Nearly all of earth's plants respond to increases in the air's CO2 content by exhibiting enhanced rates of photosynthesis and biomass production. In addition, leguminous species possess the special ability to form symbiotic relationships with nitrogen-fixing bacteria, which can indirectly increase plant responses to elevated CO2 by making more nitrogen available for plant uptake and utilization. In this review, we summarize the results of some of the studies we have reviewed that report the effects of atmospheric CO2 enrichment on these phenomena in woody plants.
Olesniewicz and Thomas (1999) grew black locust (Robinia pseudoacacia) seedlings for approximately two months in controlled environment chambers maintained at atmospheric CO2 concentrations of 350 and 710 ppm, determining that the elevated CO2 increased total plant biomass by 180%. In addition, the extra CO2 increased nitrogen-fixation by 69%, nodule mass by 92%, and the amount of seedling nitrogen derived from nitrogen-fixation by 212%. Working with the same species under much the same conditions, Uselman et al. (1999) additionally determined that between 1 and 2% of the total symbiotically-fixed nitrogen is exuded from the tree's roots to become available for uptake by neighboring vegetation.
Schortemeyer et al. (1999) grew seedlings of Acacia melanoxylon, a leguminous nitrogen-fixing tree native to south-eastern Australia, for six weeks in growth cabinets maintained at atmospheric CO2 concentrations of 350 and 700 ppm in hydroponic solutions with nitrogen concentrations ranging from 3 to 6,400 mmol m-3. Although atmospheric CO2 enrichment did not stimulate symbiotic nitrogen fixation, averaged across all nitrogen treatments, the seedlings grown in elevated CO2 displayed net photosynthetic rates that were 22% higher than those of control seedlings, and they did not exhibit any signs of photosynthetic acclimation. These positive responses likely contributed to the doubled final biomass observed in the CO2-enriched seedlings in all but the two lowest nitrogen concentrations, where final biomass was unaffected by elevated CO2.
In a subsequent study of seven Acacia species native to Australia, Shortemeyer et al. (2002) once again grew seedlings in environmental chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm, but this time for nearly five months, finding that the elevated CO2 enhanced rates of net photosynthesis by 19 to 56% among all species and led to an average total plant dry weight increase of 86%. In addition, the elevated CO2 increased the total amount of nitrogen fixed per plant by an average of 65%.
Last of all, Temperton et al. (2003) measured total biomass and a number of physiological processes of N2-fixing Alnus glutinosa or common alder trees that were grown for three years (1994-1996) in open-top chambers maintained at either ambient or elevated (ambient + 350 ppm) concentrations of atmospheric CO2 and two soil nitrogen regimes (full nutrient solution or no fertilizer), while they measured nitrogen fixation by Frankia spp. in the root nodules of the trees. They found that nitrogenase activity was consistently higher in the elevated CO2 treatment in both 1995 and 1996. In addition, they report that "in October 1996, elevated CO2 had a significant effect on total nodule dry mass, and there was a trend toward heavier nodules in the elevated CO2 treatment than in the ambient CO2 treatment." With respect to these findings, they report that "most single-species studies on the effect of elevated CO2 on N2-fixing species have reported stimulation of growth, nodule mass and nitrogenase activity (Norby, 1987; Arnone and Gordon, 1990; Hibbs et al., 1995; Vogel and Curtis, 1995; Tissue et al., 1997; Vogel et al., 1997; Thomas et al., 2000)," which is similar to what they observed.
In light of these several findings, it would appear that continued increases in the air's CO2 content will greatly enhance the growth of earth's leguminous trees, while stimulating their fixation of nitrogen and increasing their root exudations of nitrogenous substances, all of which phenomena bode well for the biosphere.
References
Arnone, J.A. and Gordon, J.C. 1990. Effect of nodulation, nitrogen fixation and CO2 enrichment on the physiology, growth and dry mass allocation of seedlings of Alnus rubra Bong. New Phytologist 116: 55-66.
Hibbs, D.E., Chan, S.S., Castellano, M. and Niu, C.-H. 1995. Response of red alder seedlings to CO2 enrichment and water stress. New Phytologist 129: 569-577.
Norby, R.J. 1987. Nodulation and nitrogenase activity in nitrogen fixing woody plants stimulated by CO2 enrichment of the atmosphere. Physiologia Plantarum 71: 77-82.
Olesniewicz, K.S. and Thomas, R.B. 1999. Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide. New Phytologist 142: 133-140.
Schortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R. 1999. The impact of elevated atmospheric CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of Acacia melanoxylon. Australian Journal of Plant Physiology 26: 737-774.
Schortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R. 2002. N2 fixation by Acacia species increases under elevated atmospheric CO2. Plant, Cell and Environment 25: 567-579.
Temperton, V.M., Grayston, S.J., Jackson, G., Barton, C.V.M. , Millard, P. and Jarvis, P.G. 2003. Effects of elevated carbon dioxide concentration on growth and nitrogen fixation in Alnus glutinosa in a long-term field experiment. Tree Physiology 23: 1051-1059.
Thomas, R.B., Bashkin, M.A. and Richter, D.D. 2000. Nitrogen inhibition of nodulation and N2 fixation of a tropical N2-fixing tree (Gliricida sepium) grown in elevated atmospheric CO2. New Phytologist 145: 233-243.
Tissue, D.T., Thomas, R.B. and Strain, B.R. 1997. Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4-year experiment in the field. Plant, Cell and Environment 20: 1123-1134.
Uselman, S.M., Qualls, R.G. and Thomas, R.B. 1999. A test of a potential short cut in the nitrogen cycle: The role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature. Plant and Soil 210: 21-32.
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.
Vogel, C.S., Curtis, P.S. and Thomas, R.B. 1997. Growth and nitrogen accretion of dinitrogen-fixing Alnus glutinosa (L.) Gaertn. under elevated carbon dioxide. Plant Ecology 130: 63-70.