COS 22-6
Will Tropical symbiotic nitrogen fixers face increasing P and Mo limitation with CO2 fertilization?

Tuesday, August 6, 2013: 9:50 AM
L100B, Minneapolis Convention Center
Annette Trierweiler, Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ
Klaus Winter, Smithsonian Tropical Research Institute, Panama
Nina Wurzburger, Odum School of Ecology, University of Georgia, Athens, GA
Lars Hedin, Ecology and Evolutionary Biology, Princeton University, Princeton, NJ

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.


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.