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Plant Growth Response to CO2 and Nitrogen
(Grasses) -- Summary

In controlled experiments, enriching the air with elevated concentrations of atmospheric CO2 nearly always results in increased plant photosynthetic rates and biomass production, especially when environmental conditions are favorable to growth.  Some individuals, however, have questioned whether atmospheric CO2 enrichment will enhance plant growth under conditions of low soil nitrogen availability, which often induces stress in earth’s vegetation.  In this summary, we review the responses of various grasses and grassland communities to atmospheric CO2 enrichment under varying concentrations of soil nitrogen to determine if low soil nitrogen availability negates the positive effects of elevated CO2 on their photosynthesis and growth.

Perennial ryegrass (Lolium perenne L.) has been used as a model species in many experiments to help elucidate grassland responses to atmospheric CO2 enrichment and soil nitrogen availability.  In the FACE study of Rogers et al. (1998), for example, plants exposed to 600 ppm CO2 exhibited a 35% increase in their photosynthetic rates without regard to soil nitrogen availability.  However, when ryegrass was grown in plastic ventilated tunnels at twice-ambient concentrations of atmospheric CO2, the
CO2-induced photosynthetic response was about 3-fold greater in a higher, as opposed to a lower, soil nitrogen regime (Casella and Soussana, 1997).  Similarly, in an open-top chamber study conducted by Davey et al. (1999), it was reported that an atmospheric CO2 concentration of 700 ppm stimulated photosynthesis by 30% in this species when it was grown with moderate, but not low, soil nitrogen availability.  Thus, CO2-induced photosynthetic stimulations in perennial ryegrass can be influenced by soil nitrogen content, with greater positive responses typically occurring under higher, as opposed to lower, soil nitrogen availability.

With respect to biomass production, van Ginkel and Gorissen (1998) reported that a doubling of the atmospheric CO2 concentration increased shoot biomass of perennial ryegrass by 28%, regardless of soil nitrogen concentration.  In the more revealing six-year FACE study of Daepp et al. (2000), plants grown at 600 ppm CO2 and high soil nitrogen availability continually increased their dry matter production over that observed in ambient-treatment plots, from 8% more in the first year to 25% more at the close of year six.  When grown at a low soil nitrogen availability, however, CO2-enriched plants exhibited an initial 5% increase in dry matter production, which dropped to a negative 11% in year two; but this negative trend was thereafter turned around, and it continually rose to ultimately reach a 9% stimulation at the end of the study.  Thus, these data demonstrate that elevated CO2 increases perennial ryegrass biomass, even under conditions of low soil nitrogen availability, especially under conditions of long-term atmospheric CO2 enrichment.

If perennial ryegrass is indeed representative of most grassland species, one would anticipate the remainder of this summary to include similar positive effects of CO2 on the photosynthesis and biomass of other grasses, even under low nitrogen conditions; and that is exactly what the data reveal.  Lutze et al. (1998), for example, reported that microcosms of the C3 grass Danthonia richardsonii grown for four years in glasshouses fumigated with air containing 720 ppm CO2 displayed total photosynthetic carbon gains that were always 15-34% higher than those of ambiently-grown microcosms, depending on the soil nitrogen concentration.  In a clearer depiction of photosynthetic responses to soil nitrogen, Davey et al. (1999) noted that photosynthetic rates of Agrostis capillaries subjected to twice-ambient levels of atmospheric CO2 for two years were 12 and 38% greater than rates measured in control plants grown at 350 ppm CO2 under high and low soil nitrogen regimes, respectively.  In addition, they also reported CO2-induced photosynthetic stimulations of 25 and 74% for Trifolium repens subjected to high and low soil nitrogen regimes, respectively.  Thus, we see that the greatest CO2-induced percentage increase in photosynthesis can sometimes occur under the least favorable soil nitrogen conditions.

With respect to biomass production, Navas et al. (1999) reported that 60 days’ exposure to 712 ppm CO2 increased biomass production of Danthonia richardsonii, Phalaris aquatica, Lotus pedunculatus, and Trifolium repens across a large soil nitrogen gradient.  With slightly more detail, Cotrufo and Gorissen (1997) reported average CO2-induced increases in whole-plant dry weights of Agrostis capillaries and Festuca ovina that were 20% greater than those of their respective controls, regardless of soil nitrogen availability.  In the study of Ghannoum and Conroy (1998), three Panicum grasses grown for two months at twice-ambient levels of atmospheric CO2 and high soil nitrogen availability displayed similar increases in total plant dry mass that were about 28% greater than those of their respective ambiently-grown controls.  At low nitrogen, however, elevated CO2 had no significant effect on the dry mass of two of the species, while it actually decreased that of the third species.

In summary, it is clear that atmospheric CO2 enrichment stimulates photosynthesis and biomass production in grasses and grassland species when soil nitrogen availability is high and/or moderate.  Under lower soil nitrogen conditions, it is also clear that atmospheric CO2 enrichment can have the same positive effect on these parameters, but that it can also have a reduced positive effect, no effect, or (in one case) a negative effect.  In light of the one long-term study that lasted six years, however, it is likely that – given enough time – grasslands have the ability to overcome soil nitrogen limitations and produce positive CO2-induced growth responses.  Thus, because the rising CO2 content of the air is likely to continue for a long time to come, occasional nitrogen limitations on the aerial fertilization effect of atmospheric CO2 enrichment of grasslands will likely become less and less restrictive as time progresses.

Casella, E. and Soussana, J-F.  1997.  Long-term effects of CO2 enrichment and temperature increase on the carbon balance of a temperate grass sward.  Journal of Experimental Botany 48: 1309-1321.

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.

Davey, P.A., Parsons, A.J., Atkinson, L., Wadge, K. and Long, S.P.  1999.  Does photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency?  A study of three native UK grassland species in open-top chambers.  Functional Ecology 13: 21-28.

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.

Lutze, J.L. and Gifford, R.M.  1998.  Carbon accumulation, distribution and water use of Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth.  Global Change Biology 4: 851-861.

Navas, M.-L., Garnier, E., Austin, M.P. and Gifford, R.M.  1999.  Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along a nitrogen gradient: differences between isolated plants, monocultures and multi-species mixtures.  New Phytologist 143: 323-331.

Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P.  1998.  Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization.  Perennial ryegrass under free-air CO2 enrichment.  Plant Physiology 118: 683-689.

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.