Data from: 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) Do species differ in their investment and regulation of nitrogen fixation? (2) Is fixation regulated via plant allocation to nodules, fixation rate, or both? and (3) Does phosphorus availability limit symbiotic nitrogen fixation? 4. All Mediterranean perennial legumes in the experiment established and grew nodulated and fixed nitrogen, even under severe nitrogen limitation. The four species reacted similarly to nitrogen supply, by strongly downregulating fixation through both decreased nodulation and lower rate of fixation. However, we found a significant inter-specific difference in fixation (both nodulation and rate), biomass production and growth rate. Our experimental species presented a range of fixation investment strategies corresponding to life history and resource partitioning patterns. Phosphorus limitation had a minor influence on both fixation and plant growth. 5. Synthesis: the high physiological cost of symbiotic fixation imposes the need to tightly regulate fixation in perennial legumes coping with severe water stress and seasonal conditions. Control of fixation allows legume species to colonize recently disturbed nitrogen-deficient habitats, cope with grazing, survive long seasonal droughts and recover nitrogen fixation latter in the wet season, and survive over time by reducing nitrogen inputs to the ecosystem.

1. 如今，绝大多数湿润陆地生态系统中，共生固氮（symbiotic dinitrogen (N₂) fixation）在调控氮-碳耦合循环方面的重要性已得到学界公认。然而，在水分受限且具有季节性干旱特征的生态系统中，共生固氮能否在其氮碳收支中发挥关键作用，仍是未解之谜。
2. 在水分受限的环境中维持共生固氮需要付出高昂的代谢与生理代价，这一调控过程极为复杂。在植物首个生长季，为应对长期季节性干旱，幼苗需将资源优先分配至深根构建与抗旱机制的构建中，此时共生固氮的代价尤为高昂。
3. 为此，我们针对适应干旱环境的豆科植物（legume），探究其共生固氮的资源分配策略如何随土壤氮素有效性变化而改变。我们选取4种具有显著季节生长特性的地中海常见豆科灌木幼苗，在可控的氮、磷有效性水平下开展实验，并提出以下三个研究问题：(1) 不同物种在固氮投入与调控机制上是否存在显著差异？(2) 固氮过程是通过植物对根瘤的资源分配调控，还是通过固氮速率调控，抑或两者共同参与调控？(3) 磷素有效性是否会限制共生固氮？
4. 本实验中的所有地中海多年生豆科植物均成功定植并形成根瘤、完成固氮，即便在严重氮素限制条件下亦是如此。4个供试物种对氮素供给的响应模式高度相似：均通过减少根瘤形成与降低固氮速率，显著下调固氮过程。但我们发现，不同物种在固氮（包括根瘤形成与固氮速率）、生物量积累及生长速率方面均存在显著的种间差异。供试物种展现出一系列与自身生活史策略及资源分配模式相匹配的固氮投入策略。磷素限制对固氮过程与植物生长的影响均较为微弱。
5. 综合结论：共生固氮高昂的生理代价，要求应对严重水分胁迫与季节性干旱的多年生豆科植物必须严格调控固氮过程。通过精准调控固氮，豆科物种能够定植于近期受干扰的氮素匮乏生境、应对放牧压力、熬过长期季节性干旱，并在雨季恢复固氮；同时，通过减少向生态系统输入的氮素，可实现长期种群存续。