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