Nutrient acquisition strategies augment growth in tropical N₂ -fixing trees in nutrient-poor soil and under elevated...

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.