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

生态学家仍然不知道为什么在生产力受到N限制的陆地生态系统中，固定大气氮(N)并将其转化为植物可利用形式的土壤细菌和植物如此罕见。大型哺乳动物草食动物对豆类的偏好消费可能会限制N的固定，但这在过去还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草食动物是否以及如何刺激或限制塞伦盖蒂地区豆类和土壤细菌的固氮。塞伦盖蒂地区是世界上最后的大型放牧生态系统之一，也是典型的热带草原和稀树草原。我们的结果应该表明，在发展中国家特有的半干旱草原和稀树草原上，食草动物如何限制或维持食物和纤维的生产。我们的研究将为不同的管理行动提供信息，这些行动可能会廉价地增加自然氮的固定。应该有助于更好地估计生态系统中自然氮输入相对于包括美国在内的世界各地人类造成的氮沉积增加的贡献。我们的项目将雇用坦桑尼亚本土人来帮助建设坦桑尼亚管理其自然资源和减少对外国援助的依赖的能力。

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.