Reference
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.
What was done
The authors grew 90-day-old well-watered seedlings of two Acacia species (Acacia auriculiformis Cunn. Ex Benth and Acacia magium Willd) for an additional 30 days in pots containing a 1-to-1 mixture of granite regosol and perlite at substrate phosphorus concentrations of 30, 150 and 300 mg P per pot (low, medium and high P, respectively) in a growth chamber study where the plants were maintained in either ambient air or air enriched to a CO2 concentration of 800 ppm.
What was learned
Nguyen et al. report that "under ambient CO2, plant growth and the amount of N fixed symbiotically in [the] N2-fixing seedlings decreased with the decrease of supplied P." However, they say that "this relationship did not occur under elevated CO2." More specifically, the three researchers found that in going from the high P to low P substrate treatment, plant biomass in the ambient-air growth chamber decreased by approximately 22% in A. auriculiformis and by about 25% in A. mangium. In the elevated-CO2 growth chamber, on the other hand, plant biomass actually increased - by approximately 6% (though non-significantly) in both species - in going from the high P to low P substrate treatment. Likewise they determined that in going from the high P to low P substrate treatment, plant symbiotic nitrogen fixation in the plants growing in ambient air dropped by about 25% in A. auriculiformis and by approximately 30% in A. mangium, while there was no statistically significant difference in nitrogen fixation between the high P and low P substrate treatments in either species in the plants growing in the CO2-enriched chamber.
What it means
As Nguyen et al. note in their concluding paragraph, "elevated CO2 alleviated low P-induced reduction in plant growth mainly by increasing the use efficiency of internal P for plant growth and N2 fixation," which helps to allay the fear, expressed in their introduction, that "the advantages enjoyed by symbiotic N2-fixing plants may be lost under sub-optimal conditions of degraded land having depleted concentrations of major mineral elements."