Nutrient Acquisition Strategies Augment Growth in Tropical N2‐Fixing Trees in Nutrient‐Poor Soil and Under Elevated CO2

Tropical forests play a dominant role in the global carbon (C) cycle, and models predict increases in tropical net primary productivity (NPP) and C storage in response to rising atmospheric carbon dioxide (CO2) concentrations. The extent to which increasing CO2 will enhance NPP depends in part on the availability of nitrogen (N) and phosphorus (P) to support growth. Some tropical trees can potentially overcome nutrient limitation by acquiring N via symbiotic dinitrogen (N2) fixation, which may provide a benefit in acquiring P via investment in N‐rich phosphatase enzymes or arbuscular mycorrhizal (AM) fungi. We conducted a seedling experiment to investigate the effects of elevated CO2 and soil nutrient availability on the growth of two N2‐fixing and two non‐N2‐fixing tropical tree species. We hypothesized that under elevated CO2 and at low nutrient availability (i.e., low N and P), N2 fixers would have higher growth rates than non‐N2 fixers because N2 fixers have a greater capacity to acquire both N and P. We also hypothesized that differences in growth rates between N2 fixers and non‐N2 fixers would decline as nutrient availability increases because N2 fixers no longer have an advantage in nutrient acquisition. We found that the N2 fixers had higher growth rates than the non‐N2 fixers under elevated CO2 and at low nutrient availability, and that the difference in growth rates between the N2 and non‐N2 fixers declined as nutrient availability increased, irrespective of CO2. Overall, N2 fixation, root phosphatase activity, and AM colonization decreased with increasing nutrient availability, and increased under elevated CO2 at low nutrient availability. Further, AM colonization was positively related to the growth of the non‐N2 fixers, whereas both N2 fixation and root phosphatase activity were positively related to the growth of the N2 fixers. Though our results indicate all four tree species have the capacity to up‐ or down‐regulate nutrient acquisition to meet their stoichiometric demands, the greater capacity for the N2 fixers to acquire both N and P may enable them to overcome nutritional constraints to NPP under elevated CO2, with implications for the response of tropical forests to future environmental change