Nitrogen-fixing legume plants have the potential to sustain the productivity and nitrogen-richness of ecosystems, and to modulate the response of natural ecosystems to disturbance and anthropogenic change. This is primarily due to the symbiotic relationship between the legume plant and symbiotic dinitrogen-fixing bacteria. Nitrogen-fixing plants adopted a suite of strategies for regulating their investment in fixation as a function of the availability of nitrogen in the soil. Our study asks how these different strategies influence plant fitness and persistence in diverse plant communities. We evaluated the strategies of dinitrogen fixation in a diversity of Mediterranean legumes, and how fixation strategies influence individual-plant fitness and survival. We measured plant investment in fixation at different levels of nutrient supplementation, both in controlled experiments and in the field, and evaluated plant abundance and fitness. We further analysed patterns of biomass and nitrogen allocation to aboveground, belowground, and reproductive tissues.
Results/Conclusions
We found all the different nitrogen fixation strategies in a selection of species of annual herbaceous legumes that often coexist in the same habitats. The most common herbaceous Mediterranean legumes include species that maintain a constant rate of fixation regardless of soil nitrogen availability (obligate strategy, e.g., Medicago truncatula), species that downregulate fixation when soil nitrogen is available (facultative strategy, e.g., Hymenocarpus circinatus), and even species that seem to be able fix more nitrogen when soil nitrogen is available (e.g., species of Vicia). Fixed nitrogen contributed directly to both biomass growth and to fitness via seed production, but the allocation of soil nitrogen vs. fixed nitrogen differed between obligate and facultative fixers. Surprisingly, annual legumes allocated no more than half of their nitrogen to seeds, indicating that large amounts of this expensive fixed nitrogen are lost at the end of the season. Our results suggest that herbaceous legumes evolved a diversity of strategies of nitrogen fixation and nitrogen allocation that allow their coexistence and persistence in diverse Mediterranean plant communities.