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.
