Effects of CO2 and nutrient enrichment on tissue quality of two California annuals

The effects of CO₂ enrichment and soil nutrient status on tissue quality were investigated and related to the potential effect on growth and decomposition. Two California annuals, Avena fatua and Plantago erecta, were grown at ambient and ambient plus 35 Pa atmospheric CO₂ in nutrient unamended and amended serpentine soil. Elevated CO₂ led to significantly increased Avena shoot nitrogen concentrations in the nutrient amended treatment. It also led to decreased lignin concentrations in Avena roots in both nutrient treatments, and in Plantago shoots and roots with nutrient addition. Concentrations of total nonstructural carbohydrate (TNC) and carbon did not change with elevated CO₂ in either species. As a consequence of increased biomass accumulation, increased CO₂ led to larger total pools of TNC, lignin, total carbon, and total nitrogen in Avena with nutrient additions. Doubling CO₂ had no significant effect on Plantago. Given the limited changes in the compounds related to decomposibility and plant growth, effects of increased atmospheric CO₂ mediated through tissue composition on Avena and Plantago are likely to be minor and depend on site fertility. This study suggests that other factors such as litter moisture, whether or not litter is on the ground, and biomass allocation among roots and shoots, are likely to be more important in this California grassland ecosystem. CO₂ could influence those directly as well as indirectly.     

Individual vs. population plastic responses to elevated CO2, nutrient availability, and heterogeneity: a microcosm experiment with co-occurring species

We conducted an experiment to evaluate the plastic phenotypic responses of individuals, growing under intra-specific competition, and populations of three co-occurring grassland species (Lolium perenne, Plantago lanceolata, and Holcus lanatus) to joint variations in atmospheric CO₂ partial pressure (P _CO2; 37.5 vs. 70 Pa), nutrient availability (NA; 40 vs. 120 mg N added as organic material), and the spatial pattern of nutrient supply (SH; homogeneous vs. heterogeneous nutrient supply). At both the population and individual levels, the aboveground biomass of the three species significantly increased when the nutrients were heterogeneously supplied. Significant two- (SH × NA) and three-term (P _CO2 × NA × SH) interactions determined the response of traits measured on populations (aboveground biomass and below: aboveground biomass ratio, BAR) and individuals (aboveground biomass and specific leaf area). The combination of a high SH and NA elicited the highest plasticity of aboveground biomass in populations and individuals of the three species evaluated, and of BAR in Holcus. Soil heterogeneity and elevated P _CO2 elicited the highest plasticity in the SLA of Plantago and Lolium individuals. Our results show that populations, and not only individuals, respond to soil heterogeneity in a plastic way, and that plastic responses to elevated P _CO2 are complex since they vary across traits and species, and are influenced by the availability of nutrients and by their spatial distribution. They also emphasize the importance of soil heterogeneity as a modulator of plant responses to global change drivers. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Responsible Editor: Angela Hodge     

Effects of CO2 enrichment,nutrient addition,and fungal endophyte-infection on the growth of two grasses

Summary Increasing atmospheric carbon dioxide (CO₂) concentration is expected to increase plant productivity and alter plant/plant interactions, but little is known about its effects on symbiotic interactions with microorganisms. Interactions between perennial ryegrass, Lolium perenne (a C3 plant), and purpletop grass, Tridens flavus (a C4 plant), and their clavicipitaceous fungal endophytes (Acremonium lolii and Balansia epichloe, respectively) were investigated by growing the grasses under 350 and 650 l l^– 1 CO₂ at two nutrient levels. Infected and uninfected perennial ryegrass responded with increased growth to both CO₂ enrichment and nutrient addition. Biomass and leaf area of infected and uninfected plants responded similarly to CO₂ enrichment. When growth analysis parameters were calculated, there were significant increases in relative growth rate and net assimilation rate of infected plants compared to uninfected plants, although the differences remained constant across CO₂ and nutrient treatments. Growth of purpletop grass did not increase with CO₂ enrichment or nutrient addition and there were no significant differences between infected and uninfected plants. CO₂ enrichment did not alter the interactions between these two host grasses and their endophytic-fungal symbionts.     

Limitations to CO2-induced growth enhancement in pot studies

Recently, it has been suggested that small pots may reduce or eliminate plant responses to enriched CO₂ atmospheres due to root restriction. While smaller pot volumes provide less physical space available for root growth, they also provide less nutrients. Reduced nutrient availability alone may reduce growth enhancement under elevated CO₂. To investigate the relative importance of limited physical rooting space separate from and in conjunction with soil nutrients, we grew plants at ambient and double-ambient CO₂ levels in growth containers of varied volume, shape, nutrient concentration, and total nutrient content. Two species (Abutilon theophrasti, a C₃ dicot with a deep tap root andSetaria faberii, a C₄ monocot with a shallow diffuse root system) were selected for their contrasting physiology and root architecture. Shoot demography was determined weekly and biomass was determined after eight and ten weeks of growth. Increasing total nutrients, either by increasing nutrient concentration or by increasing pot size, increased plant growth. Further, increasing pot size while maintaining equal total nutrients per pot resulted in increased total biomass for both species. CO₂-induced growth and reproductive yield enhancements were greatest in pots with high nutrient concentrations, regardless of total nutrient content or pot size, and were also mediated by the shape of the pot. CO₂-induced growth and reproductive yield enhancements were unaffected by pot size (growth) or were greater in small pots (reproductive yield), regardless of total nutrient content, contrary to predictions based on earlier studies. These results suggest that several aspects of growth conditions within pots may influence the CO₂ responses of plants; pot size, pot shape, the concentration and total amount of nutrient additions to pots may lead to over-or underestimates of the CO₂ responses of real-world plants.     

Effects of elevated CO2 and nitrogen on growth ofPoa pratensis (L.)

The growth responses of a grass,Poa pratensis, to elevated CO₂ and nitrogen were investigated. Light-saturated photosynthetic rate per unit leaf area increased with exposure to elevated CO₂, while dry weight did not respond to increased CO₂. Patterns of biomass allocation within plants, including leaf area, leaf area ratio, specific leaf area, and root to shoot ratios, were not altered by elevated CO₂, but changed considerably with N treatment Shoot and whole-plant tissue N concentrations were significantly diluted by elevated CO₂ (Tukey test, P < 0.05). Total N content did not differ significantly among CO₂ treatments. The absence of a concomitant increase in N uptake under elevated CO₂ may have caused a dilution in plant tissue [N], probably negating the positive effects of increased photosynthesis on biomass accumulation.     

Responses in stomatal conductance to elevated CO2 in 12 grassland species that differ in growth form

Responses in stomatal conductance (g _st ) and leaf xylem pressure potential ( _leaf ) to elevated CO₂ (2x ambient) were compared among 12 tallgrass prairie species that differed in growth form and growth rate. Open-top chambers (OTCs, 4.5 m diameter, 4.0 m in height) were used to expose plants to ambient and elevated CO₂ concentrations from April through November in undisturbed tallgrass prairie in NE Kansas (USA). In June and August, _leaf was usually higher in all species at elevated CO₂ and was lowest in adjacent field plots (without OTCs). During June, when water availability was high, elevated CO₂ resulted in decreased g _st in 10 of the 12 species measured. Greatest decreases in g _st (ca. 50%) occurred in growth forms with the highest potential growth rates (C₃ and C₄ grasses, and C₃ ruderals). In contrast, no significant decrease in g _st was measured in the two C₃ shrubs. During a dry period in September, reductions in g _st at elevated CO₂ were measured in only two species (a C₃ ruderal and a C₄ grass) whereas increased g _st at elevated CO₂ was measured in the shrubs and a C₃ forb. These increases in g _st were attributed to enhanced _leaf in the elevated CO₂ plants resulting from increased soil water availability and/or greater root biomass. During a wet period in September, only reductions in g _st were measured in response to elevated CO₂. Thus, there was significant interspecific variability in stomatal responses to CO₂ that may be related to growth form or growth rate and plant water relations. The effect of growth in the OTCs, relative to field plants, was usually positive for g _st and was greatest (>30%) when water availability was low, but only 6–12% when _leaf was high.The results of this study confirm the importance of considering interactions between indirect effects of high CO₂ of plant water relations and direct effects of elevated CO₂ on g _st , particularly in ecosystems such as grasslands where water availability often limits productivity. A product of this interaction is that the potential exists for either positive or negative responses in g _st to be measured at elevated levels of CO₂.     

The effect of long term CO2 enrichment on the growth,biomass partitioning and mineral nutrition of Sitka spruce (Picea sitchensis (Bong.) Carr.)

Sitka spruce [Picea sitchensis (Bong.) Carr.] seedlings were grown for 3 years in an outside control plot or in ambient (355 mol mol^-1) or elevated (ambient + 350 mol mol^-1) atmospheric CO₂ environments, within open top chambers (OTCs) at the Institute of Terrestrial Ecology, Edinburgh. Sequential harvests were carried out at the end of each growing season and throughout the 1991 growing season, five in all. Plants grown in elevated CO₂ had, (i) 35 and 10% larger root/shoot ratios at the end of the first and third season, respectively, (ii) significantly higher summer leader extension relative growth rates, which declined more rapidly in early autumn than ambient grown plants, (iii) after three growing seasons a significantly increased mean annual relative growth rate, (iv) consistently lower foliar nutrient concentrations, and (v) after two growing seasons smaller total projected needle areas. Plants grown inside OTCs were taller, heavier and had a smaller root/shoot ratio than those grown outside the chambers. There was no effect of CO₂ concentration on Sitka spruce leaf characteristics, although leaf area ratio, specific leaf area and leaf weight ratio all fell throughout the course of the 3 year experiment.     

Effects of Elevated Atmospheric CO2 Concentrations on CH4 and N2O Emission from Rice Soil: An Experiment in Controlled-environment Chambers

The effects of elevated concentrations of atmospheric CO₂ on CH₄ and N₂O emissions from rice soil were investigated in controlled-environment chambers using rice plants growing in pots. Elevated CO₂ significantly increased CH₄ emission by 58% compared with ambient CO₂. The CH₄ emitted by plant-mediated transport and ebullition–diffusion accounted for 86.7 and 13.3% of total emissions during the flooding period under ambient level, respectively; and for 88.1 and 11.9% of total emissions during the flooding period under elevated CO₂ level, respectively. No CH₄ was emitted from plant-free pots, suggesting that the main source of emitted CH₄ was root exudates or autolysis products. Most N₂O was emitted during the first 3 weeks after flooding and rice transplanting, probably through denitrification of NO₃⁻ contained in the experimental soil, and was not affected by the CO₂ concentration. Pre-harvest drainage suppressed CH₄ emission but did not cause much N₂O emission (< 10 μg N m⁻² h⁻¹) from the rice-plant pots at both CO₂ concentrations.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

Cardiorespiratory responses of the woodchuck and porcupine to CO2 and hypoxia

Summary The burrow-dwelling woodchuck (Marmota monax) (mean body wt.=4.45±1 kg) was compared to a similar-sized (5.87±1.5 kg) but arboreal rodent, the porcupine (Erithrizon dorsatum), in terms of its ventilatory and heart rate responses to hypoxia and hypercapnia, and its blood characteristics.V _T,f,T _I andT _E were measured by whole-body plethysmography in four awake individuals of each species. The woodchuck has a longerT _E/T _TOT (0.76±0.03) than the porcupine (0.61±0.03). The woodchuck had a higher threshold and significantly smaller slope to its CO₂ ventilatory response compared to the porcupine, but showed no difference in its hypoxic ventilatory response. The woodchuck P₅₀ of 27.8 was hardly different from the porcupine value of 30.7, but the Bohr factor, –0.72, was greater than the porcupine's, –0.413. The woodchuck breathing air has PaCO₂=48 (±2) torr, PaO₂=72 (±6), pHa=7.357 (±0.01); the porcupine blood gases are PaCO₂=34.6 (±2.8), PaO₂=94.9 (±5), pHa=7.419 (±0.03), suggesting a difference in PaCO₂/pH set points. The woodchuck exhibited no reduction in heart rate with hypoxia, nor did it have the low normoxic heart rate observed in other burrowing mammals.     

Involvement of ethylene in the morphological and developmental response of rice to elevated atmospheric CO2 concentrations

We tested the hypothesis that increased carbohydrate flux under elevatedCO₂ regulates accelerated development using rice (Oryzasativa L. cv. Jarrah). Plants were grown either in flooded soil orsolution culture at either 360 or 700 L CO₂L^–1. Total dry mass, shoot elongation rates (SER),tiller appearance rates (TAR) and ethylene release from intact rice seedlingswere measured from 5 to 42 days after planting (DAP). At maturity, shoot andsheath length, tiller number and grain mass were also measured. ElevatedCO₂ had a profound effect on growth, morphology and development andthe effects were more pronounced during the early growth phase. Total aboveground biomass increased at elevated CO₂ and this was accounted for by enhanced tiller number. Grain yield was increased by 56% under elevated CO₂mainly due to increased tiller number and hence panicle number. TAR and SERwereenhanced at elevated CO₂ but SER increased only untill 25 DAP.Elevated CO₂ stimulated a 2-3-fold increase in endogenous andACC-mediated ethylene release but the ACC concentration in the leaves waslittleaffected showing that rates of ACC synthesis matched its oxidation. Inhibitionof ethylene action by 1-aminocyclopropane (1-MCP) had a more pronouncedinhibitory effect on ethylene release in plants that were grown at 700 ascompared to 360 L CO₂ L^–1. Feedingsucrose to intact plants enhanced ethylene synthesis and these results areconsistent with the hypothesis that increased accumulation of sucrose atelevated CO₂ may enhance expression of genes in the ethylenebiosynthetic pathway. We conclude that increase in ethylene release may becentral in promoting accelerated development under elevated CO₂ andthis coincides with the release of auxiliary buds and accelerated rates oftiller appearance hence increased grain yield at elevated CO₂.     

Effects of elevated CO2 and nitrogen on nutrient uptake in ponderosa pine seedlings

This paper reports on the results of a controlled-environment study on the effects of CO₂ (370, 525, and 700 mol mol^-1) and N [0, 200, and 400 g N g soil^-1 as (NH₄)SO₄] on ponderosa pine (Pinus ponderosa) seedlings. Based upon a review of the literature, we hypothesized that N limitations would not prevent a growth response to elevated CO₂. The hypothesis was not supported under conditions of extreme N deficiency (no fertilizer added to a very poor soil), but was supported when N limitations were less severe but still suboptimal (lower rate of fertilization). The growth increases in N-fertilized seedlings occurred mainly between 36 and 58 weeks without any additional N uptake. Thus, it appeared that elevated CO₂ allowed more efficient use of internal N reserves in the previously-fertilized seedlings, whereas internal N reserves in the unfertilized seedlings were insufficient to allow this response. Uptake rates of other nutrients were generally proportional to growth. Nitrogen treatment caused reductions in soil exchangeable K⁺, Ca²⁺, and Mg²⁺ (presumably because of nitrification and NO₃ ^- leaching) but increases in extractable P (presumably due to stimulation of phosphatase activity).The results of this and other seedling studies show that elevated CO₂ causes a reduction in tissue N concentration, even under N-rich conditions. The unique response of N is consistent with the hypothesis that the efficiency of Rubisco increases with elevated CO₂. These results collectively have significant implications for the response of mature, N-deficient forests to evevated CO₂.     

The evolution of plant response to herbivory: simultaneously considering resistance and tolerance in Brassica rapa

Although the evolution of plant response to herbivory can involve either resistance (a decrease in susceptibility to herbivore damage) or tolerance (a decrease in the per unit effect of herbivory on plant fitness), until recently few studies have explicitly incorporated both of these characters. Moreover, theory suggests these characters do not evolve independently, and also that the pattern of natural selection acting on resistance and tolerance depends on their costs and benefits. In a genotypic selection analysis on an experimental population of Brassica rapa (Brassicaceae) I found a complex set of correlational selection gradients acting on resistance and tolerance of damage by flea beetles (Phyllotreta cruciferae: Chrysomelidae) and weevils (Ceutorhynchus assimilis: Curculionidae), as well as directional and stabilizing selection on resistance to attack by weevils. Evolution of response to flea beetle attack is constrained by a strong allocation cost of tolerance, and this allocation cost may be caused by a complex correlation among weevil resistance, weevil tolerance, flea beetle resistance, and flea beetle tolerance. Thus, one important conclusion of this study is that ecological costs may involve complex correlations among multiple characters, and for this reason these costs may not be detectable by simple pairwise correlations between characters. The evolution of response to weevil attack is probably constrained by a series of correlations between weevil resistance, weevil tolerance, and fitness in the absence of weevil damage, and possibly by a cost of tolerance of weevil damage. However, the nature of these constraints is complicated by apparent overcompensation for weevil damage. Because damage by both flea beetles and weevils had non-linear effects on plant fitness, standard measures of tolerance were not appropriate. Thus, a second important contribution of this study is the use of the area under the curve defined by the regression of fitness on damage and damage-squared as a measure of tolerance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO₂ is evaluated from the results of over 40 studies focusing on the effect of long-term CO₂ enrichment on the short-term response of photosynthesis to intercellular CO₂ (the A/C_i response). The effect of CO₂ enrichment on the A/C_i response was dependent on growth conditions, with plants grown in small pots (< 5 L) or low nutrients usually exhibiting a reduction of A at a given C_i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C_i following a doubling or tripling of atmospheric CO₂ during growth. Using theoretical interpretations of A/C_i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C_i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO₂ enrichment usually had little effect or increased the value of A at all C_i. However, a minority of species grown at elevated CO₂ exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO₂ (the C_i/C_a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C_i/C_a exhibited no consistent change in a variety of C₃ species, indicating no stomatal acclimation. Under drought or humidity stress, C_i/C_a declined in high-CO₂ grown plants, indicating stomata will become more conservative during stress episodes in future high CO₂ environments.Abbreviations A net CO₂ assimilation rate - C_i (C_a) intercellular (ambient) partial pressure of CO₂ - operational C_i intercellular partial pressure of CO₂ at a given ambient partial pressure of CO₂ - g_s stomatal conductance - normal CO₂ current atmospheric mole fraction of CO₂ (330 to 355 mol mol^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

Effect of CO2 enrichment and nitrogen availability on resource acquisition and resource allocation in a grass,Bromus mollis

Summary The effects of CO₂ enrichment on the growth, biomass partitioning, photosynthetic rates, and leaf nitrogen concentration of a grass, Bromus mollis (C₃), were investigated at a favorable and a low level of nitrogen availability. Despite increases in root: shoot ratios, leaf nitrogen concentrations were decreased under CO₂ enrichment at both nitrogen levels. For the low-nitrogen treatment, this resulted in lower photosynthetic rates measured at 650 l/l for the CO₂-enriched plants, compared to photosynthetic rates measured at 350 l/l for the non-enriched plants. At higher nitrogen availability, photosynthetic rates of plants grown and measured at 650 l/l were greater than photosynthetic rates of the non-enriched plants measured at 350 l/l. Water use efficiency, however, was increased in enriched plants at both nitrogen levels. CO₂ enrichment stimulated vegetative growth at both high and low nitrogen during most of the vegetative growth phase but, at the end of the experiment, total biomass of the high and low CO₂ treatments did not differ for plants grown at low nitrogen availability. While not statistically significant, CO₂ tended to stimulate seed production at high nitrogen and to decrease it at low nitrogen.     

Soil drying and its effect on leaf conductance and CO2 assimilation of Vigna unguiculata (L.) Walp

Summary Well watered plants of Vigna unguiculata (L.) Walp cv. California Blackeye No. 5 had maximum photosynthetic rates of 16 mol m^-2 s^-1 (at ambient CO₂ concentration and environmental parameters optimal for high CO₂ uptake). Leaf conductance declined with increasing water vapour concentration difference between leaf and air (w), but it increased with increasing leaf temperature at a constant small w. When light was varied, CO₂ assimilation and leaf conductance were correlated linearly. We tested the hypothesis that g was controlled by photosynthesis via intercellular CO₂ concentration (c _i). No unique relationship between (1) c _i, (2) the difference between ambient CO₂ concentration (c _a) and c _i, namely c _a-c _i, or (3) the c _i/c _a ratio and g was found. g and A appeared to respond to environmental factors fairly independently of each other. The effects of different rates of soil drying on leaf gas exchange were studied. At unchanged air humidity, different rates of soil drying were produced by using (a) different soils, (b) different irrigation schemes and (c) different soil volumes per plant. Although the soil dried to wilting point the relative leaf water content was little affected. Different soil drying rates always resulted in the same response of photosynthetic capacity (A _max) and corresponding leaf conductance (g(_Amax)) when plotted against percent relative plant-extractable soil water content (W _e %) but the relationship with relative soil water content (W _e ) was less clear. Above a range of W _e of 15%–25%, A _max and g(_Amax) were both high and responded little to decreasing W _e . As soon as W _e fell below this range, A _max and g(_Amax) declined. The data suggest root-to-leaf communication not mediated via relative leaf water content. However, g(_Amax) was initially more affected than A _max.List of abbreviations A CO₂ assimilation - A _max photosynthetic capacity at favourable ambient conditions - c _a CO₂ concentration of the air in the leaf chamber - c _i intercellular - CO₂ concentration - E transpiration - g leaf conductance - g(_Amax) leaf conductance corresponding to photosynthetic capacity - I photon flux rate - T _l leaf temperature - W _e relative plant-extractable soil water content - W _e absolute plant-extractable soil water content - W _l relative leaf water content - W _s relative soil water content - w difference in water vapour mole fraction between leaf and air - leaf water potential     

二氧化碳储存通量对森林生态系统碳收支的影响

涡度相关系统观测高度以下的CO₂储存通量对准确评价森林生态系统与大气间净CO₂交换量（NEE）有着重要的影响.本研究以长白山阔叶红松林为研究对象,利用2003年的涡度相关观测数据以及CO₂浓度廓线数据,分析了CO₂储存通量的变化规律及其对碳收支过程的影响.结果表明：涡度相关观测高度以下的CO₂储存通量具有典型的日变化特征,其最大变化量出现在大气稳定与不稳定层结转换期.利用涡度相关系统观测的单点CO₂浓度变化方法与利用CO₂浓度廓线方法计算的CO₂储存通量差异不显著.忽略CO₂储存通量,在半小时尺度上会造成对夜间和白天的NEE分别低估25%和19%,在日和年尺度上,会对NEE低估10%和25%;忽略CO₂储存通量,会低估Michaelis-Menten光响应方程及Lloyd-Taylor呼吸方程的参数,并且对表观初始量子效率α和参考呼吸R_ref的低估最大;忽略CO₂储存通量,在半小时、日及年尺度上,均会对总光合作用（GPP）和生态系统呼吸（R_e）低估约20%.     

Response of tussock tundra to elevated carbon dioxide regimes: analysis of ecosystem CO2 flux through nonlinear modeling

Summary The response of tussock tundra to elevated atmospheric concentrations of CO₂ was measured at Toolik Lake, Alaska in the summer of 1983. Computer-controlled greenhouses were used to determine diurnal ecosystem flux of CO₂ under four treatments: 340 ppm, 510 ppm, and 680 ppm CO₂, as well as 680 ppm CO₂ with a four degree centrigrade increase in temperature. For the seven days of data analyzed, net daily CO₂ flux was significantly different between treatments. Net uptake was positively correlated with CO₂ concentration in the chamber and negatively correlated with temperature. A nonlinear model was used to analyze this data set and to determine some of the reasons for different net CO₂ flux. This model allowed an estimation of light utilization efficiency, total conductance of CO₂, and a comparable measure of total respiration. From this analysis we conclude that nutrient limitations in the arctic decrease the capacity of tundra plants to make use of elevated CO₂ concentrations. The plants respond by decreasing conductance in the presence of elevated CO₂, which results in approximately equal gross uptake rates for the three CO₂ treatments. Apparent changes in system respiration result in higher net uptake under elevated CO₂ but this may be due to biases in the data. The treatment with increased temperature exhibited higher conductances and, consequently, higher gross uptake of CO₂ than the other treatments. Higher temperatures, however, also increase respiration with the result being lower net uptake than would be expected in the absence of temperature inscreases.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.     

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N₂, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO₂. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO₂ response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO₂, and (3) elevated CO₂ stimulates symbiotic N₂ fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO₂ concentration (365 and 700 mol mol^–1) and N addition on whole plant growth and C and N acquisition in an N₂-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO₂ response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m^–2 yr^–1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N₂ fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO₂enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO₂ increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO₂ regardless of N treatment. Although partial photosynthetic acclimation to CO₂ enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO₂ (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO₂. Elevated CO₂ increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N₂ fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N₂-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO₂ that are independent of soil N supply. The enhanced amount of N derived from N₂ fixation under elevated CO₂ presumably helps meet the increased N demand in N₂-fixing species. This response may lead to modified roles of N₂-fixers and N₂-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO₂.