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

草原在维持人类生计方面至关重要，例如农业和畜牧业，以及将温室气体作为土壤碳储存在地球表面的大片区域。其中许多地区的土壤非常贫瘠，植物的养分含量很低，比如氮和磷，但牧草的生长往往比预期的要高。保持出人意料的高草生长的一个关键过程被称为固氮，植物从空气中获得氮。在东非塞伦盖蒂国家公园之前的研究中，人们发现草根，就像我们更熟悉的豆类或豆科成员一样，拥有丰富的活性细菌，可以固定大量的氮。其结果是形成了一个由各种固氮菌、其他相关土壤微生物、寄主草和各种放牧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的生态研究基础上，将探索放牧、降雨和其他土壤养分的有效性如何结合起来影响固氮。这项工作还将探索菌根真菌是否与草根共生，并在其他方面使植物受益，是否与固氮菌竞争宿主植物提供的糖和其他形式的碳，从而减少细菌的固氮。氮限制了大多数陆地生态系统的生产力和生态系统服务，而与草根相关的细菌的固氮可能被证明是许多热带和亚热带草原和稀树草原生产力、生态系统服务和农村人类生计的最重要的氮源和驱动力。这些成果将用于与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，以封存土壤碳，改善可持续畜牧业生产，并可能在自愿市场上产生碳信用销售。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。该项目将首次探索以固氮为中心的放牧动物、植物和有益土壤微生物之间的关键相互作用。它包括四个部分：(1)确定不同草种在一系列土壤、放牧和降雨条件下固氮的发生和程度；(2)进行实验，测试放牧和水、氮、磷对牧草固氮的影响是否与相同条件下的豆科植物不同；(3)在控制AM真菌和细菌的大盆栽中进行实验，以测试这些不同微生物组之间的竞争；以及(4)开发和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、或养分限制互作控制固氮和牧草生产力。这一新的模型将展示固氮细菌和有益真菌的丰富如何改变作为土壤有机质的碳的储存，并导致草食-植物-互惠网络的发展，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在塞伦盖蒂验证的土壤碳动态模型联系起来，以了解固氮与可能的土壤碳固定和从大气中消除温室气体之间的联系。

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.