White lupin (Lupinus albus L.) exposed to elevated atmospheric CO2 requires additional phosphorus for N2 fixation

Purpose: The steady rise of atmospheric CO2 concentrations enhances symbiotic N2 fixation and plant growth. However, it is largely unknown whether more P is required to enhance N2 fixation under elevated CO2. We examined if plants under elevated CO2 require more P to support enhanced N2 fixation. Methods: White lupin (Lupinus albus L. cv. Kiev) was grown in nutrient solution under ambient (380 ppm) and elevated (700 ppm) CO2 levels at 0, 2, 5, 12, 25 and 50 µM P. Nodulation, nodule function, N2 fixation and plant growth were assessed at 4 and 6 weeks post inoculation. Excess cation uptake and proton release were also analysed due to its importance in P acquisition. Results: Elevated CO2 increased the critical P concentration in nutrient solution for N2 fixation, indicating plants under elevated CO2 need to acquire additional P to support greater N2 fixation. Elevated CO2 improved N2 fixation in P-deficient plants by increasing specific nitrogenase activity. Elevated CO2 also enhanced excess cation uptake which increased proton release by roots. Conclusions: As elevated CO2 increased the plant demand for P, it increased the critical P concentration in nutrient solution for N2 fixation, thereby plants require more P to mitigate the effect of P deficiency on N2 fixation. Despite this, elevated CO2 could enhance N2 fixation in P-limiting environments.