Free-air CO2 Research Articles

Flower power

The binomial classification of flowering plants is strongly based on differences in flower design. Most species therefore differ in some observable design characteristics, a feature that evidently advertises diversification by natural selection, as extensively and experimentally investigated by Charles Darwin in the 19th century. His experimental approach to investigating the evolutionary consequences of floral mechanisms that lead to outbreeding and inbreeding continues to the present. Skimming through papers in a couple of recent New Phytologist issues shows that current methods of investigation are considerably more diverse, with opportunities to resolve the evolutionary quirks that have come into play to develop the magnificent diversity of floral design. Floral design is an observable indicator of complex reproductive mechanisms that are generally difficult to identify and quantify. For example, the same red colouration in Mimulus flowers is achieved by different genetic pathways (Cooley & Willis, 2009). Flowers that trap their pollinating insects are easily recognizable, but even here the mechanisms are complex. In Aristolochia (Oelschägel et al., 2009), the inner walls of the mouth and long tube of the flower possess downward-facing trichomes that allow an easy entrance of insects and the opportunity to transfer pollen from another flower to the specialized stigma of the flower. At this stage the insect cannot escape past the downward-facing trichomes, but this does become possible during male anthesis when the trichomes wilt. The inducement of pollinators to visit flowers correlates with the production of nectar rewards; for the flower the key process is the loading of the pollinator with pollen and the lodging of pollen on the stigmatic surface. Ambruster et al. (2009) suggest that the dual requirements of pollen transfer from the flower to the pollinator and the forced transfer from the pollinator to the flower, en route to the nectar, should select for equal distances for reward to anther and to stigma distances, an adaptive relationship that should scale with flower size. However, many plant species have showy flowers but an absence of nectar rewards for pollinators. This is particularly the case for orchids, but in Dactylorhiza sufficient visits by naïve pollinators still have the potential to select for enhanced display and mechanical fit between flower and pollinator (Sletvold et al., 2010). Larger flower size is also selected in Mimulus guttatus when occurring in nonnative habitats (Murren et al., 2009), while it appears to be the general rule that invasion of a new habitat can also induce changes in flowering phenology (Levin, 2009). Variation in flower size and mating system is also found in cleistogamous species, such as the annual Impatiens capensis (McGoey & Stinchcombe, 2009) that produces large outcrossing, but small self-fertile, flowers, ensuring an annual production of fertile seed. Pollen limitation is often the cause of the reproductive decline incurred as population size decreases, a major problem for the continued survival of rare species (Le Cadre et al., 2008). Pinus chiapensis has a restricted distribution in Central America and is classed as vulnerable. The species is self-fertile and suffers from inbreeding depression, with high variation within populations (del Castillo & Trujillo, 2008), reducing seed viability. In Pinus taeda (Williams, 2008) the selective elimination of selfed embryos and presumably deleterious alleles, occurs after fertilization up to embryo maturity, with a strong maternal influence. The loss, or virtual loss, of sexual reproduction that might occur at range boundaries or in isolated populations of species is expected to diminish adaptive evolution and increase the load of deleterious alleles, indicating an inevitable road to local extinction. However, work on the Oenothera genus (Johnson et al., 2010) and on clonal Vaccinium stamineum (Yakimowski & Eckert, 2008) indicates, like many other attempts at generalizations concerning plant reproduction, that the evolutionary consequences of reduced sexual reproduction at range boundaries are not that restrictive. Flower production in populations at their current low temperature limits is likely to be responsive to increasing temperatures and to the continuing increases in atmospheric CO2 concentration. Free-air CO2 and temperature-enrichment experiments in temperate grassland (Hovenden et al., 2008) showed precocious flowering in response to warming, but < 15% of the species exhibited any response to CO2 enrichment. Research on flowers and their evolutionary and ecological interactions seems, more than any other area in plant research, to show that species differ, with a wide range of species-specific responses frequently showing quite different routes to the same apparent end point. The use of molecular techniques can identify how species differences arise but they require the application of a wide range of ecological, developmental and theoretical techniques to identify why such differences may have emerged.

Effects of free-air CO 2 enrichment on energy traits and seed quality of oilseed rape

Previous experiments have not considered in situ evaluations of CO 2 enrichment on oilseed rape (OSR) under European climates with regard to energetic value and seed quality. Therefore, Brassica napus cv. Campino was grown in a free-air CO 2 enrichment (FACE) field experiment at Stuttgart-Hohenheim (Germany) under ambient (388 ± 9 μl l −1) and elevated (494 ± 16 μl l −1) CO 2 concentrations. Aboveground biomass production and energy yields of both straw and seeds increased, while energy contents per dry weight were identical in both CO 2 treatments. Among the mineral concentrations in seeds, sulphur, boron and cadmium were significantly reduced and zinc and manganese tended to decrease (0.1 ≥ P > 0.05) under elevated CO 2. Total protein concentration declined significantly by 4.6% with CO 2 enrichment, although increased seed yield (+18.4%) compensated for the change on a production basis. Accordingly, several proteinogenic amino acids per unit meal weight were significantly reduced (asparagine/aspartic acid, glycine, alanine, cysteine, valine, isoleucine, and leucine) or tended to decrease (glutamine/glutamic acid, methionine) in the high-CO 2 treatment, resulting in higher carbon/nitrogen ratios. Amino acids on a per protein basis and total oil concentrations were unaffected, while oil yield per unit ground area tended to increase by 19.8%. Elevated CO 2 slightly changed the composition of fatty acids on a per dry weight basis, which was more pronounced for unsaturated fatty acids. The concentration of major unsaturated fatty acids such as oleic acid increased significantly, while linolenic acid, nervonic acid and the group of essential fatty acids were decreased. There was also a negative trend for cis-11-eicosenic acid. The same holds true for the composition of fatty acids on a per oil basis. Among glucosinolates, only progoitrin tended to decrease under CO 2 enrichment. The results indicate that elevated CO 2 levels may have implications for food and feed quality and use for industrial processing of OSR in the future.

Simulation of free air CO 2 enriched wheat growth and interactions with water, nitrogen, and temperature

Agricultural system simulation models are key tools for assessment of possible impacts of climate change on crop production and environmental quality. In this study, the CERES-Wheat 4.0 module in the RZWQM2 model was calibrated and validated for simulating spring wheat grown under elevated CO 2 conditions in the FACE (Free Air CO 2 Enrichment) experiments conducted at Maricopa, Arizona, USA from 1992 to 1997. The validated model was then used to simulate the possible impacts of climate change on the crop for a 16-year period centered on 2050 with a projected atmospheric CO 2 concentration of 550 ppm. Sixteen General Circulation Model (GCM) projections of climate in response to this CO 2 concentration were used for this purpose. In the FACE experiment, the crops were grown under ambient (365–370 ppm) and elevated (∼550 ppm) CO 2 concentrations with two irrigation treatments (wet and dry) in 1992–1993 and 1993–1994, and two nitrogen (N) treatments (high and low N) in 1995–1996 and 1996–1997 crop seasons. The model simulated crop growth and grain yield, and soil water responses to CO 2 reasonably well, reproducing variations due to the treatments. Under ambient CO 2 in 1992–1993 and 1995–1996, biomass was simulated better in the dry and low N treatments with root mean square difference (RMSD) of 181 and 161 kg ha −1, respectively, compared to the wet and high N treatments with RMSD of 259 and 268 kg ha −1, respectively. The effects of water and N treatments were higher than those of CO 2, and the model reproduced these effects well. Elevated CO 2 effects on crop growth were counter-balanced by temperature effects, and projected precipitation had little effect on the simulated crop. The model results provide reasonable confidence for simulations of possible impacts of projected climate change on wheat crop growth in the region, within normal field data uncertainties.

Differential anatomical responses to elevated CO2 in saplings of four hardwood species

To determine whether an elevated carbon dioxide concentration ([CO(2)]) can induce changes in the wood structure and stem radial growth in forest trees, we investigated the anatomical features of conduit cells and cambial activity in 4-year-old saplings of four deciduous broadleaved tree species - two ring-porous (Quercus mongolica and Kalopanax septemlobus) and two diffuse-porous species (Betula maximowicziana and Acer mono) - grown for three growing seasons in a free-air CO(2) enrichment system. Elevated [CO(2)] had no effects on vessels, growth and physiological traits of Q. mongolica, whereas tree height, photosynthesis and vessel area tended to increase in K. septemlobus. No effects of [CO(2)] on growth, physiological traits and vessels were seen in the two diffuse-porous woods. Elevated [CO(2)] increased larger vessels in all species, except B. maximowicziana and number of cambial cells in two ring-porous species. Our results showed that the vessel anatomy and radial stem growth of Q. mongolica, B. maximowicziana and A. mono were not affected by elevated [CO(2)], although vessel size frequency and cambial activity in Q. mongolica were altered. In contrast, changes in vessel anatomy and cambial activity were induced by elevated [CO(2)] in K. septemlobus. The different responses to elevated [CO(2)] suggest that the sensitivity of forest trees to CO(2) is species dependent.

Effect of free air carbon dioxide enrichment combined with two nitrogen levels on growth, yield and yield quality of sugar beet: Evidence for a sink limitation of beet growth under elevated CO 2

The increase in atmospheric CO 2 concentration [CO 2] has been demonstrated to stimulate the growth of C 3 crops. However, little information exists about the effect of elevated [CO 2] on biomass production of sugar beet, and data from field experiments are lacking. In this study, sugar beet was grown within a crop rotation over two rotation cycles (2001, 2004) at present and elevated [CO 2] (375 μl l −1 and 550 μl l −1) in a free air CO 2 enrichment (FACE) system and at two levels of nitrogen supply [high (N2), and 50% of high (N1)], in Braunschweig, Germany. The objective of the present study was to determine the CO 2 effect on seasonal changes of leaf growth and on final biomass and sugar yield. Shading treatment was included to test whether sugar beet growth is sink limited under elevated [CO 2]. CO 2 elevation did not affect leaf number but increased individual leaf size in early summer resulting in a faster row closure under both N levels. In late summer CO 2 enrichment increased the fraction of senescent leaves under high but not low N supply, which contributed to a negative CO 2 effect on leaf area index and canopy chlorophyll content under high N at final harvest. Petioles contained up to 40% water-soluble carbohydrates, which were hardly affected by CO 2 but increased by N supply. More N increased biomass production by 21% and 12% in 2001 and 2004, respectively, while beet and sugar yield was not influenced. Concentration of α-amino N in the beet fresh weight was increased under low N and decreased under high N by CO 2 enrichment. The CO 2 response of total biomass, beet yield and white sugar yield was unaffected by N supply. Averaged over both N levels elevated [CO 2] increased total biomass by 7% and 12% in 2001 and 2004, respectively, and white sugar yield by 12% and 13%. The shading treatment in 2004 prevented the decrease in leaf area index under elevated [CO 2] and high N in September. Moreover, the CO 2 effect on total biomass (24%) and white sugar yield (28%) was doubled as compared to the unshaded conditions. It is concluded that the growth of the storage root of sugar beet is not source but sink limited under elevated [CO 2], which minimizes the potential CO 2 effect on photosynthesis and beet yield.

Stomatal uptake of O 3 in aspen and aspen-birch forests under free-air CO 2 and O 3 enrichment

Rising atmospheric carbon dioxide (CO 2) may alleviate the toxicological impacts of concurrently rising tropospheric ozone (O 3) during the present century if higher CO 2 is accompanied by lower stomatal conductance ( g s), as assumed by many models. We investigated how elevated concentrations of CO 2 and O 3, alone and in combination, affected the accumulated stomatal flux of O 3 (AFst) by canopies and sun leaves in closed aspen and aspen-birch forests in the free-air CO 2–O 3 enrichment experiment near Rhinelander, Wisconsin. Stomatal conductance for O 3 was derived from sap flux data and AFst was estimated either neglecting or accounting for the potential influence of non-stomatal leaf surface O 3 deposition. Leaf-level AFst (AFst l) was not reduced by elevated CO 2. Instead, there was a significant CO 2 × O 3 interaction on AFst l, as a consequence of lower values of g s in control plots and the combination treatment than in the two single-gas treatments. In addition, aspen leaves had higher AFst l than birch leaves, and estimates of AFst l were not very sensitive to non-stomatal leaf surface O 3 deposition. Our results suggest that model projections of large CO 2-induced reductions in g s alleviating the adverse effect of rising tropospheric O 3 may not be reasonable for northern hardwood forests.

Responses of canopy and soil climate in a six year free-air CO 2 enrichment study with spring crops

Besides increased growth, plants cultivated under elevated carbon dioxide (CO 2) show reduced transpiration and improved water use efficiency due to decreased stomatal conductances. While growth profits from the longer availability of soil water under CO 2 enrichment, increased canopy temperature may counteract these positive effects. Here we report on time series of soil temperatures and moistures from six years in which spring crops were cultivated in free-air CO 2 enrichment (Mini-FACE) experiments. Besides air and soil climate, temperature and relative humidity were determined in wheat canopies. Measurements rested on five replicates per treatment, representing a control (CON), an ambient air (AMB) and a FACE treatment. While the CON and AMB plots did not receive additional CO 2, concentrations were moderately elevated by 150 μl l −1 in the FACE plots. Plant growth differed among years due to the different climate and duration of individual experiments. Total biomass production was increased in the FACE treatments but significant effects were found only in one out of six years. In most of the years, soil temperatures tended to be reduced and soil moistures remained higher under elevated CO 2. Because the observed differences recurred during the growing season, we conclude that CO 2 enrichment was responsible for changes of the soil microclimate. At the same time vapour pressure deficit in the canopy significantly differed between the treatments for some days. While canopy heating due to CO 2 enrichment occurred in the early growing season these effects disappeared later suggesting that the stronger increase in leaf area index in the FACE treatments mitigated heating effects over time. The results support the supposed effects of CO 2 enrichment on the canopy climate and indicate a ‘microclimatic paradox’ with higher soil water availability due to the reduced transpiration and stronger canopy heating in FACE plots at least early in the season.

Effect of Free Air CO&lt;SUB&gt;2&lt;/SUB&gt; Enrichment (FACE) on phenolic acid metabolism in winter wheat leaf under different nitrogen levels

利用开放式空气CO₂浓度升高(Free Air Carbon-dioxide Enrichment, FACE)平台, 研究了低氮(LN)和常氮(NN)水平下, 大气CO2浓度升高对冬小麦叶片酚酸类物质代谢的影响。结果表明, CO₂浓度升高对小麦叶片水杨酸、对羟基苯甲酸、肉桂酸、阿魏酸和香草酸含量的影响随供氮水平的不同而有所差异。低氮下小麦通过提高叶片苯丙氨酸解氨酶(PAL)活性(30.1%)而使其含量均显著增加, 增幅分别达33.7%、119.6%、26.7%、39.9%和28.6%; 而常氮下PAL活性和酚酸类含量变化均未达显著水平。可见, 大气CO₂浓度升高对冬小麦酚酸类物质代谢的影响受氮水平的调控, 在未来CO₂浓度升高条件下, 选择适宜的施肥水平将显得更为重要。此外, 总酚含量与水杨酸、对羟基苯甲酸、肉桂酸、阿魏酸和香草酸等含量变化趋势基本一致, 且总酚含量变化的79.6%~151.4%是由这几种酚酸含量变化引起的, 说明CO₂浓度升高使水杨酸、对羟基苯甲酸、肉桂酸、阿魏酸和香草酸等含量增加是总酚含量增加的直接原因。低氮条件下大气CO₂浓度升高将通过改变酚酸类物质代谢而间接影响小麦与伴生杂草的关系。

Effects of free-air CO 2 enrichment and nitrogen supply on grain quality parameters and elemental composition of wheat and barley grown in a crop rotation

With respect to European growth conditions very little information is available on how future atmospheric CO 2 concentrations [CO 2] might affect quality characteristics of important crops. Winter wheat cv. ‘Batis’ and winter barley cv. ‘Theresa’ were grown for two growing seasons each under ambient [CO 2] (ca. 375 μmol mol −1) and elevated [CO 2] (550 μmol mol −1) with two different nitrogen (N) fertilization levels (adequate N supply / ca. 50% of adequate N) in the course of a six year crop rotation. Effects on grain quality and grain elemental composition were investigated. Grain crude protein concentrations were lowered under elevated [CO 2] by −4% to −13% in wheat and by −11% to −13% in barley. Reduced N supply decreased crude protein concentrations in wheat and barley by −14% to −22% and by −12% to −19%, respectively. In both species, starch concentration was increased by +4% on average due to reduced N fertilization. In wheat, both CO 2 enrichment and low N supply reduced the activity of total and soluble β-amylase (−11% and −7%), Hagberg falling number (−7%), and single kernel hardness (−18%). In barley, both of the treatments induced reductions in the viscosity of the water extract (−25% on average). Concerning minerals, sulphur concentrations were depleted under both elevated [CO 2] and low N supply by averages of −5% in wheat and −14% in barley. Reduced N supply caused −8% lower means of wheat grain calcium concentrations and reduced zinc concentrations on average by −23%. According to these results, flour from cereal grains grown under elevated [CO 2] and/or low N fertilization will have a diminished nutritional and processing quality and an altered elemental composition.

Challenges in elevated CO 2 experiments on forests

Current forest Free Air CO(2) Enrichment (FACE) experiments are reaching completion. Therefore, it is time to define the scientific goals and priorities of future experimental facilities. In this opinion article, we discuss the following three overarching issues (i) What are the most urgent scientific questions and how can they be addressed? (ii) What forest ecosystems should be investigated? (iii) Which other climate change factors should be coupled with elevated CO(2) concentrations in future experiments to better predict the effects of climate change? Plantations and natural forests can have conflicting purposes for high productivity and environmental protection. However, in both cases the assessment of carbon balance and how this will be affected by elevated CO(2) concentrations and the interacting climate change factors is the most pressing priority for future experiments.

Re‐assessment of plant carbon dynamics at the Duke free‐air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development

*The potential for elevated [CO(2)]-induced changes to plant carbon (C) storage, through modifications in plant production and allocation of C among plant pools, is an important source of uncertainty when predicting future forest function. Utilizing 10 yr of data from the Duke free-air CO(2) enrichment site, we evaluated the dynamics and distribution of plant C. *Discrepancy between heights measured for this study and previously calculated heights required revision of earlier allometrically based biomass determinations, resulting in higher (up to 50%) estimates of standing biomass and net primary productivity than previous assessments. *Generally, elevated [CO(2)] caused sustained increases in plant biomass production and in standing C, but did not affect the partitioning of C among plant biomass pools. Spatial variation in net primary productivity and its [CO(2)]-induced enhancement was controlled primarily by N availability, with the difference between precipitation and potential evapotranspiration explaining most interannual variability. Consequently, [CO(2)]-induced net primary productivity enhancement ranged from 22 to 30% in different plots and years. *Through quantifying the effects of nutrient and water availability on the forest productivity response to elevated [CO(2)], we show that net primary productivity enhancement by elevated [CO(2)] is not uniform, but rather highly dependent on the availability of other growth resources.

Effects of elevated atmospheric CO 2 and tropospheric O 3 on tree branch growth and implications for hydrologic budgeting

The forest hydrologic budget may be impacted by increasing CO 2 and tropospheric O 3. Efficient means to quantify such effects are beneficial. We hypothesized that changes in the balance of canopy interception, stem flow, and through-fall in the presence of elevated CO 2 and O 3 could be discerned using image analysis of leafless branches. We compared annual stem flow to the results of a computerized analysis of all branches from the 2002, 2004, and 2006 annual growth whorls of 97 ten-year-old trees from the Aspen Free-Air CO 2 and O 3 Enrichment (Aspen FACE) experiment in Rhinelander, WI. We found significant effects of elevated CO 2 and O 3 on some branch metrics, and that the branch metrics were useful for predicting stem flow from birch, but not aspen. The results of this study should contribute to development of techniques for efficient characterization of effects on the forest hydrologic budget of increasing CO 2 and tropospheric O 3.

Canopy CO2 exchange of sugar beet under different CO2 concentrations and nitrogen supply: results from a free‐air CO2 enrichment study

Sugar beet (Beta vulgaris ssp. altissima Döll) was grown in the field under free-air CO(2) enrichment (FACE, 550 ppm) and different nitrogen (N) supply (2001: 126 (N100) and 63 kg.ha(-1) (N50); 2004: 156 (N100) and 75 kg.ha(-1)) during two crop rotations. Canopy CO(2) exchange rates (CCER) were measured during the main growth phase (leaf area index > or =2) using a dynamic chamber system. Canopy CO(2) exchange data were analysed with respect to treatment effects on seasonal means and light use efficiency and light response characteristics. CO(2) enrichment enhanced CCER throughout the season. However, in both years, CCER declined after the second half of August independent of radiation and [CO(2)]. Elevated [CO(2)] strongly stimulated CCER on a seasonal basis, whereas the reduction of CCER caused by low N was below 10% and not significant. There were no effects of N on daily radiation use efficiency of carbon gain calculated from CCER data, but a strong enhancement by CO(2) enrichment. CCER closely tracked diurnal variations in incident photosynthetic photon flux density (PPFD, mumol.m(-2).s(-1)). The relationship between CCER and incident PPFD was curvilinear. In both seasons, initial slopes and maximum rates (CCER(max)) were determined from two 6-day periods using these relationships. The first period was measured after canopy closure (first half of July) and the second in the second half of August. In the first period, elevated [CO(2)] increased the initial slopes. Low N supply affected neither the initial slopes nor their response to elevated [CO(2)] in either period. In contrast to initial slopes, N stress limited the [CO(2)] response of CCER(max) in the first period. In the second period, however, this interaction of [CO(2)] and N on CCER(max) was completely dominated by a general decline of CCER(max) whereas no general decline of the initial slopes occurred in the second period. This response of light response parameters to [CO(2)] and N suggests that, in sugar beet, the decline of CCER in the late season may rely on limiting mechanisms such as photosynthetic acclimation responses to elevated [CO(2)] caused by sink limitations.

CO2 Enhancement of Forest Productivity Constrained by Limited Nitrogen Availability

AbstractStimulation of terrestrial productivity by rising CO~2~ concentration is projected to reduce the airborne fraction of anthropogenic CO~2~ emissions; coupled climate-carbon (C) cycle models, including those used in the IPCC Fourth Assessment Report (AR4), are sensitive to this negative feedback on atmospheric CO~2~^1^. The representation of the so-called CO~2~ fertilization effect in the 11 models used in AR4 and subsequent models^2,3^ was broadly consistent with experimental evidence from four free-air CO~2~ enrichment (FACE) experiments, which indicated that net primary productivity (NPP) of forests was increased by 23 +/- 2% in response to atmospheric CO~2~ enrichment to 550 ppm^4^. Substantial uncertainty remains, however, because of the expectation that feedbacks through the nitrogen (N) cycle will reduce the CO~2~ stimulation of NPP^5,6^; these feedbacks were not included in the AR4 models and heretofore have not been confirmed by experiments in forests^7^. Here, we provide new evidence from a FACE experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee, USA, that N limitation has significantly reduced the stimulation of NPP by elevated atmospheric CO~2~ concentration (eCO~2~). Isotopic evidence and N budget analysis support the premise that N availability in this forest ecosystem has been declining over time, and declining faster in eCO~2~. Model analyses and evidence from leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to eCO2. These results provide a strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.

Leaf size and surface characteristics of Betula papyrifera exposed to elevated CO 2 and O 3

Betula papyrifera trees were exposed to elevated concentrations of CO 2 (1.4 × ambient), O 3 (1.2 × ambient) or CO 2 + O 3 at the Aspen Free-air CO 2 Enrichment Experiment. The treatment effects on leaf surface characteristics were studied after nine years of tree exposure. CO 2 and O 3 increased epidermal cell size and reduced epidermal cell density but leaf size was not altered. Stomatal density remained unaffected, but stomatal index increased under elevated CO 2. Cuticular ridges and epicuticular wax crystallites were less evident under CO 2 and CO 2 + O 3. The increase in amorphous deposits, particularly under CO 2 + O 3, was associated with the appearance of elongated plate crystallites in stomatal chambers. Increased proportions of alkyl esters resulted from increased esterification of fatty acids and alcohols under elevated CO 2 + O 3. The combination of elevated CO 2 and O 3 resulted in different responses than expected under exposure to CO 2 or O 3 alone.

Elevated carbon dioxide does not offset loss of soil carbon from a corn–soybean agroecosystem

The potential for storing additional C in U.S. Corn Belt soils – to offset rising atmospheric [CO 2] – is large. Long-term cultivation has depleted substantial soil organic matter (SOM) stocks that once existed in the region's native ecosystems. In central Illinois, free-air CO 2 enrichment technology was used to investigate the effects of elevated [CO 2] on SOM pools in a conservation tilled corn–soybean rotation. After 5 and 6 y of CO 2 enrichment, we investigated the distribution of C and N among soil fractions with varying ability to protect SOM from rapid decomposition. None of the isolated C or N pools, or bulk-soil C or N, was affected by CO 2 treatment. However, the site has lost soil C and N, largely from unprotected pools, regardless of CO 2 treatment since the experiment began. These findings suggest management practices have affected soil C and N stocks and dynamics more than the increased inputs from CO 2-stimulated photosynthesis.

Effect of elevated CO 2 on soil N dynamics in a temperate grassland soil

The response of terrestrial ecosystems to elevated atmospheric CO 2 is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO 2 Enrichment (FACE) for six years. A 15N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO 2 on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a 15N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH 4 + and from organic-N to NO 3 − and analysed the rate of dissimilatory NO 3 − reduction to NH 4 + (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO 2. At the same time the gross rate of NH 4 + immobilization increased by 20%, while NH 4 + oxidation to NO 3 − was reduced by 25% under elevated CO 2. The NO 3 − dynamics under elevated CO 2 were characterized by a 52% increase in NO 3 − immobilization and a 141% increase in the DNRA rate, while NO 3 − production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH 4 + pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH 4 + under elevated CO 2. The advantage of such a shift is that NH 4 + is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO 2.

Elevated CO2 significantly delays reproductive development of soybean under Free-Air Concentration Enrichment (FACE)

The effect of rising atmospheric concentration of carbon dioxide [CO(2)] on the reproductive development of soybean (Glycine max. Merr) has not been evaluated under open-air field conditions. Soybeans grown under Free-Air CO(2) Enrichment (FACE) exhibit warmer canopies due to decreased latent heat loss because of decreased stomatal conductance. According to development models based on accumulated thermal time, or growing degree days ( degrees Cd), increased canopy temperature should accelerate development. The SoyFACE research facility (Champaign, Illinois, USA) was used to test the hypothesis that development is accelerated in soybean when grown in [CO(2)] elevated to 548 micromol mol(-1). Canopy temperature was measured continuously with infrared thermometry, and used in turn to calculate GDD. Opposite to expectation, elevated [CO(2)], while increasing canopy temperature, delayed reproductive development by up to 3 days (P <0.05). Soybean grown in elevated [CO(2)] required approximately 49 degrees Cd more GDD (P <0.05) to complete full bloom stage (R2) and approximately 52 degrees Cd more GDD (P <0.05) to complete the beginning seed (R5) stage, but needed approximately 46 degrees Cd fewer GDD (P <0.05) to complete seed filling (R6). Soybeans grown in elevated [CO(2)] produced significantly more nodes (P <0.01) on the main stem than those grown under current [CO(2)]. This may explain the delay in completion of reproductive development and final maturation of the crop under elevated [CO(2)]. These results show a direct effect of rising [CO(2)] on plant development that will affect both projections of grain supply and may be significant to other species including those in natural communities.

Atmospheric CO 2 enrichment changes the wheat grain proteome

Spring wheat ( Triticum aestivum L. cv. Triso) was grown in a free-air CO 2 enrichment (FACE) field experiment in order to gain information on CO 2-induced effects on grain composition and quality at maturity. A proteome analysis was performed using two-dimensional gel electrophoresis (2-DE) and protein identification was done with mass spectrometry (MALDI-TOF MS). In elevated CO 2 (526 μl l −1), an increase of 13.5% in grain yield was observed relative to 375 μl l −1 at a low level of significance ( P = 0.528). Total grain protein concentration was decreased by 3.5% at a high level of statistical significance. Most importantly, a number of statistically significant changes within the grain proteome were observed, as the levels of 32 proteins were affected by elevated CO 2: 16 proteins were up-regulated and 16 were down-regulated. Our experiment demonstrates that high-CO 2 can markedly affect the proteome of mature wheat grain. The potential role of the proteins, changed in response to CO 2 enrichment, is discussed as some may affect grain quality. For the task of selecting cultivars resistant to CO 2-induced quality loss, we propose to consider the proteins affected by elevated CO 2 identified in this work here.

Climate change and the flowering time of annual crops

Crop production is inherently sensitive to variability in climate. Temperature is a major determinant of the rate of plant development and, under climate change, warmer temperatures that shorten development stages of determinate crops will most probably reduce the yield of a given variety. Earlier crop flowering and maturity have been observed and documented in recent decades, and these are often associated with warmer (spring) temperatures. However, farm management practices have also changed and the attribution of observed changes in phenology to climate change per se is difficult. Increases in atmospheric [CO(2)] often advance the time of flowering by a few days, but measurements in FACE (free air CO(2) enrichment) field-based experiments suggest that elevated [CO(2)] has little or no effect on the rate of development other than small advances in development associated with a warmer canopy temperature. The rate of development (inverse of the duration from sowing to flowering) is largely determined by responses to temperature and photoperiod, and the effects of temperature and of photoperiod at optimum and suboptimum temperatures can be quantified and predicted. However, responses to temperature, and more particularly photoperiod, at supraoptimal temperature are not well understood. Analysis of a comprehensive data set of time to tassel initiation in maize (Zea mays) with a wide range of photoperiods above and below the optimum suggests that photoperiod modulates the negative effects of temperature above the optimum. A simulation analysis of the effects of prescribed increases in temperature (0-6 degrees C in +1 degree C steps) and temperature variability (0% and +50%) on days to tassel initiation showed that tassel initiation occurs later, and variability was increased, as the temperature exceeds the optimum in models both with and without photoperiod sensitivity. However, the inclusion of photoperiod sensitivity above the optimum temperature resulted in a higher apparent optimum temperature and less variability in the time of tassel initiation. Given the importance of changes in plant development for crop yield under climate change, the effects of photoperiod and temperature on development rates above the optimum temperature clearly merit further research, and some of the knowledge gaps are identified herein.