Under ambient conditions, plant growth and development are typically carbon-limited, which is why plants generally exhibit increased growth and biomass production in response to atmospheric CO2 enrichment. Next to carbon, nitrogen is usually the next most limiting nutrient to plant growth, followed by phosphorus. Thus, although it is a less significant component of plant tissues than carbon and nitrogen, phosphorus is still required for successful life cycle completion in many plant species. Hence, it is prudent to investigate aspects of plant phosphorus acquisition and biomass production in response to atmospheric CO2 enrichment when phosphorus concentrations in soils are less than optimal.
To date, we have reviewed only one report that has evaluated phosphorus acquisition under conditions of atmospheric CO2 enrichment. In that study, Barrett et al. (1998) demonstrated that a doubling of the air's CO2 content under continuous phosphorus deficiency increased wheat root phosphatase activity by 30 to 40%, thus increasing the inorganic phosphorus supply for plant utilization.
A few more studies have investigated plant biomass response to atmospheric CO2 enrichment under conditions of limiting phosphorus supply. In the study of Walker et al. (1998), ponderosa pine seedlings grown for an entire year at atmospheric CO2 concentrations of 525 and 700 ppm exhibited significantly greater root, shoot and total dry weights than control plants grown at ambient CO2, with little overall influence of a superimposed phosphorus treatment (low vs. high). In the study of Niklaus et al. (1998) the authors investigated the effects of elevated CO2, nitrogen and phosphorus supply on calcareous grassland communities. At low phosphorus concentrations, biomass nitrogen contents were unaffected by elevated CO2 (600 ppm); while at high phosphorus concentrations, community biomass-nitrogen increased by 28%, suggesting that community biomass nitrogen will increase in the future only if soil phosphorus contents are increased as well. However, in a companion study of these grasslands published by Stocklin and Korner (1999), it was shown that community total biomass (the actual dry weight of plant material, not the amount of nitrogen within the plant material) increased with atmospheric CO2 enrichment even under low phosphorus concentrations, with or without nitrogen-fixing legumes present in the grassland swards. Finally, Staddon et al. (1999) demonstrated that Plantago lanceolata and Trifolium repens effectively increased their phosphorus-use efficiency under elevated CO2 conditions by reducing shoot phosphorus contents as a component of CO2-induced photosynthetic acclimation.
In conclusion, although there are only a handful of papers dealing with phosphorus-related subjects under elevated CO2 concentrations, it can still be stated that plants growing in CO2-enriched atmospheres will likely respond by increasing their biomass production, even under conditions of low soil phosphorus concentrations. Especially will this be so if plants have the ability to increase root phosphatase activity, as was noted in the study of Barrett et al. (1998) with wheat.
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.
Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C. 1998. Nutrient relations in calcareous grassland under elevated CO2. Oecologia 116: 67-75.
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.
Stocklin, J. and Korner, Ch. 1999. Interactive effects of elevated CO2, P availability and legume presence on calcareous grassland: results of a glasshouse experiment. Functional Ecology 13: 200-209.
Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T. 1998. Growth and ectomycorrhizal colonization of ponderosa pine seedlings supplied different levels of atmospheric CO2 and soil N and P. Forest Ecology and Management 109: 9-20.