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

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