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The Progressive Phosphorus Limitation Hypothesis
Reference
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.

Background
The faster and more vigorous plant growth that is typically observed in CO2-enriched air, in the words of the authors, "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 they say are "often considered to limit productivity in terrestrial ecosystems." As a result, several scientists have periodically assumed that the growth stimulation due to the aerial fertilization effect of atmospheric CO2 enrichment will not be sustained as time progresses, due to insufficient amounts of soil N and P.

What was done
Khan et al. tested 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.

What was learned
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."

What it means
Kahn et al. 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," which assessment gives us ever greater confidence that the CO2-induced greening of the earth will continue.

Reference
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.

Reviewed 13 August 2008