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Progressive Phosphorus Limitation of CO2-Stimulated Tree Growth
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.

With respect to the CO2-induced increase in the growth of plants that is generally observed in atmospheric CO2 enrichment experiments, the authors write that "if faster and more vigorous plant growth is to be sustained," such as in the case of trees that may live for many decades or even centuries, "a sufficient nutrient supply must be available" to sustain that growth, which concept underlies the well-known -- but long-discredited -- progressive nitrogen limitation hypothesis (see Nitrogen (Progressive Limitation Hypothesis in our Subject Index). And since they correctly note that "phosphorus is one of the key elements often considered to limit productivity in terrestrial ecosystems," they state that its gradual depletion in the soil may also "act as a negative feedback to increased growth."

What was done
In a test of this hypothesis as it pertains to phosphorus, Khan et al. grew three species of poplar trees (Populus alba, P. nigra and P. x euramericana) at ambient and elevated (550 ppm) CO2 concentrations at the EuroFACE facility in Viterbo (Central Italy) in a low-nitrogen soil for a period of five years. Started as stem cuttings in 1999, the trees experienced a significant CO2-induced stimulation of biomass production over the first three years of the study, at which point they were coppiced and allowed to grow for two more years, during which period they continued to exhibit a significant CO2-induced enhancement of growth, after which numerous soil samples were taken to a depth of 60 cm under each species and analyzed for phosphorus (P) availability.

What was learned
The five researchers report that rather than decreasing under elevated CO2, and contrary to their own initial hypothesis "the plant available, the weatherable mineral P pool and degradable organic P pools increased under FACE," suggesting that "the availability of P can actually increase in elevated CO2, forming a positive feedback with increased biomass production on P limited soils."

What it means
In the last paragraph of their paper, Khan et al. say their study "shows that increased tree growth under elevated CO2 has not resulted in depletion of P pools in soils, but rather in replenishment and increased storage of P in the rooting zone," and that contrary to even their own expectations before beginning the experiment, "phosphorus limitation may therefore not reduce tree growth in a high CO2 world."

Reviewed 22 December 2010