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Elevated CO2 Is Claimed to Inhibit Plant Nitrate Assimilation and Subsequent Growth ... AGAIN!!! ...

Reference
Bloom, A.J., Burger, M., Asensio, J.S.R. and Cousins, A.B. 2010. Carbon dioxide enrichment inhibits nitrate assimilation in wheat and Arabidopsis. Science 328: 899-903.
In an article published in the Los Angeles Times on 14 May 2010, entitled Plant Study Dims Silver Lining to Global Warming, Amina Khan writes that "some biologists had theorized earlier that rising greenhouse gas levels would encourage plant growth over the long term because of the increased amount of carbon dioxide in the atmosphere," but she goes on to say that "plant physiologists from UC Davis may have dashed those hopes," quoting the principal investigator of the research project that prompted her article as saying that "we thought rising carbon dioxide levels might actually have some benefit, but it proves to be wrong. ... Over a period of time, be it weeks or years, that stimulation [of photosynthetic and growth rates] disappears."

In the scientific paper describing the research, which was published the same day as the Khan article, Bloom et al. (2010) claim to have used "five independent methods with wheat and Arabidopsis to show that atmospheric carbon dioxide enrichment inhibited the assimilation of nitrate into organic nitrogen compounds," and that "this inhibition may be largely responsible for carbon dioxide acclimation, the decrease in photosynthesis and growth of plants conducting C3 carbon fixation after long exposures (days to years) to carbon dioxide enrichment."

Interestingly, this concept had a nearly identical incarnation that appeared eight years ago in the Proceedings of the National Academy of Sciences, wherein Bloom and three other collaborators (Bloom et al., 2002) made essentially the same claims as Bloom and his second set of associates are making today. And what we showed to be wrong then remains wrong now.

In the earlier study, Bloom et al. (2002) analyzed the ability of two-week-old seedlings of hydroponically-grown wheat to respond to a near-doubling of the air's CO2 content when their roots were bathed in a non-nitrogen-limiting solution of either ammonium (NH4+) or nitrate (NO3-). The results of that experiment demonstrated that the 94% increase in the air's CO2 content enhanced the biomass of the young wheat plants by 44% when the seedlings received their nitrogen in the form of NO3-. This result was a significant positive response. Nevertheless, the study's findings were widely portrayed by various organizations, websites and publications as presaging significant negative consequences in the years and decades to come for almost all of earth's vegetation, including both agricultural crops and natural ecosystems. But why?

One reason for the negativism may have resided in the fact that the positive result obtained for the plants whose roots were bathed in the NH4+ solution was even more impressive. Instead of "just" a 44% increase in plant biomass, these plants exhibited a whopping 78% increase. And thus it was that on 4 February 2002 -- one day before the Bloom et al. (2002) paper had even appeared in print -- NASA's Earth Observatory News posted an article on its website entitled High CO2 Levels Hamper Nitrate Incorporation by Plants, in which it was claimed that "nitrate fertilizer is not nearly as efficient as ammonium fertilizer when atmospheric carbon dioxide levels are unusually high," which is quite an expansive conclusion to draw from a study that lasted only two weeks, dealt with only one species, and utilized only seedlings growing only in nutrient solution.

This report was followed by several similar stories of much the same negative bent. Scientific American introduced their take on the Bloom et al. (2002) paper with an equally expansive title stating Rising CO2 Levels Could Force Shift in Fertilizer Use, which document was reproduced the very same day by the Climate Ark organization. Simultaneously, AmeriScan displayed an article entitled Rising CO2 Hampers Fertilizers, which began with the declaration that "as carbon dioxide levels rise, plant life around the globe may lose the ability to incorporate certain forms of nitrogen, like those found in most fertilizers." And it ended by saying that "as atmospheric CO2 levels continue to rise, nitrate-sensitive plant and tree species in the wild could be at a competitive disadvantage," stating that "this could change the distribution of plants in natural ecosystems."

Eerily, and almost mimicking -- in advance -- the lead-in to Amina Khan's story, some of the press reports also said that "for many years, scientists believed ... rising levels of carbon dioxide would actually benefit plants," as if to suggest that such was no longer the case. They also matter-of-factly stated that the typically-observed initial positive growth response to atmospheric CO2 enrichment observed in most experiments "wasn't sustained," dropping back to just a few percent above normal "within a few days or weeks," but which Bloom et al. (2010) have now extended to "days to years."

It was also interesting -- but not unexpected -- that the environmental press so highly hyped so many of the presumed negative ramifications of the Bloom et al. (2002) experiment for both agricultural and natural ecosystems, when the experiment upon which those presumptions were based lasted only 14 days and had been performed under sterile laboratory conditions that included no soil, no competing plants, and a totally unnatural mix of antibiotics in the water surrounding the seedlings' roots, which antibiotics were introduced to suppress naturally-occurring nitrogen-transforming processes that Bloom et al. (2002) actually admitted are "rapid in nonsterile cultures (Padgett and Leonard, 1993) and sensitive to atmospheric CO2 (Smart et al., 1997)," all of which situations are typical of the real world of nature.

So what do we learn from longer experiments that have been conducted under more realistic conditions? We first consider the three-part claim -- which, to be fair to the press, was actually made by Bloom et al. (2002) -- that (1) "a doubling of CO2 level initially accelerates carbon fixation in C3 plants by about 30%," that (2) this growth stimulation "after days to weeks" dramatically declines, and that (3) the CO2-induced growth enhancement thereafter "stabilizes at a rate that averages 12% above ambient controls."

All three parts of this contention are inadequate generalizations of what is called acclimation to CO2 enrichment. In the first instance, a doubling of the air's CO2 content will often accelerate biomass production in young C3 plants in the early stages of CO2 enrichment by much more than 30%. Even in their own experiment, Bloom et al. (2002) found that slightly less than a doubling of the air's CO2 content increased the biomass of their NO3--treated plants by fully 44%, and that it increased the biomass of their NH4+-treated plants by what we have rightly called a whopping 78%. In addition, the mini-review of Idso (1999) cites at least twenty experiments where the initial growth stimulation exceeded 100%.

With respect to the decline in growth stimulation that is claimed by Bloom et al. to follow hard on the heels of the initial CO2-induced growth enhancement, we note that it sometimes never occurs (Gunderson et al., 1993; Fernandez et al., 1998; Garcia et al., 1998). In other instances, the reverse occurs; and the CO2-induced growth stimulation increases over time (Arp and Drake, 1991; Vogel and Curtis, 1995; Jacob et al., 1995). And in those cases where there is a decline in the strength of the CO2 aerial fertilization effect, it sometimes does not begin until one or more years after the initiation of the experiment. In the long-term sour orange tree study of Idso and Kimball (2001), for example, a decline in the CO2-induced growth stimulation did not begin until the 2.5-year point of the experiment.

Last of all, the degree of CO2-induced growth stimulation at which the aerial fertilization effect of atmospheric CO2 enrichment eventually stabilizes is often significantly larger than the 12% value suggested by Bloom et al. (2002) for a doubling of the air's CO2 content. In their summary report of the Phoenix, Arizona sour orange tree study, for example, Kimball et al. (2007) write that rather than "a continual acclimation" or never-ending long-term decline in the strength of the CO2-induced aerial fertilization effect, there was a "sustained enhancement," wherein there was a near-constant 70% increase in total yearly biomass production over the entire last decade of the 17-year study in response to the 75% increase in the air's CO2 content employed throughout the experiment, which for a doubling of the air's CO2 content implies that there would have been a CO2-induced productivity enhancement on the order of 93%, which is nearly eight times greater than what Bloom et al. (2002) declared to be typical.

Having thus clarified the record with respect to the erroneously-summarized aspects of the CO2 acclimation phenomenon presented in the Bloom et al. (2002) article, there is another matter of addressing their conclusion that plants respond better to atmospheric CO2 enrichment when they obtain their nitrogen in the ammonium form as opposed to the nitrate form; for in spite of their claim that theirs may have been "the first study to examine CO2 responses under controlled levels of NH4+ vs. NO3- as sole N sources," there have, in fact, been many other studies that provide important -- and vastly different -- information about this topic, some of which are briefly highlight below.

Bauer and Berntson (2001) grew seedlings of Betula alleghanienis and Pinus strobus for 15 weeks -- as opposed to the abbreviated two weeks of the Bloom et al. (2002) experiment -- in growth chambers maintained at atmospheric CO2 concentrations of 400 and 800 ppm, while the seedlings' roots were suspended in nutrient solutions whose sole sources of N were, as in the study of Bloom et al. (2002), either NO3- or NH4+. In this experiment, the extra CO2 did not have any effect on the growth of the Pinus species in either solution; but it increased total seedling dry weight in the Betula species by 61% in the nitrate treatment and by 79% in the ammonium treatment. Although this result is qualitatively the same as that obtained by Bloom et al., the ammonium/nitrate (A/N) response ratio, i.e., 79%/61% = 1.30, was much lower than the A/N response ratio of the Bloom et al. (2002) experiment, i.e., 78%/44% = 1.77, suggesting that perhaps the A/N response ratio could be undergoing a type of acclimation as experimental duration lengthens, thereby holding out the possibility that for still longer (and more realistic) periods of differential CO2 exposure, there may well be little or no plant preference for a particular N source.

Van der Merwe and Cramer (2000) grew tomato seedlings for two weeks -- bringing them to the same age as the wheat plants studied by Bloom et al. (2002) -- in air of 360 ppm CO2, while the roots of the seedlings were enclosed in sealed vessels containing either NO3- or NH4+ solutions, after which the solutions were equilibrated with air as high in CO2 concentration as 20,000 ppm. This rhizospheric CO2 enrichment had no effect on the uptake of NH4+ by the tomato seedlings; but it resulted in an enhanced uptake of NO3-, with the maximum effect occurring at a rhizospheric CO2 concentration of 5,000 ppm, which is to be compared to a normal root-zone CO2 concentration of something less than 5,000 ppm but more than 1,000 ppm, as is typical of soil airspace in most outdoor environments. Although we cannot be confident about all the possible implications of this observation, it does indicate a preferential plant uptake of nitrate N at higher-than-normal rhizospheric CO2 concentrations, which is hard to understand if plants are supposed to prefer ammonium N at high CO2 concentrations, as suggested by the work of Bloom et al. (2002).

It can be additionally noted, at this point, that attempting to discern and characterize a possible CO2-mediated plant preference for a particular form of nitrogen must involve considerably more complex investigations than simple laboratory experiments with individual plants whose roots have never been exposed to anything other than sterile nutrient solutions. Actual nonsterile soils are required to host the plants, if one is ever going to learn how plants operate in this regard in the world of nature. Hence, this realm of investigation begins with a brief review of an intermediate sort of study conducted by Constable et al. (2001), who although not using totally natural soil at least got beyond the hydroponic stage of investigation for the major portion of their experiment, and who also dealt with the added complexity supplied by the presence of two types of fungal symbionts, which often live in close association with the roots of plants in their normal habitats and serve as a living link between them and the soil environment.

Constable et al. studied mycorrhizal- and non-mycorrhizal-infected seedlings of both sweetgum and loblolly pine seedlings that were rooted in pots filled with fine sand above a layer of mycorrhizal inoculum, or clay lacking such inoculum, and grown for six months out-of-doors in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 700 ppm. At the conclusion of this stage of their investigation, they brought the seedlings into the laboratory, washed the sand from their roots and from the fungal hyphae associated with the roots of half of the plants, and placed the roots and root/hyphae systems in hydroponic solutions of NO3- and NH4+ for N uptake evaluations that were conducted within controlled environment chambers that were maintained at the same atmospheric CO2 concentrations to which the seedlings had been exposed while growing out-of-doors.

In this part of the experiment, both tree species exhibited a greater preference for NH4+ than for NO3-, regardless of mycorrhizal treatment, as is commonly reported for trees exposed to normal atmospheric CO2 concentrations (Gessler et al., 1998; Wallenda and Read, 1999). Nevertheless, the presence of mycorrhizae clearly improved nitrogen acquisition in both species at both CO2 concentrations; and, as the authors reported, "this increase in uptake capacity was preferentially for NO3- as opposed to NH4+." Furthermore, they noted that "in loblolly pine, the relative enhancement of NO3- uptake capacity by ectomycorrhizal fungi was significantly higher at elevated CO2 compared with ambient CO2," in direct opposition to what would be expected on the basis of the Bloom et al. (2002) experiment. In sweetgum, on the other hand, the reverse was true; and the authors thus urged caution in concluding too much from observations derived from too few species of both plants and mycorrhizae -- a caution, we might add, that seems to have not occurred to either Bloom et al. (2002) or the members of the press who wrote so confidently about the global applicability of the results of their wheat experiment.

In a still more realistic set of experiments, BassiriRad et al. (1999) grew two tree species -- red maple and sugar maple -- for close to 1.5 years out-of-doors in open-top chambers (OTCs) maintained at atmospheric CO2 concentrations of ambient and ambient plus 300 ppm, as well as two crop species -- soybean and sorghum -- that were studied for one full growing season in OTCs maintained at atmospheric CO2 concentrations of ambient and ambient plus 360 ppm. The trees were planted directly into the natural soil upon which the OTCs of their experiment were constructed; while the crops were planted in natural soil that filled a 2-meter-deep bin, measuring 6 meters wide and 76 meters long, upon which the OTCs of their experiment were constructed.

In both sets of experiments, small groups of fine roots were carefully exposed, cleaned, and inserted into tubes containing known volumes of 25, 50, 75, 100, 150 and 200 ÁM solutions of NH4NO3, after which the roots were allowed to take up whatever amounts of each form of N they preferred over periods of 30 to 60 minutes. The roots were then removed from the tubes and the portions that had been immersed in the nutrient solution excised, dried and weighed; while the volume of the remaining solution in each tube was stored for later assessment of the amounts of NH4+ and NO3- that had not been removed by the roots.

The results of these experiments indicated that all four species exhibited a distinct preference for NH4+ uptake over NO3- uptake when grown in air of normal atmospheric CO2 concentration; but this preference was only to be expected, because the energy requirements associated with the uptake and assimilation of NO3- are considerably greater than those associated with the uptake and assimilation of NH4+, as demonstrated by the work of Haynes and Goh (1978), Blacquiere (1987), and Glass and Siddiqi (1995). Also noted by BassiriRad et al. in this regard was the fact that "the greater preference for NH4+ vs. NO3- is almost a universal root characteristic in tree species and is often associated with an adaptation to forest soils that are relatively low in NO3-."

So what happened with the plants in the CO2-enriched chambers? Red maple did indeed exhibit a slight enhancement of its ambient-air preference for NH4+. The other three species, however, showed no change in N preference at the higher CO2 concentration. Hence, one of the four species studied provided weak support for the hypothesis of the Bloom et al. (2002) study; but the other three species provided no support.

Where does all of this discussion lead? For one thing, the body of literature discussed above suggests that different species may well behave differently with respect to the effects of atmospheric CO2 enrichment on their N uptake kinetics. BassiriRad et al. additionally note, for example, that "using potted seedlings we have shown elsewhere (BassiriRad et al., 1997a, b) that high CO2 increased, decreased or had no significant effect on NO3- uptake kinetics depending upon species tested." And they add that in a hydroponic experiment using soybean and sunflower, they found that "root N uptake kinetics response to CO2 enrichment was highly dependent on the stages of development and root age." Hence, they say that "a 'one point in time' determination" -- such as that which comprised the study of Bloom et al. (2002) -- "is not adequate," and that "more measurements of root N uptake kinetics are necessary to draw valid conclusions about possible effects of CO2."

One couldn't agree more. The Bloom et al. (2002) experiment in no way portends any of the monumental biospheric problems that they and the climate-alarmist press have attempted to convince us will occur on the basis of the results of their severely-restricted study. And the newer study of Bloom et al. (2010) likewise provides no support whatsoever for their equally wild speculations about the future of earth's biosphere. In all five of their newest experiments, for example, the plants they studied were grown (1) hydroponically in (2) isolation under (3) sterile laboratory conditions for (4) only a few weeks, while measurements of the key processes they made were generally conducted over (5) only a matter of hours. This is not the way to attempt to determine what might actually happen over the long haul in the real world. So what is?

The key to knowledge of what really is the case resides in conducting long-term studies in as natural a real-world setting as possible. For many people this means working out-of-doors in huge open-top chambers with plants rooted directly in the ground, which is the way the 17-year Phoenix, Arizona sour orange tree study was conducted. For others it also means working out-of-doors, but doing it within the context of free-air CO2 enrichment or FACE experiments. Summarizing nine years of such work at the Duke Forest FACE facility in North Carolina (USA), where portions of an aggrading loblolly pine plantation had been continuously exposed to an extra 200 ppm of CO2 since 1996, Lichter et al. (2008) reported that the CO2-induced increase in productivity there had amounted to about 30% annually -- which roughly equates to a 45% increase for a 300 ppm increase in CO2, and even more for a true doubling of the air's CO2 content -- adding that there is "little evidence to indicate a diminished response through time," while citing, in this regard, the analysis of Finzi et al. (2007), who found the same to be true at three other long-term forest FACE studies being conducted at Rhinelander, Wisconsin (USA), Oak Ridge National Laboratory (USA), and Tuscania (Italy).

As but one example, after working at the EuroFACE facility in Central Italy for a period of several years, Davey et al. (2006) published their observations in a paper entitled "Can fast-growing plantation trees escape biochemical down-regulation of photosynthesis when grown throughout their complete production cycle in the open air under elevated carbon dioxide?" In it, they report that poplar trees exposed to a 50% increase in atmospheric CO2 concentration over four growing seasons "showed a sustained increase in photosynthesis of between 35 and 60% prior to coppicing," and that "this increase in daily photosynthesis [was] maintained during the re-growth," such that "no long-term photosynthetic acclimation to CO2 occurred in these plants," while noting that "poplar trees are able to 'escape' from long-term, acclamatory down-regulation of photosynthesis," and that "the acclamatory loss of the initial increase in photosynthetic rate under elevated CO2 is not inevitable." And in their report of another study conducted on the same trees, Calfapietra et al. (2005) have written that "photosynthetic acclimation of poplar plantations is unlikely to occur in an atmosphere enriched in CO2 and thereby will not influence the response of poplar plantations to increasing atmospheric CO2 concentrations either over the long term or under conditions of nitrogen deposition."

But perhaps the most amazing example of avoiding long-term photosynthetic acclimation in the real world comes from Paoletti et al. (2007), who measured rates of net photosynthesis during a two-week period in June of 2002 "at the end of the spring rains" when midday air temperatures rose above 40°C in upper sunlit leaves of mature holm oak trees growing close to (5 m) and further away from (130 m) a natural CO2-emitting spring near Laiatico (Pisa, Italy), where the trees had experienced lifetime exposure to atmospheric CO2 concentrations of approximately 1500 and 400 ppm, respectively. This work revealed that the net photosynthetic rates of the leaves on the trees growing closest to the CO2 spring were approximately 250% greater than those of the leaves on the trees growing 125 meters further away, where the atmospheric CO2 concentration was 1100 ppm less than it was in the vicinity of the trees nearest the spring. The four Italian researchers thus concluded that "the considerable photosynthetic stimulation at the very high CO2 site suggests no photosynthetic down-regulation over long-term CO2 enrichment." And this real-world finding demonstrates the truly amazing potential for very large increases in the air's CO2 content to greatly stimulate photosynthesis and significantly enhance the growth and development of earth's plants over the very long term.

Contrary to the strident claims of Bloom et al. (2002) and Bloom et al. (2010), therefore, it would appear that earth's trees -- including those that are thought to have access to less-than-adequate soil nitrogen supplies -- are entirely capable of maintaining the sizable increases in their growth rates that are made possible by elevated concentrations of atmospheric CO2. In the case of North Carolina's Duke Forest, for example, "even after nine years of experimental CO2 fertilization," as Lichter et al. (2008) describe it, "attenuation of the CO2-induced productivity enhancement has not been observed," as has also been noted to be the case by Finzi et al. (2006). And this finding at this particular location is extremely significant, because the growth of pine-hardwood forests in the southeastern United States often removes so much nitrogen from the soils in which they grow that they induce what Finzi and Schlesinger (2003) have described as "a state of acute nutrient deficiency that can only be reversed with fertilization," which operation, however, has not been employed in the Duke Forest FACE study, which makes it about as challenging a situation as there could possibly be for the long-term persistence of the growth-promoting aerial fertilization effect of atmospheric CO2 enrichment.

Yet the phenomenon does precisely that ... it persists!

Additional References
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BassiriRad, H., Griffin, K.L., Reynolds, J.F. and Strain, B.R. 1997a. Changes in root NH4+ and NO3- absorption rates of loblolly and ponderosa pine in response to CO2 enrichment. Plant and Soil 190: 1-9.

BassiriRad, H., Prior, S.A., Norby, R.J. and Rogers, H.H. 1999. A field method of determining NH4+ and NO3- uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species. Plant and Soil 217: 195-204.

BassiriRad, H., Reynolds, J.F., Virginia, R.A. and Brunelle, M.H. 1997b. Growth and root NO3- and PO3- uptake capacity of three desert species in response to atmospheric CO2 enrichment. Australian Journal of Plant Physiology 24: 353-358.

Bauer, G.A. and Berntson, G.M. 2001. Ammonium and nitrate acquisition by plants in response to elevated CO2 concentration: the roles of root physiology and architecture. Tree Physiology 21: 137-144.

Blacquiere, T. 1987. Ammonium and nitrate nutrition in Plantago lanceolata and P. major ssp. major. II. Efficiency of root respiration and growth. Comparison of measured and theoretical values of growth respiration. Plant Physiology and Biochemistry 25: 1775-1785.

Bloom, A.J., Smart, D.R., Nguyen, D.T. and Searles, P.S. 2002. Nitrogen assimilation and growth of wheat under elevated carbon dioxide. Proceedings of the National Academy of Sciences, USA 99: 1730-1735.

Calfapietra, C., Tulva, I., Eensalu, E., Perez, M., De Angelis, P., Scarascia-Mugnozza, G. and Kull, O. 2005. Canopy profiles of photosynthetic parameters under elevated CO2 and N fertilization in a poplar plantation. Environmental Pollution 137: 525-535.

Constable, J.V.H., BassiriRad, H., Lussenhop, J. and Zerihun, A. 2001. Influence of elevated CO2 and mycorrhizae on nitrogen acquisition: contrasting responses in Pinus taeda and Liquidambar styraciflua. Tree Physiology 21: 83-91.

Davey, P.A., Olcer, H., Zakhleniuk, O., Bernacchi, C.J., Calfapietra, C., Long, S.P. and Raines, C.A. 2006. Can fast-growing plantation trees escape biochemical down-regulation of photosynthesis when grown throughout their complete production cycle in the open air under elevated carbon dioxide? Plant, Cell and Environment 29: 1235-1244.

Finzi, A.C., Moore, D.J.P., DeLucia, E.H., Lichter, J., Hofmockel, K.S., Jackson, R.B., Kim, H.-S., Matamala, R., McCarthy, H.R., Oren, R., Pippen, J.S. and Schlesinger, W.H. 2006. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest. Ecology 87: 15-25.

Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.

Finzi, A.C. and Schlesinger, W.H. 2003. Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456.

Frenandez, M.D., Pieters, A., Donoso, C., Tezara, W., Azuke, M., Herrera, C., Fengifo, E. and Herrera, A. 1998. Effects of a natural source of very high CO2 concentration on the leaf gas exchange, xylem water potential and stomatal characteristics of plants of Spatiphylum cannifolium and Bauhinia multinervia. New Phytologist 138: 689-697.

Garcia, R.L., Long, S.P., Wall, G.W., Osborne, C.P., Kimball, B.A., Nie, G.Y., Pinter Jr., P.J., LaMorte, R.L. and Wechsung, F. 1998. Photosynthesis and conductance of spring-wheat leaves: field response to continuous free-air atmospheric CO2 enrichment. Plant, Cell and Environment 21: 659-669.

Gessler, A., Schneider, S., Von Sengbusch, D., Weber, P., Hanemann, W., Huber, C., Rothe, A., Kreutzer, K. and Rennenberg, H. 1998. Field and laboratory experiments on net uptake of nitrate and ammonium by the roots of spruce (Picea abies) and beech (Fagus sylvatica) trees. New Phytologist 138: 275-285.

Glass, A.D.M. and Siddiqi, M.Y. 1995. Nitrogen absorption by plant roots. In: Nitrogen Nutrition in Higher Plants. Srivastava, H.S. and Singh, R.P. (Eds.). Associated Publishing Co., New Delhi, India, pp. 21-56.

Gunderson, C.A., Norby, R.J. and Wullschleger, S.D. 1993. Foliar gas exchange responses of two deciduous hardwoods during three years of growth in elevated CO2: No loss of photosynthetic enhancement. Plant, Cell and Environment 16: 797-807.

Haynes, R.J. and Goh, K.M. 1978. Ammonium and nitrate nutrition of plants. Biological Reviews 53: 465-510.

Idso, S.B. 1999. The long-term response of trees to atmospheric CO2 enrichment. Global Change Biology 5: 493-495.

Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

Jacob, J., Greitner, C. and Drake, B.G. 1995. Acclimation of photosynthesis in relation to Rubisco and non-structural carbohydrate contents and in situ carboxylase activity in Scirpus olneyi grown at elevated CO2 in the field. Plant, Cell and Environment 18: 875-884.

Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour orange trees: final results. Global Change Biology 13: 2171-2183.

Lichter, J., Billings, S.A., Ziegler, S.E., Gaindh, D., Ryals, R., Finzi, A.C., Jackson, R.B., Stemmler, E.A. and Schlesinger, W.H. 2008. Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment. Global Change Biology 14: 2910-2922.

Padgett, P.E. and Leonard, R.T. 1993. Contamination of ammonium-based nutrient solutions by nitrifying organisms and the conversion of ammonium to nitrate. Plant Physiology 101: 141-146.

Paoletti, E., Seufert, G., Della Rocca, G. and Thomsen, H. 2007. Photosynthetic responses to elevated CO2 and O3 in Quercus ilex leaves at a natural CO2 spring. Environmental Pollution 147: 516-524.

Smart, D.R., Ritchie, K., Stark, J.M. and Bugbee, B. 1997. Evidence that elevated CO2 levels can indirectly increase rhizosphere denitrifier activity. Applied and Environmental Microbiology 63: 4621-4624.

Van der Merwe, C.A and Cramer, M.D. 2000. Effect of enriched rhizosphere carbon dioxide on nitrate and ammonium uptake in hydroponically grown tomato plants. Plant and Soil 221: 5-11.

Vogel, C.S. and Curtis, P.S. 1995. Leaf gas exchange and nitrogen dynamics of N2-fixing, field-grown Alnus glutinosa under elevated atmospheric CO2. Global Change Biology 1: 55-61.

Wallenda, T. and Read, D.J. 1999. Kinetics of amino acid uptake by ectomycorrhizal roots. Plant, Cell and Environment 22: 179-187.

Archived 26 May 2010