Rising CO2 shifts the balance between carbon and nutrient limitation of growth

Background/Question/Methods

Atmospheric concentrations of carbon dioxide ([CO₂]) over the past 650,000 years have ranged from 180 - 280 µmol mol^-1, but in the past 150 years have risen rapidly to 389 µmol mol^-1. This 40% increase is similar in magnitude to the increase in [CO₂] projected for the 21^st century, and understanding the factors that regulated plant responses to past increases in [CO₂] may improve our ability to predict responses to future [CO₂]. However, studies often observe proportionately larger growth and physiological responses to the transition from glacial to modern [CO₂] than from modern to future [CO₂]. This pattern may reflect several factors, including that the relative effect of short-term increases in [CO₂] on photosynthesis in C₃ plants is highest at low [CO₂], declining with rising [CO₂]. This decline in photosynthetic response as [CO₂] rises may reflect a transition from [CO₂] limitation to nutrient limitation. In this study, the interactive effects of phosphorus supply (0.004 – 0.5 mM) and rising atmospheric [CO₂] from glacial (280 µmol mol^-1) to future (700 µmol mol^-1) concentrations on growth and physiology were examined on Populus deltoides (cottonwood) seedlings grown under well watered conditions.

Results/Conclusions

Increasing [CO₂] from glacial to modern increased growth 25% across P treatments, reflecting reduced [CO₂] limitations to photosynthesis and increased A_sat. Conversely, the growth response to future [CO₂] was very sensitive to P supply. Future [CO₂] increased growth 80% in the highest P supply but only 7% in the lowest P supply, reflecting P limitations to A_sat, leaf area and LAR, compared to modern [CO₂]. Changes in the maximal Rubisco-limited rate of photosynthesis (V_cmax) and stomatal conductance (g_s) together accounted for 78% of the variation in A_sat among [CO₂] and P treatments, suggesting significant biochemical and stomatal controls on photosynthesis. These results suggest P demand will increasingly affect growth and photosynthesis in cottonwood as [CO₂] continues to rise, reflecting a transition from CO2 limitation to nutrient limitation.