Mega-Herds, Megaherbivores and Mutualists: Nitrogen Fixation in Tropical Grasslands and Savannas

生态学家仍然不知道为什么土壤细菌和植物固定大气中的氮（N），并将其转化为植物可用的形式，在陆地生态系统中，其生产力受到N的限制。大型哺乳动物食草动物优先食用豆类可能限制氮固定，但这在过去没有得到充分的研究。我们将使用基于测量稳定同位素15-N浓度的新方法来探索大型哺乳动物食草动物是否以及如何刺激或限制塞伦盖蒂的豆科植物和土壤细菌的固氮作用，塞伦盖蒂是世界上最后一个大型放牧生态系统之一，也是代表性的热带草原和稀树草原。我们的研究结果应该表明，食草动物限制或维持生产的食物和纤维在半干旱草原和稀树草原的发展中国家的特点。我们的研究将为不同的管理措施提供信息，这些措施可能会廉价地增加自然固氮。并应有助于更好地估计自然氮输入在生态系统中的贡献相对于人类引起的氮沉积在世界各地，包括美国我们的项目将雇用土著坦桑尼亚人，以帮助建立坦桑尼亚的能力，以管理其自然资源，减少对外国援助的依赖。

Ecologists still do not know why soil bacteria and plants that fix atmospheric nitrogen (N) and convert it into forms useable by plants are so often rare in land ecosystems whose productivity is limited by N. Preferential consumption of legumes by large mammal herbivores potentially limits N fixation, but this has not been adequately studied in the past. We will use new methods based on measuring concentrations of the stable isotope 15-N to explore whether and how large mammal herbivores stimulate or limit N fixation by legumes and soil bacteria in the Serengeti, one of the worlds last great grazing ecosystems and a representative tropical grassland and savanna. Our results should suggest ways that herbivores limit or sustain production of food and fiber in semi-arid grasslands and savannas characteristic of the developing world. Our research will inform different management actions that might inexpensively increase natural N fixation. And should help better estimate the contribution of natural N inputs in ecosystems relative to human-induced elevated N deposition around the world, including the U.S. Our project will employ indigenous Tanzanians to help build the capacity of Tanzania to manage their natural resources and rely less on foreign aid.