Collaborative Research: Evolution in LTER Experiments: Ecological and Evolutionary Consequences of Long-Term Nitrogen Addition for the Legume-Rhizobium Mutualism

在人为造成的全球变化中，一个鲜为人知的方面是，通过制造肥料和燃烧化石燃料，植物获得的氮量增加了一倍。氮是最强烈地限制植物在陆地上生长的元素，因此一个生态后果很可能是植物总生产力的提高。一个更令人惊讶和相反的后果可能是植物氮的主要自然来源的下降，豌豆科植物物种之间的联系，被称为豆科植物；而被称为根瘤菌的根栖细菌，能将空气中的氮气转化为铵，这是一种植物用来制造蛋白质和其他生命必需分子的氮。能够直接从土壤中获得更多氮的豆科植物与根瘤菌的联系较少，根瘤菌需要从植物中获得碳水化合物。因此，预计随着时间的推移，全球氮的变化将减少根瘤菌的丰度，并推动不太合作的根瘤菌的进化，从而减少植物可获得的氮。这可能反过来增加对合成肥料的依赖，并对全球氮循环产生反馈。该项目将利用凯洛格生物站（Kellogg Biological Station）长期生态研究（ltter）站点现有的一项已有22年历史的氮添加实验，作为未来几十年自然氮供应可能发生的变化的窗口。该项目的更广泛影响包括应用于农业、对学生的研究培训以及促进科学劳动力的多样性。许多种植制度，包括大豆和玉米的轮作，利用豆类和根瘤菌之间的联系作为氮的来源。两门大型本科课程的探究性实验将基于该项目，提交给科学教育者广泛使用的资源《生态学教学问题与实验》发表。为了促进学术界的公平和多样性，该项目将制作基于网络的材料，并组织小组讨论，指导来自不同背景的本科生申请研究生院并在研究生院取得成功。

One little-publicized aspect of human-caused, global change has been a doubling of the availability of nitrogen to plants via the manufacture of fertilizer and the burning of fossil fuels. Nitrogen is the element that most strongly limits plant growth on land, so one ecological consequence is likely to be a rise in the total productivity of plants. A more surprising and opposite consequence could be a fall in a major, natural source of nitrogen for plants, the association between species of plants in the Pea Family, known as legumes; and root-dwelling bacteria, known as rhizobia, which convert nitrogen gas in the air into ammonium, a form of nitrogen that plants use to make proteins and other molecules essential for life. Legumes that can get more nitrogen directly from the soil tend less to associate with rhizobia, which demand carbohydrates from the plants. Over time, global change in nitrogen is thus predicted to reduce the abundance of rhizobia and drive the evolution of less cooperative rhizobia that make less nitrogen available to the plants. This could in turn increase reliance on synthetic fertilizers, and feedback upon the global nitrogen cycle. This project will use an existing, 22-year-old experiment in nitrogen addition experiment at the Long-Term Ecological Research (LTER) site at the Kellogg Biological Station as a window onto what may happen to the natural nitrogen supply over the decades to come. The broader impacts of this project include applications to agriculture, research training for students, and promotion of diversity in the scientific workforce. A number of cropping systems, including the rotation of soybeans and corn, use the association between legumes and rhizobia as a source of nitrogen. Inquiry-based labs for two large undergraduate courses will be based on the project and submitted for publication by Teaching Issues and Experiments in Ecology, a widely used resource for science educators. To promote equitability and diversity in the academic community, the project will create web-based materials and organize panel discussions that will guide undergraduate students from diverse backgrounds through the process of applying to and succeeding in graduate school.