Rising CO₂ accelerates phosphorus and molybdenum limitation of N₂-fixation in young tropical trees

Background and aims: Nitrogen fixation may be critical for supplying the nitrogen (N) needed to maintain the tropical carbon sink in a world of rising atmospheric CO₂. However, we do not know whether increased CO₂ acts to exacerbate nutrient limitation on the fixation process itself. We experimentally test this idea by growing N₂-fixing plants in pre-Industrial (280 ppm), present-day (400 ppm), and doubled (800 ppm) atmospheric CO₂. Methods: In a greenhouse experiment, we grew tree seedlings from N₂-fixing species and a non-fixing species at three CO₂ concentrations with control, +P (phosphorus), +Mo (molybdenum), and + P +Mo nutrient treatments. Results: We found nutrient limitation to be minimal at pre-Industrial CO₂, but with increasing CO₂ fixer growth and fixation became increasingly limited by P and by a P-by-Mo interaction. At 400 ppm, plants with +P grew ~50% faster and fixed 10-15× more N₂ based on nodule mass and nitrogenase activity. At 800 ppm, plants with +P +Mo grew 200% more, and fixed 25× more N₂, suggesting Mo-P co-limitation at elevated CO₂. Conclusion: Our findings imply that complex patterns of nutrient limitation can develop as CO₂ rises, potentially suppressing tropical N₂-fixation and the new inputs of N needed to sustain the tropical carbon sink.