Nitrogen fixation and carbon storage by herbivore-grass-mutualist interaction webs in the Serengeti

草原对于维持人类生计至关重要，例如农业和牲畜放牧，以及在地球表面的广大地区以土壤碳的形式储存温室气体。其中许多地区的土壤非常贫瘠，对植物来说，氮和磷等营养物质的含量很低，但草的生长往往比预期的要高。维持草的高生长的一个关键过程被称为固氮，植物从空气中获取氮。在之前对东非塞伦盖蒂国家公园的研究中，人们发现，像我们更熟悉的豆类或豆类家族成员一样，草根拥有大量的活性细菌，可以固定大量的氮。结果形成了一个由各种固氮细菌、其他相关土壤微生物、寄主草和各种食草动物组成的复杂网络。该项目建立在塞伦盖蒂国家公园长期生态研究的基础上，将探索放牧、降雨和其他土壤养分的可用性如何共同影响固氮作用。这项工作还将探索菌根真菌是否与固氮细菌竞争寄主植物提供的糖和其他形式的碳，从而减少细菌的固氮作用。菌根真菌也生活在草根中，并以其他方式造福植物。氮限制了大多数陆地生态系统的生产力和生态系统服务，而在许多热带和亚热带草原和稀树草原上，与草根有关的细菌的固氮可能是最重要的氮来源，也是生产力、生态系统服务和农村人类生计的驱动因素。研究结果将用于与大自然保护协会和其他非政府组织的持续合作，帮助坦桑尼亚和肯尼亚的40多个当地人类社区采用新的牲畜和火灾管理做法，这些做法可以吸收土壤碳，改善可持续的牲畜生产，并有可能在自愿市场上销售碳信用额度。该项目还将通过暑期实地课程和参与研究来雇用和培训本科生和研究生。该项目将首次探索放牧动物、植物和有益土壤微生物之间以固氮为中心的关键相互作用。它有四个部分：(1)确定在不同的土壤、放牧和降雨条件下，许多不同的禾草物种的固氮发生和幅度；(2)进行实验，测试在相同的条件下，放牧和水、氮、磷对禾草固氮的影响是否与豆科植物不同；(3)在大型盆栽中进行实验，控制AM真菌和细菌，以测试这些不同微生物群之间的竞争。(4)开发并测试一个简单的计算机模型，该模型集成了降雨、放牧、与菌根真菌的竞争或营养限制如何相互作用，以控制固氮和草的生产力。这个新的模型将展示固氮细菌和有益真菌的丰度如何改变作为土壤有机质的碳储存，并导致草食-植物-共生网络的发展，最终产生地上和地下植物凋落物、粪便和AM真菌菌丝对土壤有机碳的关键碳输入。这些模型结果将与最近验证的塞伦盖蒂土壤碳动态模型联系起来，以了解固氮如何与可能的土壤碳固存和大气中温室气体的去除联系起来。

Grasslands are critical in sustaining human livelihoods, such as farming and livestock grazing, and storing greenhouse gases as soil carbon across vast areas of the earth surface. Many of these areas have very poor soils that are low in nutrients for plants, like nitrogen and phosphorus, but the growth of the grasses is often higher than expected. A key process in maintaining unexpectedly high grass growth is called nitrogen fixation, where plants acquire their nitrogen from the air. In previous work in Serengeti National Park in east Africa, it was discovered that grass roots, like more familiar legumes or members of the bean family, host abundant, active bacteria that can fix large amounts of nitrogen. The result is a complex web of various nitrogen-fixing bacteria, other associated soil microbes, the host grasses, and a diversity of grazing animals. This project builds on a long history of ecological research in Serengeti National Park and will explore how grazing, rainfall and the availability of other soil nutrients combine to affect nitrogen fixation. The work will also explore if mycorrhizal fungi, which also live in association with grass roots and benefit plants in other ways, compete with nitrogen-fixing bacteria for sugars and other forms of carbon supplied by their host plants and thus reduce bacterial nitrogen-fixation. Nitrogen limits productivity and ecosystem services in most terrestrial ecosystems, and nitrogen-fixation by bacteria associated with grass roots may prove to be the most important source of nitrogen and driver of productivity, ecosystem services, and rural human livelihoods in many tropical and subtropical grasslands and savannas. The results will be used in an ongoing collaboration with The Nature Conservancy and other non-government organizations to help more than 40 local human communities in Tanzania and Kenya adopt new livestock and fire management practices that sequester soil carbon, improve sustainable livestock production, and potentially generate the sale of carbon credits on voluntary markets. This project will also employ and train undergraduate and graduate students through summer field courses and participation in the research.This project will explore, for the first time, the key interactions among grazing animals, plants, and beneficial soil microbes centered on nitrogen fixation. It has four parts: (1) to determine the occurrence and magnitude of nitrogen fixation by many different grass species across a range of soil, grazing, and rainfall conditions, (2) to conduct experiments that will test if grazing and water, nitrogen, and phosphorus affect nitrogen-fixation by grasses differently than legumes under the same conditions, (3) to conduct experiments in large pots where AM fungi and bacteria are controlled to test for competition between these different groups of microbes, and (4) to develop and test a simple computer model that integrates how rainfall, grazing, competition with mycorrhizal fungi, or nutrient limitation interact to control nitrogen-fixation and grass productivity. This new modeling will show how the abundance of nitrogen-fixing bacteria and beneficial fungi changes the storage of carbon as soil organic matter and leads to the development of herbivore-plant-mutualist webs that ultimately produce the key carbon inputs of above- and belowground plant litter, dung, and AM fungal hyphae to soil organic carbon. These model outcomes will be linked with recently validated modeling of soil carbon dynamics in the Serengeti to understand how nitrogen-fixation links to possible soil carbon sequestration and removal of greenhouse gases from the atmosphere.