Will Tropical symbiotic nitrogen fixers face increasing P and Mo limitation with CO2 fertilization?

Background/Question/Methods

Biological nitrogen fixation is an important biogeochemical process and source of new nitrogen in tropical forests. Despite the importance of fixation to the linked nitrogen and carbon cycles, we know little about how humans change to carbon and nutrient cycles will affect tropical fixation and N₂-fixing plants. While rising CO₂ levels may increase growth and fixation when nutrients are abundant, the increased growth may force N₂-fixing plants into phosphorus (P) and molybdenum (Mo) limitation, both elements are scarce in tropical forests.  Fixers use P to support growth and energy required for fixation. Mo is in the nitrogenase enzyme that catalyzes fixation. This study focuses on improving our understanding on what controls fixation through a series of greenhouse and in situ field experiments. To start to address these issues, we used a greenhouse study where we can manipulate CO₂ levels and a field study in forest gaps.  In the greenhouse study we grew a N₂-fixing seedling and a non-fixing seedling at pre-industrial (280 ppm), current (400 ppm), and elevated (800 ppm) CO₂ concentrations with and without P, Mo, or both.  In the year-long field study, we applied the same nutrient treatments to seedlings planting in natural light gaps and ambient CO₂.

Results/Conclusions

In the greenhouse study, we found nutrient limitation was minimal at 280 ppm, but that limitation increased with CO₂.  Phosphorus significantly limited growth and fixation increasing with CO₂, and the additions of Mo and P together allowed for even greater growth and fixation, suggesting Mo-P co-limitation at elevated CO₂. In the year-long field study, we did not find significant differences between nutrient treatments but a correlation with canopy openness. Both studies illustrate both hypothetical limitations at higher CO₂ levels as well as the difficulty in scaling up to natural forests where herbivory and competition for light and nutrients mute clear treatment effects.