Facilitating the tropical forest carbon sink: The evolution and function of symbiotic N2 fixation

树木有能力从大气中获取氮肥（N2固定物），在支持热带森林生长和碳储存方面发挥着关键作用，随着气候变化，它们可能变得越来越重要。N2固定物是否会在未来帮助热带森林储存额外的二氧化碳——从而抵消人类的二氧化碳排放——可能取决于控制它们在生态系统中的功能和进化的因素。为了更好地理解固定物在气候变化中的作用，我们可以通过研究固定物如何对过去的重大全球变化做出反应来学习，推断固定物进化的原因以及导致它们多样化并在森林和其他生态系统中传播的因素。我将使用以下几种方法来解决这个以前未被提出的问题：1)在巴拿马热带森林中建立一个大规模的长期施肥实验，以测试养分如何限制森林从干扰中恢复，以及固氮树木促进碳储存的能力；2)利用亚马逊流域最大的固形虫流行和生物多样性数据集，分析营养物质对固形虫的控制作用；3)结合最近对固定生物进化史（系统发育）的重建与全球热带地区固定生物流行和生物多样性的数据集，确定营养物质如何控制固定生物的进化和地理传播；4)将固定物的系统发育与古代地质和气候事件的回顾相结合，以确定是什么促成了主要的进化事件，导致了固定物的现代分布。这种方法有望使人们对地球历史上固定物的功能有新的认识，并能更好地预测它们的作用在未来可能发生的变化。新的发现将使决策者和公众了解热带森林缓冲二氧化碳排放的可靠性，使科学家了解生物多样性和植物功能的重要性，使实践者了解如何保护和恢复生态系统。

Trees with the ability to access nitrogen fertilizer from the atmosphere (N2 fixers) fill a critical role in supporting tropical forest growth and carbon storage and may become increasingly important as climate changes. Whether N2 fixers will help tropical forests store additional CO2 in the future-thereby offsetting human CO2 emissions-may depend on what controls their function and evolution in ecosystems. To better understand the future role of fixers as climate changes, we can learn by examining how fixers responded to major past global change, deducing why fixers evolved and what factors caused their diversification and spread across forests and other ecosystems. I will address this previously-unposed question using a combination of approaches: 1) establishing a large-scale, long-term fertilization experiment in Panamanian tropical forests to test how nutrients limit forest recovery from disturbance and the ability of N2-fixing trees to facilitate carbon storage; 2) utilizing the largest dataset of fixer prevalence and biodiversity across the Amazon Basin to analyze how nutrients control fixers; 3) combining a recent reconstruction of the evolutionary history (phylogeny) of fixers with a dataset of fixer prevalence and biodiversity across tropical regions globally to determine how nutrients govern the evolution and geographical spread of fixers; and, 4) pairing the fixer phylogeny with a review of ancient geological and climatic events to determine what precipitated the major evolutionary events that led to the modern-day distribution of fixers. This approach is expected to lead a new understanding of the function of fixers throughout earth's history and a better ability to predict how their role may change into the future. New findings will inform policy-makers and the public about the reliability of tropical forests to buffer our CO2 emissions, scientists about the importance of biodiversity and plant function, and practitioners about how to protect and restore ecosystems.