COS 50-5 - Drought selects for nitrogen fixation

Tuesday, August 8, 2017: 2:50 PM
D132, Oregon Convention Center
Wenying Liao1, Duncan N. L. Menge2, Lars O. Hedin1, Simon A. Levin1 and Stephen W. Pacala1, (1)Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, (2)Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY
Background/Question/Methods

Nitrogen (N) is an essential limiting nutrient for primary production, constraining global carbon (C) sink. Symbiotic N fixation, a process during which root nodule bacteria convert atmospheric dinitrogen gas into plant-usable form, can be the dominant regional N input. It has become increasingly clear that symbiotic N-fixing plants may be favored in dry areas. Previous work suggests that they are abundant in dry patches of temperate and tropical forests, and tropical savannas. What remains less clear is the mechanism underlying this advantage. Previous explanations rely on characteristics of the group of plants that can fix N, such as higher water use efficiency. Because these groups differ in many ways, though, these explanations do not pinpoint N fixation itself. We lack a fundamental baseline understanding of the problem – could fixation be selected for without prescribing physiological and structural differences between fixing and non-fixing plants? Here, we use an analytically-tractable model to investigate potential mechanisms responsible for selection of fixation in dry areas. Our model deals with light and water competition in a mechanistic manner, containing three main components – N, C and water. We used adaptive dynamics to determine the evolutionarily stable strategy of fixation.

Results/Conclusions

The fitness of any plant strategy depends on net C gain, which is the difference between C gain from photosynthesis during wet (light-limited) and dry (water-limited) season and C cost of leaf and root construction and maintenance. Fixing plants differ from non-fixing plants simply by incurring additional C cost associated with N fixation. Our results suggest that increasing drought severity during dry season selects for N fixation. This is likely attributable to decreased root production as drought severity increases, thus making N fixers competitively advantageous during wet season where soil N uptake of non-fixers is limited by their low root biomass.