What was done
The authors 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 - for 30 days in 3-liter pots located within growth chambers maintained at atmospheric CO2 concentrations of either ambient or ambient + 800 ppm.

What was learned
In 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 [our italics] 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."

What it means
"In many parts of the world," to quote Nguyen et al., "Acacia species are grown for environmental protection and energy plantations on degraded [our italics] 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.