Drought-adapted plants dramatically downregulate dinitrogen fixation: evidences from Mediterranean legume shrubs

1. The importance of symbiotic dinitrogen (N2) fixation in shaping the coupled nitrogen-carbon cycle is now known for most humid terrestrial ecosystems. However, whether N2 fixation can play a key role in the nitrogen and carbon budget of water-limited and seasonally dry ecosystems remains a mystery. 2. The maintenance of metabolically and physiologically costly symbiotic fixation in water-limited environments is highly complex. These costs are particularly high during the first developmental season, when allocation to deep rooting and drought-resistance mechanisms is essential for seedling survival of prolonged seasonal drought. 3. We therefore evaluated how drought-adapted legume species change their allocation to symbiotic nitrogen fixation as a function of soil nitrogen availability. We tested this on seedlings of a suite of four common Mediterranean legume shrubs with a strong seasonal behavior, which we grew under controlled nitrogen and phosphorus availabilities. We asked: (1)...