Elevated Atmospheric CO2 Levels Research Articles

A field method of determining NH4/+ and NO3/- uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species

Models describing plant and ecosystem N cycles require an accurate assessment of root physiological uptake capacity for NH 4 + and NO 3 - under field conditions. Traditionally, rates of ion uptake in field-grown plants are determined by using excised root segments incubated for a short period in an assay solution containing N either as a radioactive or stable isotope tracer (e.g., 36C103 as a NH 4 + analogue, 14CH3NH3 as an NO3 − analogue or 15NH4 + and 15NO 3 - ). Although reliable, this method has several drawbacks. For example, in addition to radioactive safety issues, purchase and analysis of radioactive and stable isotopes is relatively expensive and can be a major limitation. More importantly, because excision effectively interrupts exchange of compounds between root and shoot (e.g., carbohydrate supply to root and N transport to shoot), the assay must be conducted quickly to avoid such complications. Here we present a novel field method for simultaneous measurements of NH 4 + and NO 3 - uptake kinetics in intact root systems. The application of this method is demonstrated using two tree species; red maple (Acer rubrum) and sugar maple (Acer saccharum) and two crop species soybean (Glycine max) and sorghum (Sorghum bicolor). Plants were grown in open-top chambers at either ambient or elevated levels of atmospheric CO2 at two separate US national sites involved in CO2 research. Absolute values of net uptake rates and the kinetic parameters determined by our method were found to be in agreement with the literature reports. Roots of the crop species exhibited a greater uptake capacity for both N forms relative to tree species. Elevated CO2 did not significantly affect kinetics of N uptake in species tested except in red maple where it increased root uptake capacity, V max for NH 4 + . The application, reliability, advantages and disadvantages of the method are discussed in detail.

Linking 13C-based estimates of land and ocean sinks with predictions of carbon storage from CO2 fertilization of plant growth

The residence times of carbon in plants, litter, and soils are required for partitioning land and ocean sinks using measurements of atmospheric δ13CO2 and also for estimating terrestrial carbon storage in response to net primary production (NPP) stimulation by elevated levels of atmospheric CO2. While 13C-based calculations of the land sink decline with increasing estimates of terrestrial carbon residence times (through the fossil fuel-induced isotopic disequilibrium term in equations describing the global atmospheric budgets of 13CO2 and CO2), estimates of land sinks based on CO2 fertilization of plant growth are directly proportional to carbon residence times. Here we used a single model of terrestrial carbon turnover, the Carnegie’Ames’Stanford Approach (CASA) biogeochemical model, to simultaneously estimate 1984–1990 terrestrial carbon storage using both approaches. Our goal was to identify the fraction of the 13CO2-based land sink attributable to CO2fertilization. Uptake from CO2 fertilization was calculated using a β factor of 0.46 to describe the response of NPP to increasing concentrations of atmospheric CO2 from 1765 to 1990. Given commonly used parameters in the 13C-based sink calculation and assuming a deforestation flux of 0.8 Pg C/y, CO2 fertilization accounts for 54% of the missing terrestrial carbon sink from 1984 to 1990. CO2 fertilization can account for all of the missing terrestrial sink only when the terrestrial mean residence time (MRT) and the land isodisequilibrium forcing are greater than many recent estimates.

Is Potential Evapotranspiration and Its Relationship with Actual Evapotranspiration Sensitive to Elevated Atmospheric CO2 Levels

The possibility is examined that potential evapotranspiration values may be sensitive to changes in atmospheric carbon dioxide content. Enhanced levels of atmospheric CO2 increase water use efficiency of vegetation by improving growth rates and suppressing transpiration per unit leaf area. Highly cultivated crops without water or nutrient constraints are able to show the greatest growth improvements. In many natural or semi-natural ecosystems, under enhanced atmospheric CO2 concentrations, limits on the availability of soil nutrients severely constrains the possibility of improvements in growth and significant increases in leaf area index that could compensate for a decrease in transpiration per unit leaf area. Thus, in many natural or semi-natural ecosystems, which often form water gathering grounds in river basins, enhanced levels of CO2 will suppress transpiration and perhaps increase the proporation of precipitation that forms runoff or ground water. In low vegetation covers, such as grassland, the rates of transpiration and also evaporation from canopies that are wet after rainfall (interception loss) are very similar. In these canopies, evapotranspiration is unlikely to be significantly increased by small increases in leaf area index. It is suggested that the suppression of potential evapotranspiration by enhanced CO2 levels will be small, but that actual transpiration from tall, slow growing vegetation covers may be significantly suppressed. Thus for some vegetation covers the relationship between actual and potential evapotranspiration may be sensitive to CO2 levels. If this is so, it could be of importance to many water balance calculations. The suppression of evapotranspiration by enhanced CO2 levels will be most noticeable in dry climates where interception loss is insignificant and largely masked in very wet climates where a large proportion of evapotranspiration consists of interception loss.

Free Air CO2 Enrichment of potato (Solanum tuberosum L.): development, growth and yield

AbstractA FACE (Free Air CO2 Enrichment) experiment was carried out on Potato (Solanum tuberosum L., cv. Primura) in 1995 in Italy. Three FACE rings were used to fumigate circular field plots of 8 m diameter while two rings were used as controls at ambient CO2 concentrations. Four CO2 exposure levels were used in the rings (ambient, 460, 560 and 660 μmol mol–1). Phenology and crop development, canopy surface temperature, above‐ and below‐ground biomass were monitored during the growing season. Crop phenology was affected by elevated CO2, as the date of flowering was progressively anticipated in the 660, 560, 460 μmol mol–1 treatments. Crop development was not affected significantly as plant height, leaf area and the number of leaves per plant were the same in the four treatments. Elevated atmospheric CO2 levels had, instead, a significant effect on the accumulation of total nonstructural carbohydrates (TNC = soluble sugars + starch) in the leaves during a sunny day. Specific leaf area was decreased under elevated CO2 with a response that paralleled that of TNC concentrations. This reflected the occurrence of a progressive increase of photosynthetic rates and carbon assimilation in plants exposed to increasingly higher levels of atmospheric CO2. Tuber growth and final tuber yield were also stimulated by rising CO2 levels. When calculated by regression of tuber yield vs. the imposed levels of CO2concentration, yield stimulation was as large as 10% every 100 μmol mol–1 increase, which translated into over 40% enhancement in yield under 660 μmol mol–1. This was related to a higher number of tubers rather than greater mean tuber mass or size. Leaf senescence was accelerated under elevated CO2 and a linear relationship was found between atmospheric CO2 levels and leaf reflectance measured at 0.55 μm wavelength. We conclude that significant CO2 stimulation of yield has to be expected for potato under future climate scenarios, and that crop phenology will be affected as well.

Below‐ground architectural and mycorrhizal responses to elevated CO2 in Betula alleghaniensis populations

1. Replicate populations of crowded, regenerating stands of Betula alleghaniensis were grown in ambient and elevated (700p.p.m.) atmospheric CO2 concentrations in monoliths of forest soil. Early in the second year the seedlings were harvested and detailed measurements of individual plant root architectural parameters and ectomycorrhizal colonization were made. 2. Comparing the average responses of individual plants within the populations, elevated CO2 had no significant effects on architectural parameters that improve a plant’s ability to forage for and acquire soil resources. In contrast, the intensity and magnitude of mycorrhizal colonization, and whole plant C/N ratios were significantly enhanced with elevated CO2. 3. The allometric scaling relationship between total plant biomass and root biomass was not affected by CO2, suggesting that relative allocation between roots and shoots was not affected. However, the allometric scaling relationships between root architectural parameters and plant biomass, and between fine root biomass and woody root biomass were significantly altered by elevated CO2. For all of these relationships, elevated CO2 reduced the ‘size bias’ of architectural components in relation to plant size within the populations; in elevated CO2 root architectural size (e.g. root length) per unit biomass was more similar between the smallest and largest individuals within the population than was the case for ambient CO2. 4. Overall, the results of this study suggest that the average individual seedling biomass and architectural growth responses within populations of plants exposed to elevated atmospheric CO2 levels may be unresponsive, but that mycorrhizal responses and interactions among plants within populations may be altered significantly. These findings have important implications for how we make predictions about plant growth responses to elevated CO2 in natural ecosystems. Significant increases in mycorrhizal infection rates and architecture–biomass allometries suggest that below‐ground competitive interactions within plant populations may be reduced in elevated CO2. Alterations in competitive interactions may lead to shifts in productivity and plant population structure.

Photosynthetic Gas Exchange and Water Relation Responses of Three Tallgrass Prairie Species to Elevated Carbon Dioxide and Moderate Drought

Undisturbed tallgrass prairie was exposed to ambient and elevated (twice-ambient) levels of atmospheric CO2 and experimental dry periods. Seasonal and diurnal midday leaf water potential (Ψleaf), net photosynthesis Anet, and stomatal conductance (gs) responses of three tallgrass prairie growth forms—a C4 grass, Andropogon gerardii; a broad-leaved woody C3 shrub, Symphiocarpos orbiculatus; and a C3 perennial forb, Salvia pitcheri—were assessed. Ψleaf in A. gerardii and S. orbiculatus was higher under elevated CO2, regardless of soil moisture, while Ψleaf in S. pitcheri responded only to drought. Elevated CO2 always stimulated Anet in the C3 species, while A. gerardii Anet increased only under dry conditions. However, Anet under elevated CO2 in the C3 species declined with drought but not in the C4 grass. Under wet conditions, gs reduced in elevated CO2 for all species. During dry periods, gs at elevated CO2 was sometimes higher than in ambient CO2. Our results support claims that elevated CO2 will stimulate tallgrass prairie productivity during dry periods and possibly reduce temporal and spatial variability in productivity in these grasslands.

Gas Exchange of Citrus Seedlings at Different Temperatures, Vapor-pressure Deficits, and Soil Water Contents

Midday reductions of stomatal conductance and carbon dioxide assimilation rates (Aco2) in Citrus are typically attributed to large leaf-to-air vapor-pressure differences or high atmospheric vapor-pressure deficits (VPD). This study investigated air temperature (Ta) and available soil water (ASW) level as corollary factors of atmospheric VPD that influence midday reduction of net gas exchange in citrus leaves. The influence of elevated atmospheric CO2 under conditions that inhibit net canopy Aco2 was also investigated. Net canopy Aco2 and evapotranspiration rates of Carrizo citrange [Poncirus trifoliata Raf × Citrus sinensis (L.) Osbeck] and Swingle citrumelo (P. trifoliata Raf × C. paradisii Macf.) seedlings grown in outdoor controlled-environment growth chambers were measured under two levels of Ta with concomitant changes in VPD and two levels of atmospheric CO2 concentration, which were changed in steps over time. Cyclical depletion of ASW was allowed to occur at each set of Ta/VPD and CO2 combinations. Highest net canopy Ace, rates at ambient CO2 concentration (330 μmol·mol-1) were obtained at the low Ta/VPD level (29C/2.4 kPa) and ASW >50%. Diurnal canopy CO2 uptake rates decreased at the high Ta/VPD level (37C/3.6 kPa), and midday depression of canopy Aco2 was observed at ASW levels <50%. Net canopy Aco2 decreased at higher levels of ASW under the high Ta/VPD treatment than at the low Ta/VPD treatment. At the elevated CO2 concentration (840 μmol·mol-1) net canopy CO2 uptake rates were double those that occurred at ambient CO2 levels and they did not exhibit midday reduction. Our data indicate that, when soil water is not readily available, citrus seedlings are more sensitive to high levels of Ta and VPD which results in reduction of CO2 uptake. The inhibitory effects of elevated VPD and reduced ASW on citrus net Aco2 were lessened at the elevated atmospheric CO2 level.

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO2 levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C4 grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m−2 s−1 versus 350 μmol m−2 s−1 shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO2 when compared to plants at ambient CO2. This was due primarily to the 50% reduction in stomatal conductance at elevated CO2, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO2. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO2 enhanced plant water status. CO2-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO2 on plant water relations in all ecosystems.

Elevated CO2 and plant nitrogen-use: is reduced tissue nitrogen concentration size-dependent?

Plants often respond to elevated atmospheric CO2 levels with reduced tissue nitrogen concentrations relative to ambient CO2-grown plants when comparisons are made at a common time. Another common response to enriched CO2 atmospheres is an acceleration in plant growth rates. Because plant nitrogen concentrations are often highest in seedlings and subsequently decrease during growth, comparisons between ambient and elevated CO2-grown plants made at a common time may not demonstrate CO2-induced reductions in plant nitrogen concentration per se. Rather, this comparison may be highlighting differences in nitrogen concentration between bigger, more developed plants and smaller, less developed plants. In this study, we directly examined whether elevated CO2 environments reduce plant nitrogen concentrations independent of changes in plant growth rates. We grew two annual plant species. Abutilon theophrasti (C3 photosynthetic pathway) and Amaranthus retroflexus (C4 photosynthetic pathway), from seed in glass-sided growth chambers with atmospheric CO2 levels of 350 μmol·mol-1 or 700 μmol·mol-1 and with high or low fertilizer applications. Individual plants were harvested every 2 days starting 3 days after germination to determine plant biomass and nitrogen concentration. We found: 1. High CO2-grown plants had reduced nitrogen concentrations and increased biomass relative to ambient CO2-grown plants when compared at a common time; 2. Tissue nitrogen concentrations did not vary as a function of CO2 level when plants were compared at a common size; and 3. The rate of biomass accumulation per rate of increase in plant nitrogen was unaffected by CO2 availability, but was altered by nutrient availability. These results indicate that a CO2-induced reduction in plant nitrogen concentration may not be due to physiological changes in plant nitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.

Effects of long-term elevated atmospheric carbon dioxide onLolium perenne andTrifolium repens, using a simple photosynthesis model

Changes in gross canopy photo synthetic rate (PGc), produced by long-term exposure to an elevated atmospheric CO2 level (626 ± 50/μmol mol−1), were modelled for Lolium perenne L. cv. Vigor and Trifolium repens L. cv. Blanca, using a simple photosynthesis model, based on biochemical and physiological information (leaf gross CO2 uptake in saturating light, Pmax, and leaf quantum efficiency, α) and structural vegetation parameters (leaf area index, LAI, canopy extinction coefficient, k, leaf transmission, M). Correction of PGc for leaf respiration allowed comparison with previously measured canopy net CO2 exchange rates, with the average divergence from model prediction amounting to about 6%. Sensitivity analysis showed that for a three-week old canopy, the PGc increase in high CO2 could be attributed largely to changes in Pmax and α, while differences in canopy architecture were no longer important for the PGc-stimulation (which they were in the early growth stages). As a consequence of this increasing LAI withcanopy age, the gain of daytime CO2 uptake is progressively eroded by the increasing burden of canopy respiration in high-CO2 grown Lolium perenne. Modelling canopy photosynthesis in different regrowth stages after cutting (one week, two weeks,...), revealed that the difference in a 24-h CO2 balance between the ambient and the high CO2 treatment is reduced with regrowth time and completely disappears after 6 weeks.

IMPACT OF INCREASED TEMPERATURE AND CO2 ON RICE PRODUCTIVITY IN CAUVERY DELTA ZONE, INDIA-A SIMULATION ANALYSIS

Using an eco-physiological model, the changes in the growth characteristics and grain yield for elevated atmospheric CO2 level and temperature are explored. The main prediction is for harvest indices to fall. The model was used to develop strategies for mitigating the combined effects of increased temperature and CO2, and define rice ideotypes suited to changing climate.

Sensitivity of terrestrial carbon storage to CO2-induced climate change: Comparison of four scenarios based on general circulation models

The potential impacts of CO2-induced climate change on terrestrial carbon storage was estimated using the Holdridge Life-Zone Classification and four climate change scenarios derived from general circulation models. Carbon values were assigned to life-zones and their associated soils from published studies. All four scenarios suggest an increase in area occupied by forests although details of predicted patterns vary among the scenarios. There is a poleward shift of the forested zones, with an increase in the areal extent of tropical forests and a shift of the boreal forest zone into the region currently occupied by tundra. Terrestrial carbon storage increased from 0.4% (8.5 Gt) to 9.5% (180.5 Gt) above estimates for present conditions. These changes represent a potential reduction of 4 to 85 ppm on elevated atmospheric CO2 levels.

Elevated CO2 differentially alters the responses of coocurring birch and maple seedlings to a moisture gradient.

To determine the effects of elevated CO2 and soil moisture status on growth and niche characteristics of birch and maple seedlings, gray birch (Betula populifolia) and red maple (Acer rubrum) were experimentally raised along a soil moisture gradient ranging from extreme drought to flooded conditions at both ambient and elevated atmospheric CO2 levels. The magnitude of growth enhancement due to CO2 was largely contingent on soil moisture conditions, but differently so for maple than for birch seedlings. Red maple showed greatest CO2 enhancements under moderately moist soil conditions, whereas gray birch showed greatest enhancements under moderately dry soil conditions. Additionally, CO2 had a relatively greater ameliorating effect in flooded conditions for red maple than for gray birch, whereas the reverse pattern was true for these species under extreme drought conditions. For both species, elevated CO2 resulted in a reduction in niche breadths on the moisture gradient; 5% for gray birch and 23% for red maple. Species niche overlap (proportional overall) was also lower at elevated CO2 (0.98 to: 0.88: 11%). This study highlights the utility of of experiments crossing CO2 levels with gradients of other resources as effective tools for elucidating the potential consequences of elevated CO2 on species distributions and potential interactions in natural communities.

Increased photosynthetic capacity of Scirpus olneyi after 4 years of exposure to elevated CO2

Abstract. While a short‐term exposure to elevated atmospheric CO2 induces a large increase in photosynthesis in many plants, long‐term growth in elevated CO2 often results in a smaller increase due to reduced photosynthetic capacity. In this study, it was shown that, for a wild C3 species growing in its natural environment and exposed to elevated CO2 for four growing seasons, the photosynthetic capacity has actually increased by 31%. An increase in photosynthetic capacity has been observed in other species growing in the field, which suggests that photosynthesis of certain field grown plants will continue to respond to elevated levels of atmospheric CO2

Leaf area compensation and nutrient interactions in CO2‐enriched seedlings of yellow‐poplar (Liriodendron tulipifera L.)

summaryThe responses of yellow‐poplar (Liriodendron tulipifera L.) seedlings to elevated levels of atmospheric CO2 were investigated to identify attributes governing growth and physiological responses to CO2. Based on the pattern of leaf initiation and nutrient requirements of the species, it was predicted that (1) CO2, enrichment would enhance growth of yellow‐poplar seedlings both through accelerated leaf area production and through higher rates of carbon assimilation per unit leaf area; and (2) growth enhancement of yellow‐poplar by CO2 enrichment would be reduced by nutrient limitations. The hypotheses were tested in an experiment in which yellow‐poplar plants were grown from seed for 24 weeks in controlled‐environment chambers. The experimental design comprised three atmospheric CO2 concentrations (371, 493, and 787 cm3m−3), two levels of mineral nutrients (unfertilized or weekly additions of complete nutrient solution), and three harvests (6, 12, and 24 weeks). Plant growth rate, water use, foliar gas exchange, component dry weights, and nutrient contents were measured.Both hypotheses were rejected. Whole‐plant dry weight increased similarly with CO2, enrichment in plants provided with additional mineral nutrients and in unfertilized plants, although the fertilized plants grew 10‐fold larger. The increase in dry weight resulting from elevated CO2 occurred only in root systems. Although leaves were produced continuously during the experiment, leaf area was slightly reduced in elevated CO2, and the whole‐plant growth response was wholly attributable to an increase in carbon assimilation per unit leaf area. Although the compensation between photosynthesis and leaf area reduced the potential growth response to CO2, the reduction in leaf area ratio was associated with a significant increase in water‐use efficiency. This unexpected result demonstrated the importance of feedbacks and interactions between resources in shaping the response of a plant to CO2.

Influence of short-term atmospheric CO2 enrichment on growth, allocation patterns, and biochemistry of black spruce seedlings at different stages of development

Black spruce seedlings (Piceamariana Mill.) were exposed to either elevated (1000 ppm) or ambient (340 ppm) atmospheric CO2 levels at different stages of seedling development over a winter greenhouse production cycle. Seedlings germinated in early February and were placed in CO2 chambers for either 3 or 6 weeks during March, April, May, or August. Total seedling biomass increased under high CO2 conditions for the March, April, and May stages of development, but showed no significant response in August. The greater part of the CO2 response occurred during the second 3 weeks of exposure in March and April but during the first 3 weeks of exposure in May. In September, those seedlings exposed to CO2 in April and May had 30 and 14%, respectively, greater biomass than control seedlings, but seedlings from the other stages of development no longer had significant differences remaining from the CO2 treatment. This suggests that it could be very efficient to give a short well-timed CO2 pulse at the beginning of the production cycle in hopes of producing a size difference that is maintained throughout the remainder of the greenhouse production cycle under ambient levels of CO2. Short-term exposure to elevated CO2 also increased the ratio of shoot dry weight to total height for the March, April, and May stages of development. The ratio of total nonstructural carbohydrates to free amino acids was negatively correlated (r2 = 0.98) with the allocation of new growth between shoots and roots as measured by the allocation coefficient, k (milligrams shoot growth per milligrams root growth). As seedlings developed along their seasonal growth cycle, ratios of total nonstructural carbohydrates to free amino acids increased and the values for k decreased. The effect of CO2 enrichment on these two factors is discussed. Monitoring total nonstructural carbohydrate and free amino acid concentrations in foliage could have potential as a method to predict the percentage of carbon allocated to root systems of entire forest stands as well as of individual tree seedlings.

Photosynthesis and Stomatal Conductance with CO2‐Enrichment of Containerand Field‐Grown Soybeans1

AbstractThe present experiment was an attempt to study the differences between photosynthetic responses of pot‐grown and field‐grown soybean [Glycine max(L.) Merr., ‘Bragg’] plants to atmospheric CO2enrichment at six levels of irradiance and four leaf temperatures. Plants were grown in field soil or in pots containing an artificial substrate at five elevated atmospheric CO2levels and two water regimes in open top field chambers. Under well‐watered conditions plants grown in the pots had lower stomatal conductances and lower rates of net photosynthesis than plants grown in the field. However, the field‐grown plants responded to a less extent to increasing atmospheric CO2concentration than pot‐grown plants. The photosynthesis‐ irrdiance relationships were linear in the field‐grown plants and curvilinear in the pot‐grown plants. At the highest irradiance level tested (1400 µmol m−2s−1), the field‐grown plants had maximum net photosynthetic rates of 43 and 48 mg CO2dm−2h−1at 332 and 623 µL L−1CO2respectively. The pot‐grown plants were irradiance saturated at a level of 700 µmol m−2s−1and net leaf photosynthetic rates were 18 and 28 mg CO2dm−2h−1at 332 and 623 µL L−1CO2, respectively. Water stress caused a reduction in both stomatal conductance and net photosynthetic rate in the leaves of pot‐grown plants. The effect of leaf temperature on net photosynthethis and stomatal conductance depended upon the atmospheric CO2concentration and rooting media. High temperature increased net photosynthetic response to CO2enrichment at high CO2concentrations but not at low CO2concentrations.