Most species of plants respond to increases in the air's CO2 content by displaying enhanced rates of photosynthesis and biomass production. Oftentimes, the resulting growth stimulation is preferentially expressed belowground, thereby causing significant increases in fine-root numbers and surface area. This phenomenon tends to increase total nutrient uptake under CO2-enriched conditions, which further stimulates plant growth and development. In this summary, we thus review how the acquisition of plant nutrients - primarily nitrate and phosphate - is impacted by atmospheric CO2 enrichment. We do not discuss the effects of elevated CO2 on nitrogen fixation, however, as we address this subject elsewhere on our website (see Nitrogen Fixation).
In the study of Zak et al. (2000), aspen seedlings grown for 2.5 years at twice-ambient CO2 concentrations displayed an average total seedling nitrogen content that was 13% greater than that displayed by control seedlings grown in ambient air, in spite of an average reduction in tissue nitrogen concentration of 18%. Thus, elevated CO2 enhanced total nitrogen uptake from the soil, even though tissue nitrogen concentrations in the CO2-enriched plants were diluted by the enhanced biomass of the much larger CO2-enriched seedlings.
On a per-unit-biomass basis, Smart et al. (1998) noted there were no differences in the total amounts of nitrogen within CO2-enriched and ambiently-grown wheat seedlings after three weeks of exposure to atmospheric CO2 concentrations of 360 and 1,000 ppm. Nevertheless, the CO2-enriched seedlings exhibited greater rates of soil nitrate extraction than did the ambiently-grown plants. Similarly, BassiriRad et al. (1998) reported that a doubling of the atmospheric CO2 concentration doubled the uptake rate of nitrate in the C4 grass Bouteloua eriopoda. However, they also reported that elevated CO2 had no effect on the rate of nitrate uptake in Prosopis, and that it actually decreased the rate of nitrate uptake by 55% in Larrea. Nonetheless, atmospheric CO2 enrichment increased total biomass in these two species by 55 and 69%, respectively. Thus, although the uptake rate of this nutrient was depressed under elevated CO2 conditions in the latter species, the much larger CO2-enriched plants likely still extracted more total nitrate from the soil than did the ambiently-grown plants of the experiment.
It is also interesting to note that Nasholm et al. (1998) determined that trees, grasses and shrubs can all absorb significant amounts of organic nitrogen from soils. Thus, plants do not have to wait for the mineralization of organic nitrogen before they extract the nitrogen they need from soils to support their growth and development. Hence, the forms of nitrogen removed from soils by plants (nitrate vs. ammonium) and their abilities to remove different forms may not be as important as was once thought.
With respect to the uptake of phosphate, Staddon et al. (1999) reported that Plantago lanceolata and Trifolium repens plants grown at 650 ppm CO2 for 2.5 months exhibited total plant phosphorus contents that were much greater than those displayed by plants grown at 400 ppm CO2, due to the fact that atmospheric CO2 enrichment significantly enhanced plant biomass. Similarly, Rouhier and Read (1998) reported that enriching the air around Plantago lanceolata plants with an extra 190 ppm of CO2 for a period of three months led to increased uptake of phosphorus and greater tissue phosphorus concentrations than were observed in plants growing in ambient air.
Greater uptake of phosphorus can also occur due to CO2-induced increases in root absorptive surface area or enhancements in specific enzyme activities. In addressing the first of these phenomena, BassiriRad et al. (1998) reported that a doubling of the atmospheric CO2 concentration significantly increased the belowground biomass of Bouteloua eriopoda and doubled its uptake rate of phosphate. However, elevated CO2 had no effect on uptake rates of phosphate in Larrea and Prosopis. Because the CO2-enriched plants grew so much bigger, however, they still removed more phosphate from the soil on a per-plant basis. With respect to the second phenomenon, phosphatase - which is the primary enzyme responsible for the conversion of organic phosphate into usable inorganic forms - had its activity increased by 30 to 40% in wheat seedlings growing at twice-ambient CO2 concentrations (Barrett et al., 1998).
In summary, as the CO2 content of the air increases, the experimental data that have been accumulated to date suggest that much of earth's vegetation will likely display increases in biomass; and there is considerable evidence suggestive of the further likelihood that the larger plants thereby produced will develop more extensive root systems and extract enhanced amounts of mineral nutrients from the soils in which they are rooted.
References
Barrett, D.J., Richardson, A.E. and Gifford, R.M. 1998. Elevated atmospheric CO2 concentrations increase wheat root phosphatase activity when growth is limited by phosphorus. Australian Journal of Plant Physiology 25: 87-93.
BassiriRad, H., Reynolds, J.F., Virginia, R.A. and Brunelle, M.H. 1998. Growth and root NO3- and PO43- uptake capacity of three desert species in response to atmospheric CO2 enrichment. Australian Journal of Plant Physiology 24: 353-358.
Nasholm, T., Ekblad, A., Nordin, A., Giesler, R., Hogberg, M. and Hogberg, P. 1998. Boreal forest plants take up organic nitrogen. Nature 392: 914-916.
Rouhier, H. and Read, D.J. 1998. The role of mycorrhiza in determining the response of Plantago lanceolata to CO2 enrichment. New Phytologist 139: 367-373.
Smart, D.R., Ritchie, K., Bloom, A.J. and Bugbee, B.B. 1998. Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations. Plant, Cell and Environment 21: 753-763.
Staddon, P.L., Fitter, A.H. and Graves, J.D. 1999. Effect of elevated atmospheric CO2 on mycorrhizal colonization, external mycorrhizal hyphal production and phosphorus inflow in Plantago lanceolata and Trifolium repens in association with the arbuscular mycorrhizal fungus Glomus mosseae. Global Change Biology 5: 347-358.
Zak, D.R., Pregitzer, K.S., Curtis, P.S., Vogel, C.S., Holmes, W.E. and Lussenhop, J. 2000. Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloides. Ecological Applications 10: 34-46.