Biogeochemical controls of symbiotic N2 fixation across broad spatial scales: An evaluation of four hypotheses

Background/Question/Methods

Symbiotic N₂-fixing trees fill a critical role in the nitrogen cycle, yet the abundance of fixers varies widely both within and across biomes. A dominant idea proposed to resolve this variation is that fixation and fixers are controlled by soil phosphorus, although the mechanisms of how fixers depend on phosphorus remain largely unexplored and have different consequences for the distribution of fixers. We consider four hypotheses about the influence of phosphorus on fixers relative to non-fixers, and examine the consequences of these hypotheses for how fixers differ from non-fixers in phosphatase activity, abundance, and nutrient limitation status across a gradient in phosphorus availability. We use a theoretical plant-soil ecosystem model that includes different ideas about the phosphorus costs of fixation, competitive abilities of fixers relative to non-fixers, and phosphorus uptake strategies. We evaluate the predictions with empirical data from our own work and the literature and new analyses of forest inventory data from tropical forests.

Results/Conclusions

For all hypotheses, our model predicts that forests shift from more phosphorus-limited in the lowest phosphorus soils to more nitrogen-limited in the higher phosphorus soils, with fixers equally or more phosphorus-limited than non-fixers due to their ability to fix large amounts of nitrogen.  The degree of phosphorus vs. nitrogen limitation differs, however, depending on the hypothesized mechanisms of how fixers demand and take up phosphorus, with some phosphorus strategies making fixers better able to withstand low-phosphorus conditions than others.  The model is able to make distinctive predictions between hypotheses about how fixers differ relative to non-fixers in their phosphorus strategies and abundance across soil phosphorus gradients. 

Thus, our model provides a rigorous theoretical framework that we can evaluate using published data.  We find empirical support for some of our predictions but not others.  A resolution to these hypotheses about fixation and phosphorus is critical for our understanding of the importance of fixation in the global carbon cycle.