Nitrogen, phosphorus, and the biogeochemical niche of symbiotic di-nitrogen fixers in tropical rainforests

Background/Question/Methods

Symbiotic di-nitrogen (N₂) fixation is a critical process that confers advantages to plants while alleviating nitrogen limitation in terrestrial ecosystems; yet, what controls it remains unclear.  Of particular importance is to reconcile sets of ideas derived for the ecosystem versus individual organism point of view.  Critical issues include whether facultative fixers could contribute to tropical N richness through time lags in responses to changing soil conditions, whether nitrogen, phosphorus, or a complex interaction of both limit fixation, and how plants use internal and external strategies of nutrient adjustment to optimize growth and, ultimately, reproductive success.  In order to address these pressing questions, we grew seedlings of an N₂ fixer native to Panamanian tropical forests in different treatments of nitrogen and phosphorus to examine how the above issues interact to derive the biogeochemical niche of ecologically important N₂ fixers.  We measured the change in investment in fixation, the response of fixation and growth to nutrient treatments, and the change in strategies of investment in mycorrhizae, extracellular phosphatases, plant tissue carbon:nitrogen:phosphorus stoichiometries, and root to shoot allocation.  

Results/Conclusions

We found that plants closely track environmental conditions and down-regulate fixation rapidly.  Testing the hypotheses of single versus multiple resource controls on fixation and growth, we found that a combination of N and P controls fixation, depending upon the stoichiometry of the nutrient additions.  Finally, we found that plants can make a range of nutrient adjustments to deal with differing local nutrient conditions.  In addition to fixation itself, these strategies include investment in arbuscular mycorrhizal colonization, adjustments in tissue stoichiometries, and shifts in root to shoot ratio.  Phosphatase activity showed distinct patterns with N₂ fixation.  Except in extreme conditions of starvation, plants displayed unexpected homeostasis in carbon to nitrogen to phosphorus tissue stoichiometry.  However, in comparison to the responsiveness of the strategy of fixation, the other strategies had only very limited capacity to adjust to nutrient conditions while maintaining fitness.  These insights into the function of individual organisms navigating heterogeneous environments have importance for how we understand the functional role of nitrogen fixers and fixation in lowland tropical rainforest and the implications for ecosystem scale processes.