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

人类造成的全球变化的一个鲜为人知的方面是，通过制造化肥和燃烧化石燃料，植物获得的氮素增加了一倍。氮是最强烈地限制土地上植物生长的元素，因此一个生态后果可能是植物总生产力的提高。更令人惊讶和相反的后果可能是植物的一种主要的自然氮源--豌豆科植物物种之间的联系，即豆科植物；以及根瘤菌--它将空气中的氮气转化为铵，这是一种氮的形式，植物用来制造蛋白质和其他生命所必需的分子。可以直接从土壤中获得更多氮素的豆类往往较少与根瘤菌联系在一起，根瘤菌需要从植物中获得碳水化合物。因此，随着时间的推移，预计全球氮素变化将减少根瘤菌的丰度，并推动合作程度较低的根瘤菌的进化，从而使植物获得的氮素更少。这反过来可能会增加对合成化肥的依赖，并对全球氮循环产生反馈。该项目将使用凯洛格生物站长期生态研究(LTER)地点现有的、已有22年历史的氮素添加实验，作为了解未来几十年自然氮素供应可能发生的情况的窗口。该项目的更广泛影响包括农业应用、对学生的研究培训和促进科学劳动力的多样性。包括大豆和玉米轮作在内的许多种植制度利用豆类和根瘤菌之间的联系作为氮素的来源。两门大型本科课程的探究式实验室将以该项目为基础，并由生态教学问题和实验提交出版，这是一种广泛使用的科学教育资源。为了促进学术界的公平性和多样性，该项目将创建基于网络的材料，并组织小组讨论，指导来自不同背景的本科生完成申请研究生院并取得成功的过程。

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.