In an early study of the subject, 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. As phosphatase is the primary enzyme responsible for the mineralization of organic phosphate, which thereby makes phosphorus available for plant use, an increase in its activity with elevated CO2 could facilitate sustained plant growth responses to the ongoing rise in the air's CO2 content, even in areas where growth is currently limited by phosphorous deficiencies. Furthermore, because these increases in phosphatase activity were also observed under sterile growing conditions, this observation indicates that this response can be mediated directly by plant roots without involving soil microorganisms, which are already known to aid in phosphorus mineralization.

As the air's CO2 content thus continues to rise, phosphatase activity in wheat roots should increase, thereby converting organic phosphorus into inorganic forms that can be used to support the increased plant growth and development that is stimulated by higher CO2 concentrations. And because a similar increase in phosphatase activity at elevated CO2 has already been reported for a native Australian pasture grass, these results may be applicable to most of Earth's vegetation. If this is indeed the case, then plants that are currently phosphorus limited in their growth might increase their phosphorous acquisition from soil organic supplies as the atmospheric CO2 concentration increases; and this phenomenon, in turn, may allow them to sequester even greater amounts of carbon from the air as the atmosphere's CO2 concentration climbs ever higher.

Other studies have also investigated plant biomass responses to atmospheric CO2 enrichment under conditions of limiting phosphorus supply. Staddon et al. (1999), for example, 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 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 effects of elevated CO2, nitrogen and phosphorus supply on calcareous grassland communities were explored. 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.

In another paper, Nguyen et al. (2006) grew seedlings of two N-fixing woody plants (Acacia auriculiformis Cunn. ex Benth and Acacia mangium Willd) that were well irrigated and fertilized - except for phosphorus (P), of which there were three treatments (low, medium and high) comprised of 10, 50 and 100 mg P/liter of soil mixture - in growth chambers maintained at atmospheric CO2 concentrations of either ambient or ambient + 800 ppm. The results they obtained indicated that in the case of A. auriculiformis, plant biomass was enhanced by 19%, 21% and 57%, respectively, at high, medium and low P; while in A. mangium it was enhanced by 5%, 32% and 47% for the same respective P concentrations. Nguyen et al. also report that "in both species the increase in plant growth [caused] by elevated CO2 was accompanied by increased P use efficiency," as well as by "increased N use efficiency and total N accumulation." In addition, they say that "elevated CO2 also increased P use efficiency for N2 fixation." Consequently, under ambient CO2, in the words of the three researchers, "plant growth and the amount of N fixed symbiotically in N2-fixing seedlings decreased with the decrease of supplied P," but "this relationship did not [italics added] occur under elevated CO2," because "elevated CO2 alleviated [the] low P-induced reduction in plant growth," mainly by "increasing the use efficiency of internal P for plant growth and N2 fixation."

In describing the implications of their findings, Nguyen et al. state that in many parts of the world "Acacia species are grown for environmental protection and energy plantations on degraded [italics added] soils." In light of their findings, therefore, there is reason to believe that these soils' low nutrient levels may not impede the growth of these important plants, and that they may significantly increase their productivities in a CO2-accreting atmosphere, such as the earth has possessed over the course of the Industrial Revolution and is anticipated to possess for a long time to come.

In introducing their study of the subject, Khan et al. (2008) note that the faster and more vigorous plant growth that is typically observed in CO2-enriched air "has to be sustained by a sufficient nutrient supply," for "if increased biomass production is to continue, [nutrient] availability in the soil has to match increasing demand for major nutrients," such as nitrogen (N) and phosphorus (P), which are two of the elements that they say are "often considered to limit productivity in terrestrial ecosystems." Khan et al. therefore set out to test this hypothesis as it pertains to phosphorus at the EuroFACE facility near Viterbo in central Italy, where three genotypes of Populus - P. alba, P. nigra and P. x euroamericana - were grown under ambient and elevated (ambient + 200 ppm) atmospheric CO2 concentrations for a period of five years.

The four UK researchers say their investigation showed that "increased tree growth under elevated CO2 has not resulted in the depletion of phosphorus pools in soils as originally hypothesized, but rather in the replenishment and increased storage of P in the rooting zone," such that "P may not, therefore, limit tree growth in a high CO2 world." Kahn et al. therefore conclude that "biogenically driven weathering of primary minerals in the rooting zone is sufficient to maintain the replenishment of plant available inorganic P," and that "since future levels of elevated CO2 may stimulate biomass production in a diverse range of forests (Norby et al., 2005), this increase of P availability is of global consequence."

Similar findings were reported two years later by Kahn et al. (2010), who in reporting on the same EuroFACE experiment, suggested that "the availability of P can actually increase in elevated CO2, forming a positive feedback with increased biomass production on P limited soils," ultimately concluding that "phosphorus limitation may therefore not reduce tree growth in a high CO2 world."

In light of the above findings, it would appear that plants growing in CO2-enriched air will respond by increasing their biomass production, even under conditions of low soil phosphorus concentration; and especially will this be so if plants have the ability to increase root phosphatase activity, as was observed 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.

Khan, F.N., Lukac, M., Miglietta, F., Khalid, M. and Godbold, D.L. 2010. Tree exposure to elevated CO2 increases availability of soil phosphorus. Pakistan Journal of Botany 42: 907-916.

Khan, F.N., Lukac, M., Turner, G. and Godbold, D.L. 2008. Elevated atmospheric CO2 changes phosphorus fractions in soils under a short rotation poplar plantation (EuroFACE). Soil Biology & Biochemistry 40: 1716-1723.

Nguyen, N.T., Mohapatra, P.K. and Fujita, K. 2006. Elevated CO2 alleviates the effects of low P on the growth of N2-fixing Acacia auriculiformis and Acacia mangium. Plant and Soil 285: 369-379.

Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C. 1998. Nutrient relations in calcareous grassland under elevated CO2. Oecologia 116: 67-75.

Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King J.S., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., Angelis, P.D., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarascia-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences USA 102: 18,052-18,056.

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.