Nutrient acquisition strategies augment growth in tropical N -fixing trees in nutrient-poor soil and under elevated CO.

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 (CO ) concentrations. The extent to which increasing CO 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 (N ) 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 CO and soil nutrient availability on the growth of two N -fixing and two non-N -fixing tropical tree species. We hypothesized that under elevated CO and at low nutrient availability (i.e., low N and P), N fixers would have higher growth rates than non-N fixers because N fixers have a greater capacity to acquire both N and P. We also hypothesized that differences in growth rates between N fixers and non-N fixers would decline as nutrient availability increases because N fixers no longer have an advantage in nutrient acquisition. We found that the N fixers had higher growth rates than the non-N fixers under elevated CO and at low nutrient availability, and that the difference in growth rates between the N and non-N fixers declined as nutrient availability increased, irrespective of CO . Overall, N fixation, root phosphatase activity, and AM colonization decreased with increasing nutrient availability, and increased under elevated CO at low nutrient availability. Further, AM colonization was positively related to the growth of the non-N fixers, whereas both N fixation and root phosphatase activity were positively related to the growth of the N 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 N fixers to acquire both N and P may enable them to overcome nutritional constraints to NPP under elevated CO , with implications for the response of tropical forests to future environmental change.