differ between, ecosystem types and biomes. Such experiments inform modelling approaches at larger scales. Future ecosystem models should better include this evolving knowledge of common patterns of aboveground–belowground interactions. Improved ecosystem models are necessary tools to reduce the uncertainty in the information that assists us in the sustainable management of our environment in a changing world.

Zusammenfassung

Rate und Ausmaß menschen-gemachter Veränderungen wirken sich auf Ökosysteme, die Arten die diese zusammensetzen und Ökosystemfunktionen von denen die Menschheit abhängt aus. Angesichts der Geschwindigkeit dieser Veränderungen ist es eine der großen Herausforderungen der Gesellschaft miteinander und in Ökosystemen zu leben und deren Ökosystemfunktionen nachhaltig zu nutzen. Die Auswirkungen plausibler Szenarien des Globalen Wandels auf Ökosystemfunktionen können mit Hilfe von Ökosystemmodellen untersucht werden. Solche Modelle sollten die Ökosystemprozesse oberhalb und unterhalb der Erdoberfläche („oberirdisch und unterirdisch“) und die Interaktionen zwischen diesen Prozessen angemessen abbilden. Auf Skalenebenen, die von global bis lokal reichen, erkunden wir in diesem Artikel Möglichkeiten solche Interaktionen in Modelle einzubauen. Auf der regionalen bis globalen Ebene schlagen wir vor das Konzept der funktionellen Pflanzentypen (Pflanzenarten, die aufgrund von physiologischen Ähnlichkeiten in Gruppen zusammengefasst sind) auszudehnen, so dass Typen von oberirdisch-unterirdischen Interaktionen mitenthalten sind. Auf der Skalenebene eigenständiger Pflanzengesellschaften könnten prozessbasierte und organsimen-orientierte Modelle zu „Hybridmodellen“verschmolzen werden, die organismen-orientierte, mechanistische Abbildungen einiger trophischer Interaktionen enthalten, aber ansonsten prozess-basiert sind. Der Einfluss des Globalen Wandels auf die Häufigkeit und Aktivität von Organismen und die Ökosystemprozesse, die sie bestimmen, ist sehr wahrscheinlich häufig nicht-linear, so dass im Idealfall explizites Wissen über die Organismen und ihre Reaktionen in Modellen enthalten sein sollte. Auf der Skalenebene der einzelnen Pflanze hat sich ein gebräuchliches, organismen-basiertes Konzeptmodell der oberirdisch-unterirdisch Interaktionen herausgebildet. Dies erleichtert die Formulierung von Hypothesen und Fragestellungen in Experimenten, die nach gemeinsamen Mustern innerhalb von Ökosystemen und Unterschieden zwischen Ökosystemtypen und Biomen suchen. Dies ist die Basis für Modellierungsansätze auf größeren Skalenebenen. Zukünftige Ökosystemmodelle sollten die gemeinsamen Muster oberirdisch-unterirdischer Interaktionen besser berücksichtigen, die sich neuerdings abzuzeichnen beginnen. Verbesserte Ökosystemmodelle sind notwendige Werkzeuge um die Unsicherheit in der Information zu vermindern, auf der nachhaltiges Umweltmanagement in einer sich wandelnden Welt beruht.     

Elevated atmospheric CO2 increases microbial growth rates in soil: results of three CO2 enrichment experiments

Increasing the belowground translocation of assimilated carbon by plants grown under elevated CO₂ can cause a shift in the structure and activity of the microbial community responsible for the turnover of organic matter in soil. We investigated the long‐term effect of elevated CO₂ in the atmosphere on microbial biomass and specific growth rates in root‐free and rhizosphere soil. The experiments were conducted under two free air carbon dioxide enrichment (FACE) systems: in Hohenheim and Braunschweig, as well as in the intensively managed forest mesocosm of the Biosphere 2 Laboratory (B2L) in Oracle, AZ. Specific microbial growth rates (μ) were determined using the substrate‐induced respiration response after glucose and/or yeast extract addition to the soil. For B2L and both FACE systems, up to 58% higher μ were observed under elevated vs. ambient CO₂, depending on site, plant species and N fertilization. The μ‐values increased linearly with atmospheric CO₂ concentration at all three sites. The effect of elevated CO₂ on rhizosphere microorganisms was plant dependent and increased for: Brassica napus=Triticum aestivum<Beta vulgaris<Populus deltoides. N deficiency affected microbial growth rates directly (N limitation) and indirectly (changing the quantity of fine roots). So, 50% decrease in N fertilization caused the overall increase or decrease of microbial growth rates depending on plant species. The μ‐value increase was lower for microorganisms growing on yeast extract then for those growing on glucose, i.e. the effect of elevated CO₂ was smoothed on rich vs. simple substrate. So, the r/K strategies ratio can be better revealed by studying growth on simple (glucose) than on rich substrate mixtures (yeast extract). Our results clearly showed that the functional characteristics of the soil microbial community (i.e. specific growth rates) rather than total microbial biomass amount are sensitive to increased atmospheric CO₂. We conclude that the more abundant available organics released by roots at elevated CO₂ altered the ecological strategy of the soil microbial community specifically a shift to a higher contribution of fast‐growing r‐selected species was observed. These changes in functional structure of the soil microbial community may counterbalance higher C input into the soil under elevated atmospheric CO₂ concentration.     

Elevated CO2 alters carbon fluxes in early successional Mediterranean ecosystems

Annual carbon budgets of ecosystems are central to our understanding of the biotic control of atmospheric composition, but they are not available under elevated CO₂ for most vegetation types. Using gas exchange techniques, we assessed carbon fluxes of four early successional Mediterranean model communities, consisting of grasses, legumes and composites. The assemblages were grown on the same monoliths for three consecutive years in greenhouses tracking field conditions except for CO₂ maintained at ambient (370 μmol mol^?1) or elevated (700 μmol mol^?1) concentration. During the third year of study, CO₂ enrichment consistently shifted the annual carbon balance towards lower efflux, with displacements between 4.3 and 26.2 mol m^?2 y^?1 (one assemblage became a net CO₂ sink, another just reached equilibrium, and the remaining two remained as a CO₂ source). At least 50% of the shift under elevated CO₂ originated from a decrease in belowground respiration. This indicates that, during this year, CO₂ enrichment did not predominantly enhance C‐cycling, but on the contrary inhibited root respiration or microbial C‐utilization. Although elevated‐CO₂‐grown systems acted as a net CO₂ sink during a longer period of the year (4–7 months) compared with ambient‐CO₂‐grown systems (3–3.5 months), gross canopy photosynthesis was modified only to a limited extent (between ?5.9 and + 14.8%). Interaction between the carbon and the water cycle was apparently responsible for this weak stimulation. In particular, reduced evapotranspiration under elevated CO₂ coincided with inhibited canopy photosynthesis in early spring, most likely resulting from water saturation of the soil. In addition, only the earliest‐planted assemblages had an increased gross canopy photosynthesis during late autumn and early winter. This suggests that a longer summer drought, by delaying the establishment of such an annual type of vegetation, would reduce the positive impact of elevated CO₂ on productivity. Water regime appears to strongly govern the influence of CO₂ on the carbon fluxes in Mediterranean ecosystems with annual herbaceous vegetation.     

Elevated CO2 increases nitrogen rhizodeposition and microbial immobilization of root-derived nitrogen

With this study, we aimed to determine how elevated CO(2) affects rhizodeposition and the cycling of rhizodeposited nitrogen (N) in the soil under C(3) and C(4) plants. In addition, we examined how cultivated genotypes of wheat (Triticum turgidum) and maize (Zea mays) responded to elevated CO(2) in comparison with their wild relatives. By constructing an N-transfer experiment we could directly assess cycling of the rhizodeposited N and trace the fate of rhizodeposited N in the soil and in receiver plants. Biomass production, rhizodeposition and cycling of root-borne N in maize genotypes were not affected by elevated CO(2). Elevated CO(2) stimulated above- and below-ground biomass production of the wheat genotypes on average by 38%, and increased rhizodeposition and immobilization of root-derived N on average by 30%. Concurrently, elevated CO(2) reduced mineral (15)N and re-uptake of the root-derived N by 50% in wheat. This study shows that elevated CO(2) may enhance N limitation by increasing N rhizodeposition and subsequent immobilization of the root-derived N.