Nutrient acquisition strategies promote high growth in tropical nitrogen fixing trees under elevated CO2

Background/Question/Methods

Tropical forests play a dominant role in the global carbon (C) cycle, and global models predict increases in tropical net primary productivity (NPP) and C storage in response to rising atmospheric carbon dioxide concentrations ([CO₂]). Despite such predictions, the extent to which increasing [CO₂] will enhance NPP depends on nutrient availability, and multiple lines of evidence suggest that NPP in tropical forests may already be phosphorus (P) limited. However, some tropical tree species can potentially overcome nutrient limitation by acquiring nitrogen (N) from the atmosphere via symbiotic N fixation, which may, in turn, enhance soil P acquisition via mineralizing (phosphatases) enzymes and/or arbuscular mycorrhizal (AM) fungi. We conducted a seedling experiment using enriched [CO₂] mesocosms to investigate the effects of elevated [CO₂] and soil nutrient availability on the growth of two N fixing and two non-N fixing tropical tree species. We hypothesized that under elevated [CO₂] and low soil nutrient availability, N fixing trees would have higher growth rates than non-N fixing trees because N fixers have a greater capacity to acquire both N and P. However, we also hypothesized that differences in growth rates would decline as soil nutrient availability increased.

Results/Conclusions

Growth rates of all four tree species were higher under elevated [CO₂] relative to ambient [CO₂]. In addition, growth rates of the two N fixing tree species were higher than the non-N fixing tree species at low soil nutrient availability, but differences decreased as soil nutrient availability increased. Symbiotic N fixation rates increased under elevated [CO₂], and for all species, root phosphatase enzyme activity and AM colonization increased under elevated [CO₂], but decreased as nutrient availability increased. Our results indicate that all four tree species have the capacity to up- or down-regulate nutrient acquisition to meet their stoichiometric demands. More, N and P acquisition strategies, together, explained the greatest amount of variation in the growth rates of N fixing trees under low nutrient conditions, indicating that a greater capacity to acquire N and P facilitated higher growth rates when soil nutrients were scarce. Our results suggest that at least some tropical N fixing tree species may be equipped to overcome nutritional constraints on NPP with elevated [CO₂], with implications for tropical forest community composition, NPP, C storage, and global climate.