As the CO2 content of the air increases, most plants exhibit modifications in their physiology. One common change is increased photosynthesis. With greater amounts of CO2 in the air – and greater amounts of CO2 thus diffusing into leaves – the primary carboxylating enzyme of most plants, i.e. rubisco, performs its biochemical functions more efficiently, leading to reductions in photorespiratory carbon losses and increases in carbohydrate synthesis; and with additional carbohydrate production, most plants exhibit enhanced rates of growth and biomass accumulation. In this summary we thus review the effects of elevated atmospheric CO2 concentrations on biomass production in individual grassland species.
In two related studies, perennial ryegrass (Lolium perenne) was grown in controlled environmental chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm for approximately 14 weeks. After assessing plant growth, the authors reported CO2-induced increases in shoot (van Ginkel and Gorissen, 1998) and root (van Ginkel et al., 2000) biomass of 28 and 41%, respectively. Hodge et al. (1998) also reported, for the very same species, that plants grown at an atmospheric CO2 concentration of 720 ppm for a mere 21 days exhibited total biomass values that were a whopping 175% greater than those observed for control plants exposed to air of 450 ppm CO2.
In the study of Cotrufo and Gorissen (1997), three grasses (Lolium perenne, Agrostis capillaries, and Festuca ovina) were grown at atmospheric CO2 concentrations of 350 and 700 ppm for approximately two months before harvesting. On average, atmospheric CO2 enrichment increased plant biomass by approximately 20%, with greater carbon partitioning to roots, as opposed to shoots. Also, in a much shorter two-week study performed on tall fescue (Festuca ovina), Newman et al. (1999) reported that twice-ambient levels of atmospheric CO2 increased plant biomass by 37% relative to plants grown in ambient air.
To investigate the possible interactions of elevated CO2 and soil nitrogen on biomass production, Ghannoum and Conroy (1998) grew one C3 grass (Panicum laxum) and two C4 grasses (Panicum coloratum and Panicum antidotale) in controlled environments subjected to atmospheric CO2 concentrations of 360 and 710 ppm and low and high soil nitrogen contents for about two months. At high soil nitrogen, elevated CO2 increased total plant biomass by approximately 27% in all three species. However, at low soil nitrogen, elevated CO2 had no effect on biomass production in Panicum laxum and Panicum antidotale, while it actually decreased growth in Panicum coloratum.
Taken alone, these results seem to suggest that nitrogen deficiency may preclude CO2-induced growth responses in these grasses. However, the results of a six-year study of Lolium perenne caution us against making hasty conclusions based on the results of short-term studies. In this much more lengthy experiment, for example, Daepp et al. (2000) reported that plant biomass slowly increased from 8 to 25% over the six-year period under conditions of high soil nitrogen and a 250 ppm increase in atmospheric CO2 concentration. However, under conditions of low soil nitrogen, they observed an initial biomass increase of 5%, which dropped to –11% in year two, before it slowly began increasing to a final CO2-induced enhancement of 9% at the close of year six. Hence, low soil nitrogen levels may preclude CO2-induced growth responses in the short-term; but in the long-term, CO2-enriched plants seem to be able to find the nutrients they need to increase their growth.
A few studies have also investigated the effects of elevated CO2 and high air temperature on biomass production in grassland species. Norton et al. (1999), for example, grew ten populations of Agrostis curtisii at ambient and elevated (700 ppm) atmospheric CO2 concentrations in combination with ambient and elevated (ambient + 3°C) air temperature. After one year, no significant effects of elevated CO2 or temperature could be detected, although plant biomass tended to be greater under the combined elevated CO2/elevated air temperature treatment. Greer et al. (2000), however, grew five pasture species for one month at atmospheric CO2 concentrations of 350 and 700 ppm in combination with air temperatures of 18 and 28 °C, finding that the CO2-induced increase in average biomass rose from 8% at 18 °C to 95% at 28 °C.
Lastly, in the study of Lilley et al. (2001), Trifolium subterraneum and Phalaris aquatica were grown in mixed swards exposed to atmospheric CO2 concentrations of 380 and 690 ppm and air temperatures of ambient and ambient + 3.4 °C for an entire year. They discovered that elevated CO2 increased average plant biomass by 35% at the ambient air temperature; and although the high temperature reduced this effect, plants exposed to elevated CO2 and elevated air temperature still exhibited a biomass enhancement that was 23% greater than that of plants subjected to ambient CO2 and elevated air temperature.
The paper of Wand et al. (1999) serves as a good summary of these findings. They compiled and analyzed 40 and 80 individual responses of C4 and C3 grasses, respectively, to atmospheric CO2 enrichment, determining that twice-ambient levels of atmospheric CO2 increased C4 and C3 plant biomass by an average of 33 and 44%, respectively, under a wide range of experimental conditions. Hence, as the air’s CO2 content continues to increase, it is likely that individual grassland species will respond by exhibiting enhanced rates of photosynthesis, which invariably enhances their ability to produce greater amounts of biomass, the carbon of a portion of which is almost always sequestered in the ground.
References
Cotrufo, M.F. and Gorissen, A. 1997. Elevated CO2 enhances below-ground C allocation in three perennial grass species at different levels of N availability. New Phytologist 137: 421-431.
Daepp, M., Suter, D., Almeida, J.P.F., Isopp, H., Hartwig, U.A., Frehner, M., Blum, H., Nosberger, J. and Luscher, A. 2000. Yield response of Lolium perenne swards to free air CO2 enrichment increased over six years in a high N input system on fertile soil. Global Change Biology 6: 805-816.
Ghannoum, O. and Conroy, J.P. 1998. Nitrogen deficiency precludes a growth response to CO2 enrichment in C3 and C4 Panicum grasses. Australian Journal of Plant Physiology 25: 627-636.
Greer, D.H., Laing, W.A., Campbell, B.D. and Halligan, E.A. 2000. The effect of perturbations in temperature and photon flux density on the growth and photosynthetic responses of five pasture species. Australian Journal of Plant Physiology 27: 301-310.
Hodge, A., Paterson, E., Grayston, S.J., Campbell, C.D., Ord, B.G. and Killham, K. 1998. Characterization and microbial utilisation of exudate material from the rhizosphere of Lolium perenne grown under CO2 enrichment. Soil Biology and Biochemistry 30: 1033-1043.
Lilley, J.M., Bolger, T.P. and Gifford, R.M. 2001. Productivity of Trifolium subterraneum and Phalaris aquatica under warmer, higher CO2 conditions. New Phytologist 150: 371-383.
Newman, J.A., Gibson, D.J., Hickam, E., Lorenz, M., Adams, E., Bybee, L. and Thompson, R. 1999. Elevated carbon dioxide results in smaller populations of the bird cherry-oat aphid Rhopalosiphum padi. Ecological Entomology 24: 486-489.
Norton, L.R., Firbank, L.G., Gray, A.J. and Watkinson, A.R. 1999. Responses to elevated temperature and CO2 in the perennial grass Agrostis curtisii in relation to population origin. Functional Ecology 13: 29-37.
van Ginkel, J.H., Gorissen, A. and Polci, D. 2000. Elevated atmospheric carbon dioxide concentration: effects of increased carbon input in a Lolium perenne soil on microorganisms and decomposition. Soil Biology & Biochemistry 32: 449-456.
van Ginkel, J.H. and Gorissen, A. 1998. In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide. Soil Science Society of America Journal 62: 951-958.
Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S. 1999. Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions. Global Change Biology 5: 723-741.