Department of Ecology, Evolution, and Environmental Biology, Columbia University, New York, New York 10027, USA.
Institute of the Environment and Sustainability, University of California, Los Angeles, California 90095, USA.
Nat Plants. 2015 Jun 1;1:15064. doi: 10.1038/nplants.2015.64.
Symbiotic N2 fixation (SNF) brings nitrogen into ecosystems, fuelling much of the world's agriculture(1) and sustaining carbon storage(2,3). However, it can also cause nitrogen saturation, exacerbating eutrophication and greenhouse warming(4-7). The balance of these effects depends on the degree to which N2-fixing plants adjust how much N2 they fix based on their needs (their SNF 'strategies')(5,6). Genetic, biochemical and physiological details of SNF are well known for certain economically important species(8,9), but the diversity of N2-fixing plants(10) and bacteria(11) is enormous, and little is known about most N2-fixing symbioses in natural ecosystems(12). Here, we show that co-occurring, closely related herbs exhibit diverse SNF strategies. In response to a nitrogen supply gradient, four species fixed less N2 than they needed (over-regulation), two fixed what they needed (facultative) and two did not downregulate SNF (obligate). No species downregulated but fixed more N2 than it needed (under-regulation or incomplete downregulation), but some species under-regulated or incompletely downregulated structural allocation to SNF. In fact, most species maintained nodules (the root structures that house symbionts) when they did not fix N2, suggesting decoupling of SNF activity and structure. Simulations showed that over-regulation of SNF activity is more adaptive than under-regulation or incomplete downregulation, and that different strategies have wildly different effects on ecosystem-level nitrogen cycling.
共生固氮 (Symbiotic N2 fixation, SNF) 将氮带入生态系统，为世界上大部分农业提供动力（1）并维持碳储存（2,3）。然而，它也会导致氮饱和，加剧富营养化和温室变暖（4-7）。这些影响的平衡取决于固氮植物根据自身需求调整固定多少氮的程度（它们的 SNF“策略”）（5,6）。某些具有经济重要性的物种的 SNF 的遗传、生化和生理细节已经众所周知（8,9），但固氮植物（10）和细菌（11）的多样性是巨大的，并且对大多数自然生态系统中的固氮共生关系知之甚少（12）。在这里，我们表明，共存的、密切相关的草本植物表现出多样化的 SNF 策略。在氮供应梯度的响应中，有四个物种固定的氮少于它们所需的氮（过度调节），有两个物种固定了它们所需的氮（兼性），有两个物种没有下调 SNF（必需）。没有物种下调但固定的氮多于它们所需的氮（过度调节或不完全下调），但有些物种下调或不完全下调 SNF 的结构分配。事实上，当大多数物种不固定氮时，它们仍维持着根瘤（容纳共生体的根结构），这表明 SNF 活性和结构的解耦。模拟表明，SNF 活性的过度调节比下调或不完全下调更具适应性，并且不同的策略对生态系统水平的氮循环有截然不同的影响。
